MCNP MODELING OF PROSTATE BRACHYTHERAPY AND ORGAN DOSIMETRY. A Thesis SUSRUT RAJANIKANT USGAONKER

Transcription

1 MCNP MODELING OF PROSTATE BRACHYTHERAPY AND ORGAN DOSIMETRY A Thesis by SUSRUT RAJANIKANT USGAONKER Submitted to the Offie of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE May 2003 Major Subjet: Health Physis

2 MCNP MODELING OF PROSTATE BRACHYTHERAPY AND ORGAN DOSIMETRY A Thesis by SUSRUT RAJANIKANT USGAONKER Submitted to Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Approved as to style and ontent by: John W. Poston Sr. (Chair of Committee) Ian S. Hamilton (Member) Mihael Walker (Member) William Burhill (Head of Department) May 2003 Major Subjet: Health Physis

3 iii ABSTRACT MCNP Modeling of Prostate Brahytherapy and Organ Dosimetry. (May 2003) Susrut Rajanikant Usgaonker, B.S., Texas A&M University Chair of Advisory Committee: Dr. John W. Poston Sr. Using the omputer ode Monte Carlo N-Partile (MCNP), doses were alulated for organs of interest suh as the large intestine, urinary bladder, testes, and kidneys while patients were undergoing prostate brahytherapy. This researh is important beause the doses delivered to the prostate are extremely high and the organs near the prostate are potentially at risk for reeiving high doses of radiation, leading to inreased probabilities of adverse health effets suh as aner. In this researh, two MCNP version 4C odes were used to alulate the imparted energies to the organs of interest delivered by 125 I and 103 Pd. As expeted, the organs nearest to the prostate reeived the highest energy depositions and the organs farthest from the prostate reeived the lowest energy depositions. One the energy depositions were alulated, the doses to the organs were alulated using the known volumes and densities of the organs. Finally, the doses to the organs over an infinite time period were alulated.

4 iv DEDICATION I dediate this work to my parents, Drs. Rajanikant S. and Rajani R. Usgaonker, my brother, Ajay R. Usgaonker, and his wife, Madhavi Sonal A. Usgaonker who have all provided their support for me throughout the ourse of my graduate studies at Texas A&M University and my researh.

5 v ACKNOWLEDGEMENTS I would like to thank the following individuals for their assistane that led to the suessful ompletion of this researh and this thesis: Dr. John W. Poston Sr., for serving as my graduate ommittee hair and assisting with my admission into the graduate program in Health Physis in the Department of Nulear Engineering and providing the assistane and support for my projet; Dr. Ian S. Hamilton for providing Sabrina Bodybuilder, whih provided invaluable assistane in reating the male phantom neessary for the modeling, utting the time neessary to omplete the projet immensely; Dr. Mihael Walker for serving on my ommittee and providing support for the projet; Dr. John Ford, Dr. Leslie Braby, and Dr. Daniel W. Reee for answering researh related questions; and Matthew G. Arno, Md. Nasir Uddin Bhuiyan, and Frank Szakaly for assisting with debugging the odes, leading to the suessful ompletion of the exeution of the programs.

6 vi TABLE OF CONTENTS Page ABSTRACT..iii DEDICATION iv ACKNOWLEDGEMENTS v TABLE OF CONTENTS..vi LIST OF FIGURES.viii LIST OF TABLES.... ix CHAPTER I INTRODUCTION... 1 Objetives Benefits vs. Risks Signifiane of the Researh II THEORY.. 4 The Prostate Gland Prostate Caner Prostate Brahytherapy Ultrasound Graphs of the Seeds in Prostate III MATERIALS AND METHODS Monte Carlo N-Partile Computer Modeling Sabrina Bodybuilder Proedures

7 vii Page IV RESULTS.. 27 MCNP Results Doses to the Critial Organs V DISCUSSION AND CONCLUSIONS Disussion Future Work.. 36 REFERENCES APPENDIX A APPENDIX B VITA

8 viii LIST OF FIGURES FIGURE Page 1 Seed plaement in prostate ross setion ( 125 I ) Seed plaement in prostate ross setion ( 103 Pd) Posterior view of prostate ( 125 I) Interior view of prostate ( 125 I) Cross-setional view of prostate ( 125 I) Retal view of prostate ( 125 I) Posterior view of prostate ( 103 Pd) Interior view of prostate ( 103 Pd) Cross-setional view of prostate ( 103 Pd) Retal view of prostate ( 103 Pd) Side view (ut at x=2) of male phantom Anterior view (ut at y=2) of male phantom Lateral view of prostate region (ut at y=2)

9 ix LIST OF TABLES TABLE Page 1 Calulated doses to organs in phantom Calulated lifetime doses to the phantom Fatal aner risk assessment

10 1 CHAPTER I INTRODUCTION. OBJECTIVES The objetive of this researh was to determine the radiation-absorbed doses to the organs that were likely to reeive signifiant doses from the implantation of radioative brahytherapy seeds in the prostate. This was ahieved by using omputer odes that were exeuted with Monte Carlo N-Partile (MCNP) to alulate the doses to the organs of interest (Briesmeister 2000). BENEFITS VS. RISKS For many deades, radiation has been benefiial in both diagnosti and therapeuti nulear mediine. Radiation has been used to diagnose and treat aner with proesses suh as ablation (as in 131 I therapy used to treat thyroid aner), brahytherapy, and external beam therapy. One of the most effetive radiation therapies is prostate brahytherapy. Brahytherapy has been a suessful mode of treatment for prostate aner for years. Although other options suh as surgery and external beam therapy exist, brahytherapy has been the best at preventing reourring tumors. Prostate brahytherapy involves the permanent plaement in the prostate gland of radioative seeds, whih emit energeti partiles through radioative deay by modes of eletron apture and emission of photons. These energeti partiles irradiate the internal tissues of the prostate, This thesis follows the style and format of Health Physis. 1 Personal Communiation: Dr. George Warner, May 2002.

11 2 diminishing the volume of the tumor and killing the aner ells, preventing them from reourring 1. The seeds used in prostate brahytherapy are iodine-125 ( 125 I) and palladium-103 ( 103 Pd). These two radionulides have relatively long half-lives ompared to most radiopharmaeutials: 59.4 days for 125 I and days for 103 Pd (Chart of the Nulides 1996). Both seeds are about the size of rie grains and are about 4.5 mm long and about 0.8 mm in diameter. The 125 I seeds onsist of 0.05 mm thik titanium walls with welded ends and silver markers that are surrounded by 125 I (Nath, et al. 1995). The 103 Pd seeds also onsist of 0.05 mm thik titanium walls, but ontain lead x-ray markers instead of silver markers (Meigooni, et al. 1990). The long-term benefit of undergoing this treatment is the minimized probability of reourring tumors, adding years to the patients lives and allowing them to proeed with their regular daily ativities. Although there are signifiant benefits to undergoing this therapy, there are also ertain issues that must be onsidered. These issues inlude side effets and radiation safety. The fous of this researh is to the dose delivered to other organs while undergoing this therapy, sine the seeds are permanently implanted in the gland. The doses delivered to the prostate are large and an be as high as 145 gray (Gy) in typial treatments. This researh is important beause, sine the radiation dose to the prostate is large, the neighboring organs (i.e., urinary bladder and genitalia) are at risk of reeiving high radiation doses. Areas suh as the pelvis, kidneys, stomah, lungs, and organs in

12 3 the lower portions of the trunk of the body, as well as ertain organs in the upper portions of the trunk, may reeive signifiant doses from the prostate treatment. One disadvantage of prostate brahytherapy is that healthy prostate ells are killed along with anerous ells. As a result of irradiation of the prostate gland, the neighboring genitalia and urinary bladder are likely to be damaged. One potential risk for some men is impotene sine the radiation penetrates through the prostate and irradiates the ells in the testiles, preventing the prodution of sperm. Although this effet is not desirable, patients give it less onsideration when omparing it to the importane of living a long and aner-free life. Also, the patients generally are older men past the age of ative proreation. SIGNIFICANCE OF THE RESEARCH The signifiane of this researh is that doses to internal organs an be determined. Organs suh as the urinary bladder and the genitalia are likely to reeive the highest doses from the therapy, sine they border the prostate gland. However, many onsider the benefits of brahytherapy to greatly outweigh the risks of late effets. Beause of the omplexities of using humans to determine doses to internal organs from prostate brahytherapy, using the omputer ode MCNP in modeling the energy deposition in the organs from brahytherapy would be appropriate. Doses to the internal organs an be determined using an MCNP ode, based on atual brahytherapy ases.

13 4 CHAPTER II THEORY THE PROSTATE GLAND The prostate gland is a partly musular, partly glandular organ loated distal to the nek of the urinary bladder and proximal to the penis. The gland surrounds the proximal urethra within the pelvi avity, dorsal to the symphysis pubis and ventral to the deep layer of the triangular ligament, while immediately adjaent to the retum. The prostate has an ellipsoidal shape and normally is about the size of a hestnut. The glandular part of the prostate onsists of epithelial folliular pouhes. The prostate has arteries originating in the internal pudi, vesile, and haemorrhoidal areas. The veins form a plexus around the sides and the base of the gland. The nerves to the prostate originate from the pelvis plexus (Gray 1995). The prostate is an essential organ in the male reprodutive system that produes the fluid in semen, whih is ejaulated during sexual interourse and delivers many sperm into the female vagina while the penis is eret. The fluid is alkaline to protet the sperm from the aidity of the vagina as they travel to the female ovum. The prostate is also an essential organ for the prodution of male hormones suh as androgens (Marieb 1998). PROSTATE CANCER Prostate aner is the seond most ommon neoplasm in males over the age of 50 (Handbook of Diseases 2000). The most ommon form of prostate aner is adenoarinoma, with saroma rarely ourring. Most of the tumors originate in the

14 5 posterior prostate gland, while the rest originate near the urethra. When the prostate lesions metastasize, they invade the prostate apsule and spread along the ejaulatory duts between the seminal vesiles or perivesiular fasia (Handbook of Diseases 2000). In men, prostate aner aounts for about 18% of all aners. The inidene of prostate aner is highest in blaks and lowest in the Asian populations. The symptoms of prostate aner are diffiulty in urinating, urine retention, and unexplained ystitis. The Amerian Caner Soiety reommends that men over the age of 40 reeive a yearly digital examination. For men above the age of 50, it is reommended that blood tests be performed to detet the prostate speifi antigen (PSA). Biopsies are used to detet malignant tumors in the prostate. Elevated PSA levels may be signifiant beause serum phosphatase levels are inreased in two-thirds of men with metastati prostate aners. The treatment options available are radiation, hormone therapy, and surgery. If these options are not effetive, many physiians opt for the use of hemotherapy (Handbook of Diseases 2000). PROSTATE BRACHYTHERAPY Interstitial brahytherapy has reeived high interest due to the use of ultrasound to guide seed implantation, improved radionulides for dose delivery, exellent treatment planning software for dosimetri alulations, and effiient implantations. Due to these fators, the use of brahytherapy implants in prostates is expeted to inrease (Viini, et al. 1999). Aording to the Amerian Brahytherapy Soiety (ABS), the dose distributions after implantation often differ from the planned doses before implantation. For this reason, ABS reommends that a postimplant dosimetri

15 6 assessment be performed so those atual doses to the prostate and adjaent tissues over the treatment period an be doumented. Using these dosimetry results, physiians an assess and modify implantation tehniques and ompare results with other institutions as quality assurane for linial trials (Nag, et al. 2000). A drawbak to this treatment is the need for ustomized treatments for different patients. The main objetive in prostate brahytherapy is to irradiate the entire prostate, in addition to a margin to take into aount seed displaement and potential extraapsular disease extension (ECE). In most ases, the ECE is within 3 mm from the prostate edge. Usually, the seeds are not implanted perfetly, ausing implant-related prostate swelling. Beause of the relationship between the linear prostate dimensions and volume, there is a need for ustomized treatment planning for different patients. As the prostate volume inreases, the prostate linear dimensions hanges gradually (Ove, et al. 2001). The proess of prostate brahytherapy requires the servies of a radiation dosimetrist. One the dosimetrist determines the plaement, and ativity of the radioative seeds, and the duration of the implant, a radiation onologist follows the dosimetrist s plans and plaes the seeds into the prostate. Prior to the implantation of the seeds, the patient is given guidelines regarding side effets and minimizing radiation exposure of his family. After implantation, omputed tomography (CT) sans are usually performed for dosimetri analysis. The hallenge with CT sans is that post-implant edema usually

16 7 inreases the prostate volume by over 50%. With a typial half-life of 9 days, the edema dereases exponentially with time (Waterman & Diker 1999). The side effets of this treatment are fatigue, skin reations suh as bruising, frequent and painful urination, diarrhea, and retal bleeding. Aording to one study, retal bleeding or uleration was assoiated with treatments with doses over 100 Gy to the prostate as the soures irradiated the retal wall (Waterman & Diker 1999). Although not as frequent as from surgeries, prostate brahytherapy an lead to temporary impotene and inontinene. Many patients, however, have urinary problems suh as polyuria, urgeny, inomplete voiding, straining, and noturia. Aute urinary retention ours in approximately 3%-22% of the patients that undergo prostate brahytherapy (Merrik, et al. 2000). One the treatment is omplete, these side effets typially diminish 2. As stated above, the brahytherapy proess begins with the dosimetrist who plans the outline of the seed plaement. First, the dosimetrist obtains an ultrasound reording of the patient s prostate. Using omputer software, the dosimetrist examines the prostate and plaes a two dimensional grid on the prostate image, with the enter of the grid plaed diretly over the enter of the gland. The dosimetrist takes the three-dimensional organ and reates multiple ultrasound views of the prostate from all sides. Then, eah of the ultrasound views of the prostate is outlined to reate two-dimensional surfaes 3. 2 Personal Communiation: Dr. George Warner, May Personal Communiation: Mrs. Elizabeth Dodge, July 2002.

17 8 After the dosimetrist maps the prostate from various views, omputer software is used to determine the seed plaement. The default seed plaement is dependent on the size of the prostate and will usually plae more seeds and/or needle insertions on larger prostates. One the default seeds are displayed, isodose urves appear. The main objetive of the dosimetrist is to ensure that the entire prostate is adequately irradiated without delivering a signifiant dose to ertain ritial areas beyond the prostate. Thus, the seeds, with the needles, are distributed appropriately so that the isodose urve surrounds the prostate gland and goes beyond the prostate only by about 3-4 mm. The dosimetrist tries to minimize radiation doses to the urethra and the retal area. The dosimetrist plaes the seeds external to the prostati urethra 4. In interstitial brahytherapy planning, dose homogeneity is desired to uniformly irradiate the target tissues. Many methods have been used, suh as the inverse doserate effet and less dose distribution to the urethra and retum (D Souza & Meyer 2001). Treatment planning for prostate brahytherapy is performed by assuming that the seeds are point soure emitters in a homogeneous phantom. In the study of planning proedures, a Task Group report (TG-43) with dosimetri protools was used to standardize dosimetri parameters for speifi seed designs, whih were measured in water (DeMaro, et al.1999). Sample seed plaements for 125 I and 103 Pd seeds are given in Figs. 1 and 2, respetively (provided ourtesy of Elizabeth Dodge of Caner Therapy and Researh Center [CTRC], San Antonio). These two examples were for patients with different 4 Personal Communiation: Mrs. Elizabeth Dodge, July 2002.

18 9 prostate sizes, leading to slight differenes in their therapies. These figures show twodimensional seed plaements on the prostate ross-setion. For purposes of this researh, these sample distributions were used in the MCNP odes for alulating the doses to the ritial organs from these two soures. The geometrial shapes represent the different retrations of the needle plaements. These retrations are the distanes from the prostate surfae where the needles are to be inserted to implant the seeds. The numbers inside eah of the needle plaements represent the number of seeds in those needles. As stated above, these arrangements were based on the isodose urve, whih is alulated by the dosimetrist 5. Fig. 1: Seed plaement in prostate ross setion ( 125 I) 5 Personal Communiation: Mrs. Elizabeth Dodge, July 2002.

19 10 Fig. 2: Seed plaement in prostate ross setion ( 103 Pd) This seed arrangement ensures that the entire prostate is irradiated while restriting the dose outside the surfae of the gland. These figures show the seeds in the needles as they are plaed into the prostate as well as the retration of some of the plaements. The retration refers to the depth from the prostate surfae at whih the seeds are implanted. The urethra passes through the ross-setion of the prostate, whih is the reason for plaing the seeds peripherally. The region at the bottom of the prostate, just below the prostate s fold, is the area that rests on the retum. One the dosimetrist determines the seed plaement in the prostate with the number of seeds per needle, and the ativity per seed, the seeds and needles are ordered. The dose delivered to the prostate during brahytherapy is generally large. The plaement of the seeds is highly dependent on the size of the prostate. For the 125 I

20 11 example, there were 25 needles used with a total of 98 seeds, eah with an ativity of 0.31 mci. For the 103 Pd example, there were 29 needles used with a total of 115 seeds, eah with an ativity of 1.4 mci. Sine the seeds remain in the prostate, the dose represents that delivered by the total deay of the radionulide. ULTRASOUND GRAPHS OF THE SEEDS IN PROSTATE For the ases shown in Figs. 1 and 2, ultrasound graphs have been provided for the therapy. These ultrasound graphs show the seeds plaed in the prostate ross-setion with isodose urves for the prostate, the urinary bladder, the retum, and the urethra. The ultrasound graphs show two-dimensional views of the isodose urves from various ross-setional views of the prostate. The ultrasound views are shown in Figs. 3, 4, 5, and 6 for the 125 I ase and Figs. 7, 8, 9, and 10 for the 103 Pd ase (provided ourtesy of Elizabeth Dodge of CTRC, San Antonio). The isodose urves for the bladder, prostate, urethra, and retum were used for dosimetri purposes so that the seeds ould be arranged uniformly without signifiantly irradiating the bladder nek and the retal area. The isodose urves represent the minimum doses to the areas of the prostate where they lie. As stated above in Figs. 1 and 2, the urethra passes through the prostate s ross setion, whih is the reason for the seeds being plaed away from the enter and the isodose urves not reahing the enter. The retum lies just below the prostate and as a result, the seeds are also distributed away from the retum. Beause the 125 I and 103 Pd ases were for two different patients with different prostate sizes, the seed distributions were also different. The nulides

21 12 have different emission energies; thus it was neessary to distribute the seeds differently to protet the retum and urethra. Fig. 3: Posterior view of prostate ( 125 I)

22 Fig. 4: Interior view of prostate ( 125 I) 13

23 14 Fig. 5: Cross-setional view of prostate ( 125 I) Fig. 6: Retal view of prostate ( 125 I)

24 Fig. 7: Posterior view of prostate ( 103 Pd) 15

25 Fig. 8: Interior view of prostate ( 103 Pd) 16

26 17 Fig. 9: Cross-setional view of prostate ( 103 Pd) Fig. 10: Retal view of prostate ( 103 Pd)

27 18 CHAPTER III MATERIALS AND METHODS MONTE CARLO N-PARTICLE Monte Carlo N-Partile (MCNP) is a omputational ode that an be used for various appliations. The ode uses a alulation method alled Monte Carlo simulation to alulate the quantities of interest in various omplex geometries. Beause of this property, it was a useful tool in determining the radiation-absorbed doses to the organs of interest in the prostate brahytherapy proess. The version of MCNP used in this researh was 4C (Briesmeister 2000). The MCNP ode is setup with entries that are used to reate the model of interest. The first set of entries is referred to as a ell ard. These entries are used to setup the areas of interest and ombine surfaes by using union, intersetion, or omplement operators. These ells are important for the determining region in whih the user wishes to alulate the quantity of interest (i.e., energy deposition, energy fluene). The next set of entries is referred to as a surfae ard. These entries are used to define the surfaes that make up the ells of interest. The surfaes that MCNP an model are basi three-dimensional shapes suh as spheres, ylinders, torri, and ones. The ode an model two-dimensional planes that an be used to set upper and lower boundaries for three-dimensional geometries with infinite dimensions, suh as ylinders. The third set of entries is referred to a material ard. These entries are used to speify the material omposition of the ells, in terms of the hemial omposition.

28 19 These an be entered using the atomi numbers and mass numbers of the elements followed by either perent or atomi ompositions. One all neessary ards have been entered, the model has been setup with the surfaes and the materials. One the three main ards have been setup, the next step is to define the soures, partile types, number of partile histories, and types of tally alulations to be performed. Sine the ode an be used to alulate interations by neutrons, eletrons, and photons, the mode needs to be entered so that the ode will alulate the interations for the appropriate radiation (n=neutrons, p=photons, e=eletrons). The number of partile histories must be entered and should be seleted to provide the required statistial auray. The soures an be setup either as Cartesian soures suh as point and line soures, or as three-dimensional soures suh as spherial and ylindrial soures. The soure information must be entered with partile energies, soure positions, and emitted partile types speified by number (neutrons=1, photons=2, eletrons=3). After the soure information has been entered, tally ards are entered whih define the alulations of interest, suh as energy fluene, energy deposition in mass, and energy pulse. One the soures, partile histories, and tallies are entered, the ode is ompleted with the print ommand at the end. When all these steps are ompleted, the user is ready to exeute the odes and perform alulations. The exeution of the odes is omplete when all partile histories have been followed. COMPUTER MODELING In many ases, omputer modeling is neessary for simulating omplex situations, suh as the irradiation of a human organ beause many issues suh as

29 20 geometry and target materials must be onsidered. One form of modeling uses an independent dose-to-point alulation program for a high-dose-rate prostate brahytherapy planning system. This program an be used by hospitals to verify the doses in brahytherapy planning (Cohen, et al. 2000). A Monte Carlo tehnique an be used to alulate many parameters, suh as dose rates as a funtion of photon energies by using the photon-energy spetrum of the radionulide (Rivard 2001). SABRINA BODYBUILDER Sabrina Bodybuilder is a software pakage that an be used to model a male or a female from a newborn to a 21-year old as well as pregnant women and phantoms with male and female reprodutive organs. The software also an show graphially the phantoms that are being modeled in an MCNP ode. The software has all the equations for the major skeletal bones, organs of the gastrointestinal trat, lungs, kidneys, brain, adrenals, spleen, thyroid, thymus, and skin. For the male, the software has equations for the genitals, whih inludes the penis and the srotum in addition to their skin. For the female, the software has equations for the ovaries and the ervix. These equations reate a mathematial phantom that an be used to model various situations, from internal dosimetry in brahytherapy to external dosimetry in external beam therapy. Beause these equations are based on typial phantoms used for dosimetry, some organs, suh as the prostate, are not available in the odes. Beause of this, it would be neessary for the user to model these organs based on the geometries and the positions of the other organs modeled with Sabrina Bodybuilder.

30 21 PROCEDURES Using Sabrina Bodybuilder, an 18-year old male was modeled with most of the organs in the lower trunk as well as the major organs in the upper trunk. One the phantom was reated, the organs of interest were seleted that were likely to reeive signifiant doses from prostate brahytherapy. The organs of interest were the leg bones, the pelvis, the spine (upper, middle, lower), the ribs, the stomah wall, the small intestine wall, the large intestine wall (asending, transverse, desending, sigmoid), the kidneys, the liver, the lungs, the testiles, the urinary bladder wall, the penis and srotum (with skin), and the legs (with skin). Sine the prostate was not inluded in Sabrina Bodybuilder, it was added into the ode and was set between the urinary bladder and the genitalia as a sphere with radius of 2.5 m. Spherial shape was used in prostate modeling rather than an ellipsoidal shape to simplify the geometry by using a single radius as opposed to major and minor axes. One the organs were seleted and the prostate was added, the soure ard was setup. Point soures were used for soure definitions, were distributed inside the volume of the prostate gland and were laid out with arrangements disussed previously. Two odes were reated to simulate the therapy: one for 125 I and the other for 103 Pd. For 125 I, a total of 98 point soures were arranged with eah soure emitting photons of kev. For 103 Pd, a total of 115 point soures were arranged with eah soure emitting photons of 21 kev. Sine both radionulides deay via eletron apture, they emit Auger eletrons and photons. Beause of this deay mode, the modes onsidered in both odes were eletron and photon transport, however, the energy deposited in the organs

31 22 was due entirely to photons (Nath, et al. 1995). The oordinates for eah of the seeds used in both 125 I and 103 Pd treatments are given in Appendix A. After the soure ards were entered for both odes, the tally ards were entered to allow alulation of the energy deposited in the organs of interest. The doses from eletrons annot be alulated by using MCNP. Beause of the onstraint with eletron transport and alulating energy deposition in mass tallies, it was neessary to alulate the energy deposited (in MeV) and then alulate the doses using known densities and volumes of the tissues, given in Sabrina Bodybuilder. Finally, one the neessary ards were entered, the number of partile histories was entered for both of the odes. For both 125 I and 103 Pd, 10,000,000 partile histories were seleted. This number of histories was onsidered to provide reasonable statistial auray with minimal exeution time. One all of the neessary information was entered, the odes were set to exeute, whih alulated the energy deposited in the regions of interest. The phantom was reated in a Cartesian oordinate system (x,y,z). The origin of the oordinate system was in the enter of the phantom just below the pelvi region. The x-axis was the horizontal axis that traversed the phantom (width of phantom). The y- axis was the axis from the front of the phantom to the rear (depth of phantom). The z- axis was the vertial axis from the head to the feet (height of phantom). The prostate was designed as a sphere with a radius of 2.5 m and its enter at (0, , 2.805), just between the urinary bladder and the genitals. The position and size of the prostate is desribed by:

32 = x 2 + (y ) 2 + (z ) 2. (1) The geometry for the male phantom from the lateral view (ut at x=2) is shown in Fig. 11. An anterior view of the phantom (ut at y=2) is shown in Fig. 12. A lateral view of the mid-setion of the phantom (ut at y=2 and extent=20) with the prostate position with respet to the urinary bladder is shown in Fig. 13. The extent ommand magnifies the area of interest on the plot of the geometry. Fig. 11: Side view (ut at x=2) of male phantom

33 Fig. 12: Anterior view (ut at y=2) of male phantom 24

34 25 Fig. 13: Lateral view of the prostate region (ut at y=2) The figures show the various views of the 18-year old male phantom with the internal organs. The shapes of the organs of the phantom modeled were approximate to atual human organs. Many of the organs in the atual human body, partiularly in the pelvi region are squeezed together and are not as far apart as the figures show. Many of the atual human organs, suh as the prostate and the bladder even have lose ontat with eah other, whih is not represented by the above figures. Using this 18-year old

35 26 male phantom, the doses to the organs of interest were alulated using the Monte Carlo tehnique. Beause atual therapy ases were used in the MCNP model, these doses were onsidered to be representative of the doses the organs would reeive through this therapy.

36 27 CHAPTER IV RESULTS MCNP RESULTS One the odes were exeuted, the energy deposited (in MeV) in eah organ of interest was determined (see Appendix B for the input files). The doses to the organs of interest were onsistent with the distane from the prostate for both, 125 I and 103 Pd. Generally, the energies deposited in the organs were higher in the 125 I ase than in the 103 Pd ase as expeted due to the higher energy photons emitted by 125 I. DOSES TO THE CRITICAL ORGANS One the imparted energies were alulated, the absorbed doses per transformation (in Gy trans -1 ) were alulated using known densities and volumes of the organs. These doses represented the radiation absorbed doses per transformation in the organs of the phantom modeled in the odes. These doses are shown in Table 1.

37 28 Table 1: Calulated doses to organs in phantom 125 I Absorbed Energy Per Trans (MeV trans -1 ) 103 Pd Absorbed Energy Per Trans (MeV trans -1 ) 125 I Absorbed Dose Per Trans (Gy trans -1 ) 103 Pd Absorbed Dose Per Trans (Gy trans -1 ) Organ Organ Density Volume Organ (g m -3 ) (m 3 ) Leg Bones X X X X X10-18 Pelvis X X X X X10-18 Lower Spine X X X X Middle Spine X X X X Upper Spine X X X X Ribs X X X X X10-21 Stomah X X X X Small Intestine X X X X X10-20 Asending Colon X X X X10-20 Transverse Colon X X X X Desending Colon X X X X10-18 Sigmoid Colon X X X X10-15 Kidneys X X X X Liver X X X X Lungs X X X X X10-22 Testes X X X X10-16 Urinary Bladder X X X X10-17 Penis & Srotum X X X X X10-16 Penis & Srotum Skin X X X X10-17 Leg Skin (Left) X X X X X10-18 Leg Skin (Right) X X X X X10-19 Legs X X X X X10-17 Prostate X X X X10-15

38 29 The doses shown on Table 1 represent the doses per partile history to the organs at the time of implantation. To assess the atual doses the organs of interest, these doses must be evaluated over a period equal to total deay of the radionulide. Beause of this, it was neessary to alulate the total number of transformations that would our in the soures over this time period. The ativity, whih represents the number of transformations per seond, is given by Equation (2): A = A 0 e -λt (2) where A= Ativity at time t in bequerels (Bq); A 0 = Initial ativity in bequerels (Bq); λ= Deay onstant of the radionulide (s -1 ); and t= Time from the initial deay to the final deay (s). The deay onstant, λ, an be determined using the half-life of the radionulide and is alulated using Equation (3): ln 2 λ = (3) T 1/2 where T 1/2 represents the half-life. Using Equations (2) and (3), the number of transformations ourring an be alulated for any time period. But, in this researh, an infinite time was of interest beause the lifetime patient doses were the main interest. Using Equation (4), the total number of transformations (U s ) an be determined over an infinite time period: λt Us A0e dt = (4). 0 The solution for this equation over an infinite time interval is given by Equation (5):

39 30 A0 Us = (5). λ One the total number of transformations has been determined, the total dose (in Gy) to the organs an be alulated using Equation (6): Absorbed Dose = 1.602X10-10 i U S SEE(T S) i m -1 (6) where 1.602X10-10 = onversion fator from MeV to J and g to kg; SEE(T S) = speifi effetive (absorbed) energy transferred from soure to target (MeV), here the radiation quality fator is unity for photons; and m = mass of the target organ (g). The mass of the target organs was alulated using the known densities and volumes of the organs, reated by Sabrina Bodybuilder. One the alulations were performed, the dose to eah of the organs was alulated. The results represented the doses the male phantom reeived from these brahytherapy seeds, whih were representative of the doses the patient would reeive during his lifetime. The results of the alulations are given in Table 2.

40 31 Table 2: Calulated lifetime doses to the phantom 125 I Transformation Over Infinite Period 103 Pd Transformation Over Infinite Period 125 I Lifetime Dose (Gy) 103 Pd Lifetime Dose (Gy) Organ Leg Bones 8.3X X X10-2 Pelvis 8.3X X Lower Spine 8.3X X X Middle Spine 8.3X X X Upper Spine 8.3X X X Ribs 8.3X X X X10-5 Stomah 8.3X X X Small 1.3X10 16 Intestine 8.3X X10-4 Asending 1.3X10 16 Colon 8.3X X10-4 Transverse 1.3X10 16 Colon 8.3X Desending 1.3X10 16 Colon 8.3X X10-2 Sigmoid 1.3X10 16 Colon 8.3X Kidneys 8.3X X X Liver 8.3X X X Lungs 8.3X X X X10-6 Testes 8.3X X Urinary 1.3X10 16 Bladder 8.3X Penis & 1.3X10 16 Srotum 8.3X Penis & 1.3X10 16 Srotum Skin 8.3X Leg Skin 1.3X10 16 (Left) 8.3X X10-2 Leg Skin 1.3X10 16 (Right) 8.3X X10-3 Legs 8.3X X Prostate 8.3X X

41 32 As shown in Table 2, the 18-year old male phantom reeived prostate doses of approximately 102 Gy and 105 Gy from 125 I and 103 Pd, respetively. Sine the ativity of the 103 Pd seeds was higher than that for 125 I, a slightly higher dose to the prostate gland was expeted. Generally, 125 I delivered higher doses to the organs ompared to 103 Pd and in both ases, the organs nearest to the prostate reeived the highest doses, while the organs more distant from the prostate reeived the lowest doses, whih was expeted for these radiation therapies. The organs that reeived signifiantly high doses were the penis and srotum, the urinary bladder, the testiles, and the sigmoid olon.

42 33 CHAPTER V DISCUSSION AND CONCLUSIONS DISCUSSION These absorbed doses were approximately what patients would reeive from prostate brahytherapy from either 125 I or 103 Pd. The use of Monte Carlo alulations allowed this task to be ompleted suessfully, whih would have been extremely ompliated with onventional mathematis due to the omplex geometries as well as issues suh as attenuation and buildup. As expeted, the doses to the prostate were in the range above 100 Gy. Also as expeted, the prostate doses from both nulides were approximately the same, sine the 103 Pd ase had more seeds with lower photon energies ompared to the 125 I ase, whih had lower number of seeds with higher photon energies. The urinary bladder, sigmoid olon, testes, and penis and srotum reeived the highest doses from the radioative soures in the prostate, whih was expeted due to their proximity to the prostate. Generally, the doses to the other organs were below 1 Gy and the organs more distant from the prostate suh as those near the rib age were muh less than 1 Gy. Even though these doses were representative of the atual ases, there are some fators that must be onsidered. The phantom used to perform the modeling was based on a body of an 18-year old male. This phantom was used beause there were no phantoms representing 70 year-old males in Sabrina Bodybuilder. Beause of this, there would be slightly different values for the absorbed doses to the organs due to the varying skeleton volume, fat ontent in the trunk, and body mass. However, beause the

43 34 differenes in the sizes and positions of the organs between young adult males and elderly males are generally in the millimeter range, the differenes are minimal. Another fator that must be taken into aount is that the prostate was modeled as a sphere with a diameter of 5 m, whih is not atually the ase in most males. The sizes and shapes of prostates an vary from patient to patient, whih would lead to varying results. Presently, health effets are neither a onern to the patients, nor to the physiians beause of the overall effet in treating tumors. Although there may be a risk of getting seondary aners over many deades, this therapy is generally preferred over the ertainty of dying from prostate aner. Beause prostate aner ours during the later years of the individual s life, the probability of adverse health effets from this treatment would not be a major onern for the patients, espeially beause the treatment will save their lives from a terminal illness to whih they will definitely suumb without treatment. Beause of this, prostate brahytherapy an ontinue to be an effetive proedure in treating prostate aner and inreasing the years of life for the patients. However, risk assessments an be performed by onsidering the risks of getting fatal aners and risk oeffiients provided by the International Commission on Radiation Protetion (ICRP 1990). Using these oeffiients and the alulated doses to eah of the organs, the risk of getting fatal aner an be alulated for eah organ as well as for the individual. Beause the energies emitted were due to photons, the dose is equal to the dose equivalent, sine the radiation weighing fator is one, so 1 Gy equals 1 sievert (Sv). Sine the doses to the red bone marrow, thyroid, breast, esophagus, and

44 35 ovary were not alulated, the dose equivalents to these organs were not onsidered. Sine hereditary effets are not signifiant, the testes were not inluded in the risk assessment. The alulated risks are given in Table 3. Organs Risk Coeffiients (Sv -1 ) Table 3: Fatal aner risk assessment 125 I Organ 103 Pd Organ Dose Dose Equivalent Equivalent (Sv) (Sv) 125 I Fatal Caner Risk 103 Pd Fatal Caner Risk Bladder 3X X10-2 3X10-3 Bone 5X Marrow Bone 5X X X10-5 Surfae Breast 2X Colon 8X Liver 1.5X X X Lung 8.5X X X X X10-8 Esophagus 3X Ovary 1X Skin 2X X X10-4 Stomah 1.1X X X Thyroid 8X Kidney 5X X X Total 5X X X10-1 The alulated risks for fatal aner to the individual from 125 I and 103 Pd were about 25% and 13%, respetively. As expeted, the risk of developing fatal seondary aners was higher in 125 I beause of the higher photon energies. The organs that had the highest risk of developing fatal aners were the urinary bladder and the olon, whih was expeted sine these organs were adjaent to the prostate. However, the organ that ontributed the signifiant risk of developing fatal aner was the olon, due to the high dose delivery to the sigmoid olon by both soures.

45 36 FUTURE WORK Although this researh involved the use of atual brahytherapy ases, they do not neessarily represent the exat doses the patients are reeiving due to geometri fators and distanes between the organs. Beause of these fators, there are reommendations for future work. A reommendation for future researh is to use tissue equivalent phantoms using brahytherapy seeds and plaing dosimeters in the regions of interest. This projet would probably provide more aurate results beause this would eliminate the disrepany with the sizes of the organs, partiularly the prostate. This would also help in eliminating the ompliations of distanes between the organs. Another reommendation for future researh is to reate a phantom of an elderly male and to write an MCNP ode based on this phantom. Some potential differenes between the 18-year old and the elderly male phantoms would be in their masses, heights, fat ontent, and varying bone masses. Among many other fators, these fators may impat the results obtained in this researh beause generally people lose weight and beome shorter. Beause of this, many of the doses would probably be higher due to the loser distanes to the prostate, onsidering the redued musle ontent in the elderly males as opposed to younger males. A third reommendation is to reate a void between the organs in the pelvi region to aount for the organ surfaes ontating eah other by setting a vauum between these organs. Setting a void between these organs in the pelvi region will provide a more aurate aount of the doses to these organs from prostate

46 37 brahytherapy, whih are likely to be higher sine there will be no material for the photons to interat before reahing the organs of interest.

47 38 REFERENCES Briesmeister, JF. MCNP -A General Monte Carlo N-Partile Transport Code, version 4C. Los Alamos National Laboratories. Los Alamos, NM; Cohen, GN, Amols, HI, Zaider, M. An independent dose-to-point alulation program for the verifiation of high-dose-rate brahytherapy treatment planning. Int. J. Radiation Onology Biol. Phys., 48(4): ; DeMaro, JJ, Smathers, JB, Burison, CM, Nube, QK, Solberg, TD. CT-Based dosimetry alulations for 125 I prostate implants. Int. J. Radiation Onology Biol. Phys., 45(5): ; D Souza, WD, Meyer, RR. Dose homogeneity as a funtion of soure ativity in optimized 125 I prostate implant treatment plans. Int. J. Radiation Onology Biol. Phys., 51(4): ; Gray, Henry. Gray s Anatomy, 15 th edition. Philadelphia, PA: Barnes & Noble Handbook of Diseases, 2 nd edition. Springhouse, PA: Springhouse Corp International Commission on Radiation Protetion Report No. 60. Oxford: Pergamon. November Marieb, RN. Human Anatomy & Physiology, 4 th edition. Menlo Park, CA: Benjamin/Cummings Siene Publishing Meigooni, AS, Sabnis, S, Nath, R. Dosimetry of Palladium-103 brahytherapy soures for permanent implants. Endourietherapy/Hyperthermia Onol. 6(2): , Merrik, GS, Butler, WM, Lief, JH, Dorsey, AT. Temporal resolution of urinary morbidity following prostate brahytherapy. Int. J. Radiation Onology Biol. Phys. 47(1): ; Nag, S, Bie, W, DeWyngaert, K, Prestidge, B, Stok, R, Yan, Y. The Amerian Brahytherapy Soiety reommendations for permanent prostate brahytherapy postimplant dosimetri analyses. Int. J. Radiation Onology Biol. Phys. 46(1): ; Nath, R, Anderson, LL, Luxton, G, Weaver, KA, Williamson, JF, Meigooni, AS, et al. Dosimetry of interstitial brahytherapy soures: Reommendations of the AAPM Radiation Therapy Committee Task Group No. 43. Med Phys. 22:

48 39 Nulides and Isotopes: Chart of the Nulides, 15 th edition. General Eletri Co. and KAPL, In. San Jose, CA Ove, R, Wallner, K, Badiozamani, K, Korjsseon, T, Sutlief, S. Standardization of prostate brahytherapy treatment plans. Int. J. Radiation Onology Biol. Phys., 50(1): ; Rivard, MJ. A disretized approah to determining TG-43 brahytherapy dosimetry parameters: ase study using Monte Carlo alulations for the MED Pd soure. Appl. Rad. and Isotopes 55: ; Viini, FA, Kini, VR, Edmundson, BS, Gustafson, GS, Stromberg, J, Martinez, A. A omprehensive review of prostate aner brahytherapy: defining an optimal tehnique. Int. J. Radiation Onology Biol. Phys., 44(3): ; Waterman, FM, Diker AP. Effet of post-implant edema on the retal dose in prostate brahytherapy. Int. J. Radiation Onology Biol. Phys., 45(3): ; 1999.

49 40 APPENDIX A 125 I SEED PLACEMENTS Seed # Ativity x y z mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci

50 125 I SEED PLACEMENTS (Con t) Seed # Ativity x y z mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci

51 125 I SEED PLACEMENTS (Con t) Seed # Ativity x y z mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci

52 103 Pd SEED PLACEMENTS Seed # Ativity x y z mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci mci