Non-target dose from radiotherapy: Magnitude, Evaluation, and Impact. Stephen F. Kry, Ph.D., D.ABR.

Transcription

1 Non-target dose from radiotherapy: Magnitude, Evaluation, and Impact Stephen F. Kry, Ph.D., D.ABR.

2 Goals Compare out-of-field doses from various techniques Methods to reduce out-of-field doses Impact of out-of-field doses Second cancers, fetal dose, pacemakers Techniques to assess out-of-field doses Limitations

3 Outline Magnitude of out-of-field doses Impact of out-of-field doses Assessment of out-of-field doses

4 Outline Magnitude of out-of-field doses Impact of out-of-field doses Assessment of out-of-field doses

5 Basics conventional X-rays Sources of out-of-field radiation: Patient scatter, collimator scatter, head leakage. Fraction of Dose Leakage Patient scatter Collimator scatter 0.2 Leakage and Scatter from machine Distance from field edge (cm) Kry et al, 2009, AAPM Scatter from within patient

6 Basics conventional X-rays Doses studied extensively Trends in the data generalizations

7 Basics conventional X-rays Dose decreases ~ exponentially away from edge of field 1% at 10cm with field size Const. with energy Const. with depth Stovall et al. 1995, Medical Physics

8 Basics conventional X-rays Wedges Beam modifiers Physical wedges increase out of field dose by 2-4 times (Sherazi et al, 1985, Int J Radiat Oncol Biol Phys) Dynamic or universal wedges no increase (Li et al, 1997, Int J Radiat Oncol Biol Phys) MLC Secondary MLC no impact on out-of-field dose (Mutic et al, 2002, J Appl Clin Med Phys) Tertiary MLC is extra shielding decrease out of field dose by 30-50% (Stern, 1999, Med Phys)

9 IMRT Near target, dose reduced Treat smaller volume Farther from target, higher doses Modulation increases head leakage Leakage Patient scatter Collimator scatter Fraction of Dose Fraction of Dose Leakage Patient scatter Collimator scatter Distance from field edge (cm) Kry et al, 2009, AAPM Distance from field edge (cm)

10 IMRT Overall: generally higher doses with IMRT Extent of modulation changing Modulation factor decreasing Early 2000s: 3-5 (Followill, 1997, Int J Radiat Oncol Biol Phys. Kry, 2005, Int J Radiat Oncol Biol Phys) Clinically (2010): often <2 Dose (msv) Kry 18 MV IMRT Kry 18 MV Conv Howell 6 MV IMRT Howell 6 MV Conv Distance from central axis (cm) Kry, 2005, Int J Radiat Oncol Biol Phys Howell, 2006, Med Phys

11 High-energy photon therapy Above ~10 MV neutrons Dicentric chromosome induction in human lymphocytes Nolte, 2005 Mutagenesis in mouse fibroblast cell line Hall, 1995 Other measurements have found RBE = 1 NCRP 104 Large uncertainties in neutron RBE

12 High-energy photon therapy This RBE is usually described with: Radiation weighting factor 1.E H = w R D Neutron Fluence (A.U.) 8.E-10 6.E-10 4.E-10 Spectrum wr wr 2.E E+00 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 E (MeV) ICRP 92 0

13 High-energy photon therapy Neutron dose equivalent with beam energy Howell 2009 Med Phys ~Constant with field size, distance off-axis Dosimetry is difficult varied results in literature Neutrons: 25% of out-offield dose equivalent (18 MV IMRT) 10 MV may be optimal energy for deep tumors (Kry 2005, Int J Radiat Oncol Biol Phys) % of total dose Testes Colon Stomach Lung Thyroid Prostate Liver Esophagus Heart Distance from Central Axis (cm) Kry et al., 2009, Radiother Oncol

14 Cyberknife Other photon modalities Older version 4X higher doses than linac (Petti 2006, Med Phys) Newer versions added shielding Doses more consistent with linacs (Chuang, 2008, Med Phys) Be aware of beams along patient axis Gamma Knife Doses higher near field, lower farther away Zytkovicz, 2007, Phys Med Biol

15 Other photon modalities Tomotherapy (Hi-Art) Many more MUs but extra shielding, no flattening filter Mixed/comparable results relative to IMRT (Ramsey, 2006, J Appl Clin Med Phys) Overall: Some variation between different accelerators All in the same ballpark (factor of 2)

16 Proton therapy Sources of out-of-field dose equivalent: Mostly external neutrons Some internal neutrons A few photons Fontenot, 2008, Phys Med Biol

17 Proton therapy Neutron dose eq. varies with: Treatment energy SOBP and snout size Air gap Small change with distance or field size Zheng, 2007, Phys Med Biol Zheng, 2007, Phys Med Biol Mesoloras, 2006, Med Phys

18 Proton therapy How much dose equivalent is there? Variations in beam parameters Beam energy, SOBP, aperture, air gap Variations in experimental design Size and material of phantom, manufacturer of accelerator Challenges in Dosimetry Lack of high energy response Xu, 2008, Phys Med Biol Unique machines

19 Protons vs. Photons Conventional photon therapy Photons: More dose near treatment field Comparable dose beyond cm from field edge Xu, 2008, Phys Med Biol Stovall, 1995, Med Phys

20 Protons vs. Photons Depending on what proton data you use, you can get very different results: Hall, 2006, Int J Radiat Oncol Biol Phys Zytkovicz, 2007, Phys Med Biol

21 Protons vs. Photons Near to field, dose equivalent much lower with p+ Less lateral scatter Less exit dose Less total out of field dose Effect more pronounced at lower p+ energy Fontenot, 2008, Phys Med Biol. HT/D as a function of lateral distance (along the patient axis) from the isocenter from this work compared to IMRT values collected from Kry et al (2005) and Howell et al (2006).

22 Scanning Proton Beams Much interest in scanning beams No external neutrons Still internal neutrons, gammas Up to half of dose equivalent to near organs Negligible dose to distant organs Scanning beam is an improvement, but is not free from out-offield dose

23 Outline Magnitude of out-of-field doses Impact of out-of-field doses Assessment of out-of-field doses

24 How much dose is there? Out of field doses vary with treatment type/parameters. Typical numbers, (20 cm from field edge, 60 Gy treatment): msv, scanned proton beam msv, passive scatter protons 250 msv, Conventional photons msv, IMRT What do these doses mean?

25 How much is this dose? Put these doses in perspective msv, scanned proton beam msv, passive scatter protons 250 msv, Conventional photons msv, IMRT Courtesy of David Brenner msv

26 Late Effects Radiation has been shown to increase the risk of Cardiovascular disease Diabetes Stroke Hereditary effects Second cancers Potentially relevant for all patients

27 Hereditary effects Historically a major concern Mutant babies Gonad dose considered highest risk Not supported with data Neel Am J Hum Genet 1990, Little Br Med Bull 2003 Gonad dose no longer considered high risk

28 Heart Disease Increased risk with radiation A-bomb survivors Left-sided breast patients Clinically: Minimize dose to the heart

29 Second Cancers Female second cancer incidence. Lifetime cases/100k exposures to 0.1 Gy Cases Stomach Colon Liver Lung Breast Bladder Other Thyroid Leukemia Uterus Ovaries BEIR VII Age at exposure Much higher risk for younger populations

30 Second cancers Cancer takes at least 5 years, typically 10+ years to develop after exposure Risk continues to be elevated 50 years later Absolute Risk for 10+ year survivors: 1 in 70 Brenner Cancer 2000

31 Second Cancers Where do second cancers occur? 12% within geometric field 66% beam-bordering region 22% out-of-field (>5 cm away) Diallo IJROBP 2009 Get most second cancers in high-dose regions This is also the area we re trying to treat, so hard to reduce this dose Want to reduce the dose outside PTV

32 Therapy Symposium Late Effects from Radiation Therapy and Diagnostic Imaging Thursday, 12:30-2:20 Ballroom A

33 Fetal dose Fetus is collection of rapidly dividing cells Very sensitive to radiation damage Sensitivity and damage depend on stage of development

34 Stovall et al, 1995 Fetal dose

35 Fetal dose Important to put these risks into perspective Risk of radiation induced birth defects vs. normal birth defects Stovall et al, 1995

36 Fetal dose Treatment decision will be made by physician (treat, recommend abortion, chemo only, etc.) Physicists must carefully assess dose Impact of this dose may require discussion as well.

37 Fetal dose - shielding Shielding should always be used Even if not necessary Pb is not an effective shield against neutrons Use BPE Shielding is more effective the farther from the treatment field you go ~50% reduction Fraction of Dose 1.0 Leakage Patient scatter 0.8 Collimator scatter Distance from field edge (cm)

38 Pacemakers/Defibrillators Electronics are susceptible to radiation damage Irritation for cameras, CTs etc. Patient safety issue with pacemakers/defibrillators Can be transient effects while beam is on, or can have permanent effects

39 Pacemakers/Defibrillators Inside treatment field is clearly a high dose May be sensitive to relatively low doses: High intensity imaging procedures High LET radiation high energy X-rays/proton therapy Observed failure after 0.9 Gy of neutrons [Raitt Chest 1994]

40 Pacemakers/Defibrillators Ensure there are policies (and follow them) Determine dose to device and compare with device tolerances Monitor functionality of device during/after patient treatment Assess need to move device before treatment TG-34: Management of radiation oncology patients with implanted cardiac pacemakers TG-203: Management of radiotherapy patients with implanted cardiac pacemakers and defibrillators

41 Outline Magnitude of out-of-field doses Impact of out-of-field doses Assessment of out-of-field doses

42 Calculations: Treatment Planning Systems Distance from Field Edge (cm) Dose calc (cgy) Dose meas (cgy) Percent Difference % Howell et al, in preparation % % % This is for a simple, conventional field. As treatment complexity increases, more scatter contributing to dose, less accuracy See this with Mesothelioma cases even within the treatment field we are often off by several percent due to the limitations of the treatment planning system to handle scatter.

43 Calculations - simple TG-36 Peridose software (van der Giessen Ratiother Oncol 2001) Accuracy Pretty good (±30%) Double check with measurement Limitations of treatments Neutrons, IMRT, electrons, brachytherapy

44 Additional source Fetal dose from a large number of procedures, including radiotherapy, brachytherapy, nuclear medicine, imaging

45 Calculations - Detailed Monte Carlo studies Benchmark simulations against measurements Use detailed model Bednarz Med Phys 2008

46 Measurements Ion chamber measurements

47 Measurements In vivo measurements Taken directly on/in patient Caution: surface dose is elevated (4x)!! Cover detector with bolus TG-36 Kry Med Phys 2006

48 Additional TLD comments TLD very nice dosimeter TLD-100 over-responds to neutrons Factor of 10 in reported dose for 18 MV 2.3 mgy reported as 27.0 mgy Kry JACMP 2007 Do not use TLD-100 for out-of-field measurements for E>10 MV (OK inside treatment field) Use TLD-700, ion chamber

49 Other considerations Energy spectrum is much different outside the treatment field Energy dependent dosimeters will have a different response TLD, diodes. Even ion chambers

50 Perspective and final thoughts Out of field doses are not negligible Risk is justified Must be cognizant of, and work to minimize, stray radiation Be aware of dose assessment pitfalls Approach to assess out-of-field dose should be based on the required accuracy of the information.

51 Thank you!