DOSIMETRIC STUDY OF I-125 SEEDS USED IN PROSTATE BRACHYTHERAPY

Transcription

1 2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR ABEN ISBN: DOSIMETRIC STUDY OF I-125 SEEDS USED IN PROSTATE BRACHYTHERAPY Maíra R. Santos 1, Luiz A. R. da Rosa 1, Alessandro Facure 2, Simone C. Cardoso 3, Pedro P. Pereira 4,5 and Ademir X. da Silva 6 1 Instituto de Radioproteção e Dosimetria (IRD/CNEN - RJ) Av. Salvador Allende s/n, Recreio dos Bandeirantes Rio de Janeiro, RJ, Brazil mairasantos@ird.gov.br lrosa@ird.gov.br. 2 Comissão Nacional de Energia Nuclear (CNEN) R. Gal. Severiano 90, sala Rio de Janeiro, RJ, Brazil facure@cnen.gov.br 3 Laboratório de Física da Radiação Gama e X, Instituto de Física (IF/UFRJ) Universidade Federal do Rio de Janeiro, Ilha do Fundão Rio de Janeiro, RJ, Brazil simone@if.ufrj.br 4 Dosimetrika Avenida Olegário Maciel, 231, Sala , Rio de Janeiro RJ ppereira@dosimetrika.com.br 5 Instituto Nacional de Câncer (INCA-MS) Praça da Cruz Vermelha , Rio de Janeiro, RJ, Brazil 6 [PEN/COPPE-DNC/EE]CT, Universidade Federal do Rio de Janeiro, Ilha do Fundão , Rio de Janeiro, RJ, Brazil. ademir@con.ufrj.br ABSTRACT Among the possibilities for treatment of prostate cancer, radiotherapy is one of the most commonly used procedures. One of the radiation therapy modalities is brachytherapy, where radioactive sources are placed near or in contact with the tumor mass. Prostate Brachytherapy with iodine seed permanent implantation has become increasingly widespread because it is less invasive and offers the possibility of lower doses to organs at risk and therefore less side effects to patients. The planning for this type of treatment includes the acquisition of images and the delineation of the contours of organs at risk. Generally, important factors for dose administration as, for example, the actual composition of tissues and geometry particularities of the seeds used in the treatment are disregarded. In order to assess the impact of such approaches on the outcome of prostate permanent implant treatment, experimental results, performed with TLD detectors in a prostate phantom of solid water (RW3), and the formalism of the protocol compiled by the Task Group No.43 of the American Association of Physicists in Medicine, were compared for two different irradiation geometries. The present results indicate a good agreement between them. These dataset offers the possibility to determine correction factors that may be applied in actual treatment planning.

2 1. INTRODUCTION World resource estimates that prostate cancer is the second most common type of cancer in males [3]. In Brazil, according to estimates by the National Cancer Institute (INCA) [5], these estimates are similar to international estimates for probability of incidence. However, estimates the mortality related to prostate cancer cases are not similar to the estimates of incidence [13], indicating that in regions with greater access to better treatments, mortality can be considerably lower. Therefore the treatment appears to be quite efficient. One treatment form, which is becoming widespread, is the low dose rate brachytherapy (LDR Brachytherapy). This treatment consists of the permanent implant of low dose rate brachytherapy sources (radioactive seeds) in the patient prostate for the rest of his life. The insertion of the seeds is done after a plan that determines the placement of the seeds. Planning can be done using images generated by CT, conventional X-ray or ultrasound. The planning system uses the seeds defined positions to determine the isodose curves. The calculations carried out by the dose planning system are performed according to the formalism of the protocol compiled by the Task Group No.43 (TG-43) of the American Association of Physicists in Medicine (AAPM) [10,12]. Although the TG-43 is the main reference for current dosimetry of permanent seed implants, it proved to be incomplete with approximations that can significantly alter the dose delivered to tissue. The first approximation that the protocol does is to treat the source as a point source or a linear one. This approximation interpolates data from a table generated with a real function of the source distribution. Another approach undertaken by the protocol is the definition of dose rate in water only. Thus, the conditions of attenuation and scattering of tissue are not taken into account. The protocol TG-43 recommends the appropriate formalism for the dosimetry of a single seed. But never an implant is done with a unique seed, but with tens, even a hundred samples. Several studies [2, 8, 14, 15] have been conducted to assess the dose delivered by I-125 seeds applying thermoluminescent (TL) dosimetry, as well as the study and determination of the functions that compose the formalism recommended by TG-43 [10]. The objective of this study was to assess experimentally, using TL dosimetry, the absorbed dose in a prostate phantom treated with permanent implant prostate brachytherapy using I-125. The obtained values were compared with results based on protocol TG-43 at the same points of interest.

3 2. MATERIALS AND METHODS A geometric prostate phantom was developed in order to enable the experimental evaluation of doses during permanent implant prostate brachytherapy. The phantom was made from slabs of RW3, PTW manufacturer, model Plattenphantom These slabs are sold as square plates of 30 cm edge. These plates were cut in smaller square plates of 7 cm edge. Each slice with 0.5 cm thick, forming on a set of 14 cards a cube representing the prostate. Each plate has 169 holes, forming 13 rows and 13 columns. The holes present a distance of 0.5 cm between their centers; they are cylindrical with a radius of 0.05 cm. The holes are used to accommodate the seeds and the radiation detectors used in this investigation, namely TL detectors. This geometry was done maintaining the same reference point of the planning system and the frame of reference used during surgery. The Fig.1 and 2 show the slices forming the cube and, in detail, the front view of a slice. Figure 1 - Prostate Phantom Figure 2 - Slice Detail

4 The TL dosimeter used in this investigation was Harshaw TLD-100 square rod. Figure 3 illustrates the detail of the dosimeters. Figure 3 - TL dosimeters Initially, 60 dosimeters were selected. Their sensitivities and reproducibilities were investigated for five 1 Gy irradiations of Co-60. During irradiations the dosimeters were placed under 5.5 cm of RW3. The dosimeters presenting reproducibility better than 4% were selected. They were also sorted according to their sensitivity. The calibration factors were determined for each dosimeter. They were set according to the equation (1). (1) where is the mean reading for five exposures of the dosimeter to 1 Gy of Co-60. The energy correction factor used was defined by Weaver et al [17]. The absorbed dose measured by the TL dosimeter due to an exposure to I-125 seeds is given by equation (2). (2) The read-out conditions applied to the TL dosimeters are summarized in the table 1. Table 1 - Harshaw 3500 TL reader parameters used for TLD-100 evaluation. Pre-Heating 50 C Heating Rate 25 C/sec Maximum Temperature 300 C

5 Figure 4 - PTW-TLDO Oven Figure 5 - HARSHAW 3500 Reader I-125 Amersham 6711 seeds were used in this investigation. The schematic diagram of the seed is showed in the figure 6. This seed consists of a silver wire with silver iodide attached to this wire, armor coated titanium. The seed length is 0.45mm. This dimension allows each seed to be allocated into the plates holes without reaching the neighboring plate - and the diameter is 0.08cm, therefore narrower than the holes diameter.

6 Figure 6 Schematic diagram of Amersham 6711 seed. To determine the dose following the formalism of the TG43 protocol, the general formula was used, as shown in equation (3). (3) Where is the air kerma strength; is the dose Constant (defined for each seed model); is the geometric factor defined for each point related to the origin of the system; the anisotropic function and is the radial dose function. is Excepting, the values for all other terms presented in equation (3) were obtained from the literature [14,16,17]. The air kerma strength, as defined in the seed calibration certificate, is measured by the manufacturer. In order to evaluate the dose delivered by an unique seed, eight dosimeters were placed around it. The seed activity was mci on the day of irradiation. The dosimeters were irradiated for a period of 14 days. The whole setup was mounted at the central plate of phantom. In Figure 7, there is a layout showing the position of the seed and the eight dosimeters. The black lines represent the space division of the plate. Lines intersections correspond to the centers of the holes. For the second setup, there was the intention of avoiding align more than two dosimeters at the same line so that the radiation attenuation by the TLD would not affect the results. Thus, a group of four dosimeters was placed at 0.71 cm from the seed and a second group of four dosimeters at 1.0 cm from the seed. Additionally, four other dosimeters were placed at distances of 1.5, 2.0, 2.5 and 7.1 cm from the seed. In this setup, the seed had activity of mci on the day of irradiation and the TLDs were exposed for exactly 110 minutes.

7 Figure 7 - First Setup Figure 8 - Second Setup 3. RESULTS AND DISCUSSIONS Tables 2 and 3 present results of the comparison between TLD measurements and calculations based on TG-43 results for the two measured setups. Good agreement was achieved between dose points for the two methods with distances greater than 1.0 cm from the seed, as previous studies [14,15]. This limitation is due to the TG43 that is not able to properly define the doses less than an inch and usually underestimate their values. Another relevant factor is that the dosimeters were not calibrated to a source at this distance, but calibrated to more distant sources in which the dosimeter could be treated as punctual, which does not occur in the current situation.

8 Table 2 Comparison between TLD measurements and calculations based on TG-43 for the first setup r (cm) TLD (Gy) SD TG-43 (Gy) TLD/TG % % % % % % % % Table 3 - Comparison between TLD measurements and calculations based on TG-43 for second setup r(cm) TLD (cgy) SD TG-43 (cgy) TLD/TG % % % % % % % % % % % % CONCLUSION This study presents a comparison of two dosimetry procedures, using TL dosimetry and the protocol TG-43, for I-125 Amersham 6711 seeds, used for permanent brachytherapy of prostate cancer. In the current study it was showed that the protocol underestimates the dose at points with distances smaller than 1.0 cm from the seed. This is an important result considering that each seed is positioned in a real implant, in most of the cases, at about 0.5 cm from the next one.

9 REFERENCES 1. BOZKURT,A. Assessment of organ from expousure to neutrons using the Monte Carlo technique and na image-based anatomical model. Thesis (PhD). RENSSELAER POLYTECHNIC INSTITUTE, Source DAI-B 61/11, p New York, USA (2001). 2. CAMGÖZ, B. YEGIN, G. KUMRU, M.N. Differential dose contributions on total dose distribution of I-125 brachytherapy source. Reports of pratical Oncology and Radiotherapy 15 (2010) GLOBOCAN Cancer map. (2011) 4. HOROWITZ, Y.S. Thermoluminescence and thermoluminescent dosimetry. CRC Press, 1o Edition, Boca Raton, USA (1984). 5. INSTITUTO NACIONAL DO CÂNCER, MINISTÉRIO DA SAÚDE. Estimativa 2010: Incidência de câncer no Brasil/ Instituto Nacional do Câncer. Rio de Janeiro, Brasil. 6. KHAN, F. M. The physics of radiation therapy. Williams & Wilkins, 2ª Edição. Baltimore, USA (1994). 7. KNOLL, G.F. Radiation Detection and Measurement. Ed. John Wiley & Sons, 3 Ed., MOBIT, P. BADRAGAN, I. Response fo LiF-TLD micro-rods around I-125 radioactive seed. Phys. Med. Biol. 48 (2003) MOSCOVITCH, M., SZALANEZY, A., BRUML, W. W., et al. A TLD system based on gas heating with linear time-temperature profile. Radiation Protection and Dosimetry No 34, pp. 361, NATH,R.; ANDERSON, L.L.; LUXTON, G.; WEAVER, K.A.; WILLIANSON, J.F.; MEIGOONI, A.S. Dosimetry of interstitial brachytherapy sources: Recommendations of AAPM Radiation Therapy Committee Task Group No.43. Med. Phys. 22, , 1995.

10 11. POLLACK, A.; ZAGARS, G K.; ROSEN, I. I. Prostate cancer treatment with radiotherapy: maturing methods that minimize morbidity. Semin. Oncol. M. D. Anderson Cancer Center, 26 (2): , RIVARD, MJ et al., Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations, Med. Phys. 31 (3), , March SCHRÖDER, F.H. et al. Screening and Prostate-Cancer Mortality in a Roandomized European Study. N Engl J Med 2009; 360: SLODOBA, R.S., MENOS, G.V. Experimental determination of the anisotropy function and anisotropy factor for model 6711 I-125 seed. Med. Phys. 27 (8) August WALLACE, R.E., FAN, J.J. Evaluation of a new brachytherapy iodine-125 source by AAPM TG43 formalis. Med. Phys. 25 (11), November WEAVER, K. Anisotropy functions for I125 and Pd103 sources. Med. Phys. 25 (12) December WEAVER, K.A.; SMITH, V.; HUANG, D.; BARNETT, C.; SCHELL, M.C.; LING, C. Dose parameters of 125I and 192Ir seed sources. Med. Phys. 16, , 1989.