Dosimetry and Uncertainty Analysis in Gynecologic Brachytherapy

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Dosimetry and Uncertainty Analysis in Gynecologic Márcia Coelho 1,2, Ana Belchior 3, Pedro Vaz 3 1. Medical Consult S.A. Campo Grande, nº 56 8º A, 1700-093 Lisboa. 2. Centro Oncológico Dra.

1 Dosimetry and Uncertainty Analysis in Gynecologic Márcia Coelho 1,2, Ana Belchior 3, Pedro Vaz 3 1. Medical Consult S.A. Campo Grande, nº 56 8º A, Lisboa. 2. Centro Oncológico Dra. Natália Chaves. Rua Manuel Anastácio Alves, nº 2, Carnaxide. 3. IST/ITN, Instituto Superior Técnico, Universidade Técnica de Lisboa. Estrada Nacional 10 (ao km 139,7), Bobadela LRS. 1

2 1. Introduc,on The intracavitary brachytherapy consists in treacng tumors located in natural body cavices such as the uterus and vagina by placing one or more sealed radioaccve sources. 2

3 1. Introduc,on Allows to administer high dose to the tumor with a signiﬁcant dose reduccon to the adjacent healthy Cssues and organs1,2 (Fig.1) as opposed to EBRT (Fig.2). 2. Methodology 3. Results 4. Final 5. Bibliographical Fig.1 - Axial slice showing a vaginal brachytherapy dose distribucon. Fig.2 - Axial slice showing an external beam dose distribucon of a ginecologic pathology. PRS, September 2013, Lisbon 3

1. Introduc,on Is scll common that brachytherapy dose distribucon is based on

3 AP image on the leu side and lateral image on the right side for a gynecologic

Is frequent not to use dose calculacon algorithms that account for the

4 1. Introduc,on Is scll common that brachytherapy dose distribucon is based on radiographic images (Fig.3) instead of CT images. Fig.3 AP image on the leu side and lateral image on the right side for a gynecologic applicacon. Is frequent not to use dose calculacon algorithms that account for the interaccon of radiacon with Cssues and applicators (Fig. 4) 3. Fig.4 Dose distribucon using a shielded rectal applicator calculated with Acuros Algorithm (leu side) vs TG- 43 (right side). 4

1. Introduc,on UncertainCes in the consctuent materials of the gynecologic applicator 4,5 and in the source posiconing lead to uncertainces in dose calculacon received by the tumor and surrounding

5 1. Introduc,on UncertainCes in the consctuent materials of the gynecologic applicator 4,5 and in the source posiconing lead to uncertainces in dose calculacon received by the tumor and surrounding Cssues and organs. ObjecCves: To determine the accuracy of dose calculacon based on TG formalism obtained by the Treatment Planning System (TPS) Brachyvision. To assess the influence of source posicon uncertainces in dose calculacon received by the rectum and bladder. Monte Carlo PENELOPE 7 simulacon program 5

I. ValidaCon of the 192 Ir geometry (Varisource IX )

ionizacon chamber Farmer 0.6cm 3 (Fig. 5). a) b) c) Fig.

6 I. ValidaCon of the 192 Ir geometry (Varisource IX ) and the model implemented in Monte Carlo PENELOPE simulacon program. a) Measurements using a cylindrical phantom and the ionizacon chamber Farmer 0.6cm 3 (Fig. 5). a) b) c) Fig.5 Experimental setup: a) setup 1; b) setup 2; c) setup 3; d) setup 4. For each setup the ionizacon chamber (blue wire) is inserted into a different hole of the phantom. The source is inserted in the central hole. d) 6

7 b) ImplementaCon of the experimental model in the Monte Carlo PENELOPE simulacon program. Fig.7 - Geometry obtained by PENELOPE program for "setup 1" (display x and z axis) Legend: Material Ir Material 2 NITINOL Material 3 Air Material 4 - Aluminium Material 5 Graphite Material 6 - PMMA Fig.8 - Geometry obtained by PENELOPE program for "setup 2" (display y and z axis) 7

Fig.9 - Geometry obtained by PENELOPE program for "setup 3" (display x and z axis) Legend: Material 1 192 Ir Material 2 NITINOL Material 3 Air

8 Fig.9 - Geometry obtained by PENELOPE program for "setup 3" (display x and z axis) Legend: Material Ir Material 2 NITINOL Material 3 Air Material 4 - Aluminium Material 5 Graphite Material 6 - PMMA Fig.10 - Geometry obtained by PENELOPE program for "setup 4" (display y and z axis) 8

9 c) Comparison of the experimental results with Monte Carlo simulacons Dose (cgy) Experimental MC Penelope Graphic 1- Dose values (cgy) obtained by PENELOPE and experimentally for "setup 1" depending on the source posicon with its stacsccal uncertainty Source Posi,on (cm) 14 Dose (cgy) Experimental MC PENELOPE Graphic 2 - Dose values (cgy) obtained by PENELOPE and experimentally for "setup 2" depending on the source posicon with its stacsccal uncertainty. Source Posi,on (cm) 9

10 Dose (cgy) Source Posi,on (cm) Experimental MC Penelope Graphic 3 - Dose values (cgy) obtained by PENELOPE and experimentally for "setup 3" depending on the source posicon with its stacsccal uncertainty Dose (cgy) Experimental MC Penelope Graphic 4 - Dose values (cgy) obtained by PENELOPE and experimentally for "setup 4" depending on the source posicon with its stacsccal uncertainty Source Posi,on (cm) 10

11 In conclusion: The relacve deviacon between the measurements and the values obtained by Monte Carlo PENELOPE simulacon program was less than 5% for most of the points, therefore the model was considered validated. 11

the intencon was to get 5.5Gy in the prescripcon points considering a 3 cm diameter cylinder (Fig. 11).. Fig.

12 II. Compare a brachytherapy dose distribucon obtained with the TPS and with Monte Carlo simulacons. a) Dose distribucon was calculated by the TPS through the use of 2 radiographic images following the ICRU38 8 recommendacons.the intencon was to get 5.5Gy in the prescripcon points considering a 3 cm diameter cylinder (Fig. 11).. Fig.11 - Dose distribucon obtained by TPS for a vaginal BT treatment in the axial plane (leu image) and coronal (right image). The source posicons are represented In the center of the distribucon (A). 12

b) ImplementaCon of the geometry and materials of the source with and without the applicator in the Monte Carlo PENELOPE simulacon program (Fig. 12).

13 b) ImplementaCon of the geometry and materials of the source with and without the applicator in the Monte Carlo PENELOPE simulacon program (Fig. 12). For each geometry were created five input files for each source posicon. Fig.12 - Geometric representacon of the source without applicator (leu image) and with applicator (right image) for all source posicons. The values obtained were converted to absorbed dose considering the accvity, the emiied photons and the Cme that the source spent in each posicon. 13

III. ConstrucCon of a voxel phantom from a sequence of CBCT images (Fig. 13-14) with ImageJ 9 program.

13 - Axial, coronal and sagiial slices of a CBCT sequence of a pacent who held convenconal brachytherapy.

14 III. ConstrucCon of a voxel phantom from a sequence of CBCT images (Fig ) with ImageJ 9 program. This phantom was used to calculate the absorbed dose in organs at risk using the main program PenEasy. Fig.13 - Axial, coronal and sagiial slices of a CBCT sequence of a pacent who held convenconal brachytherapy. Fig.14 - Axial, coronal and sagiial slices of the anthropomorphic voxel phantom obtained from the CBCT images. 14

15 TPS dose distribucon vs. Monte Carlo simulacons Without applicator, the relacve differences in the calculated dose between the PENELOPE simulated values and the TPS ones were below 3% (Table 1). Table 1 - Dose values obtained at 2 cm away from the source center with TPS and with PENELOPE, for the geometry without the applicator. Central source position (cm) TPS Dose (Gy) Dose and PENELOPE uncertainty (Gy) Source without applicator Relative deviation(%) ( 0.26) ( 0.27) ( 0.26) ( 0.26) ( 0.25)

16 With the applicator, the corresponding relacve deviacon was on average - 9.4% (Table 2). Table 2 - Dose values obtained at 2 cm away from the source center with TPS and with PENELOPE, for the geometry with the applicator. Central source position (cm) TPS Dose (Gy) Dose and PENELOPE uncertainty (Gy) Source with applicator Relative deviation(%) ( 0.24) -9, ( 0.25) -10, ( 0.25) -8, ( 0.25) -8, ( 0.23) -10,06 Prescribed dose(gy) at 2cm Source posi,on (cm) in the z axis TPS Penelope without applicator Penelope with applicator 16

Source posicon uncertainces in dose calculacon received by the rectum and bladder The anterior- posterior variacon contributed to a relacve deviacon of +6.

17 Source posicon uncertainces in dose calculacon received by the rectum and bladder The anterior- posterior variacon contributed to a relacve deviacon of +6.6% in the average dose to the bladder (Graphic 6). The cranio- caudal variacon contributed to a relacve deviacon of +6.6% in the average dose to the rectum (Graphic 7). Average dose (cgy) to the bladder Reference Anterior Posterior Right LeU Cranial Caudal Source posi,on varia,on (mm) in the y axis Graphic 6- Average dose received by the bladder when performing a deviacon of 1 mm in the source posicon for the various direccons. Average dose (cgy) to the rectum Reference Anterior Posterior Right LeU 1.50 Cranial Caudal Source posi,on varia,on (mm) in the y axis Graphic 7- Average dose received by the rectum when performing a deviacon of 1 mm in the source posicon for the various direccons. 17

18 ValidaCon of the computaconal models The model of the gynecologic applicator has not been validated experimentally due to the geometric limitacons of the phantom. Considera,ons TPS dose distribucon vs. Monte Carlo simulacons The current available calculacon model TG- 43 overescmates the dose in clinical points. Compromise treatment outcome It is important the introduccon of more sophisccated algorithms that consider different densices 18

19 Source posicon uncertainces in dose calculacon The used voxel phantom was a useful tool that provided detailed informacon of the different Cssues. Uncertainty in the source posicon is a relevant factor influencing the uncertainty in the dose calculacon in gynecologic brachytherapy. Considera,ons - Source posicon verificacon before each treatment - Methods to verify the posicon of the source during treatment 10,11 Results vary: - Type of applicator - Inter and intra observer variability contouring the phantom - Sequence used represents the anatomy of only one pacent 19

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20 1. Atahan IL, Onal C, Ozyar E, Yiliz F, Selek U, Kose F: Long- term outcome and prognoscc factors in pacents with cervical carcinoma: a retrospeccve study. Int J Gynecol Cancer 2007, 17(4): Viani G, Manta G, Stefano E, de Fendi L. for cervix cancer: low- dose rate or high- dose rate brachytherapy a meta- analysis of clinical trials. J Exp Clin Cancer Res 2009 April; 28(47): hip:// 4. Ye S- J, Brezovich I, Shen S, Duan J, Popple R, Pareek Prem N. AienuaCon of intracavitary applicators in 192Ir- HDR brachytherapy. Med Phys 2004; 31: Gerardy I; EvaluaCon d`un système de planificacon pour un traitement de brachythérapie gynécologique en uclisant des techniques Monte Carlo et des measures expérimentales; Tesis Doctoral, May Nath R, Anderson L, Luxton G, Weaver K, Williamson J, Meigooni A. Dosimetry of intersccal brachytherapy sources: recommendacons of the AAPM radiacon therapy commiiee task group No. 43. Med Phys 1995 February; 22: Salvat F, Fernandez- Varea J, Sempau J. PENELOPE, a code system for Monte Carlo SimulaCon of Electron and Photon Transport. Spain : Barcelona University; InternaConal Commission on RadiaCon Units and Measurements. Dose and Volume SpecificaCon for ReporCng Intracavitary Therapy, Rep Ferreira T. ImageJ User Guide. October Nakano T, Suchowerska N, McKenzie D R, Bilek M M. Real- Cme verificacon of HDR brachytherapy source locacon: implementacon of detector redundancy. Phys Med Biol 2005; 50(2): BaCc M, Burger J, Cindro V, Kramberger G, Mandic I, Mikuz M, et al. VerificaCon of high dose rate 192 Ir source posicon during brachytherapy treatment using silicon pixel detectors. IEEE TransacCons on Nuclear Science (5):

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