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MONTE CARLO BASED PLAN VERIFICATION FOR ELECTRONIC BRACHYTHERAPY: THE DOSIMETRIC INFLUENCE OF PATIENT BOUNDARY AND TISSUE INHOMOGENEITIES
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P ROFFERED PAPER
281 oral MONTE CARLO BASED PLAN VERIFICATION FOR ELECTRONIC BRACHYTHERAPY: THE DOSIMETRIC INFLUENCE OF PATIENT BOUNDARY AND TISSUE INHOMOGENEITIES B. Guo1 , N. Papanikolaou1 , C. Shi1 1 T HE U NIVERSITY OF T EXAS H EALTH S CIENCE C ENTER AT S AN A NTONIO, Department of Radiation Oncology, San Antonio, TX, USA
Purpose: TG43 formulism, which is used in most treatment planning systems for brachytherapy, neglects the affects of patient boundary and tissue inhomogeneities in dose distribution. This has been known to cause errors in dose prediction. For low-energy electronic brachytherapy sources, the error may be too large to neglect. In this study, we quantified the influence of inhomogeneities in the dose distribution for accelerated partial breast irradiation (APBI) plans using Xoft Axxent electronic brachytherapy source. Materials: The Monte Carlo code EGSnrc was used to calculation the dose distributions. CT image sets of the patients were segmented in detail by both Hounsfield Unit (HU) numbers and the contours drawn by physicians to create the patient-specifc phantoms used in Monte Carlo simulations. An inhouse Matlab program was developed to analyze the dose distributions and generate DVHs and isodoses. 7 treatment plans for 6 patients underwent intracavitary APBI were studied. For each plan, DVHs to the target and critical structures including breasts, lungs, heart, skin and ribs and the isodose distributions on three orthogonal views were generated and compared with those calculated by TG43 formalism. Results: For all the plans we studied, comparison between Monte Carlo simulations and Calculations by TG43 formulism revealed that target coverage was significantly reduced in Monte Carlo simulation results (as an example, for patient #2, V100 was reduced from 90% by TG43 formulism to only 60% by Monte simulation). The reason is that the breast tissue has a lower average atomic number than water (Zbreast/Zwater=0.85 according to ICRP89 report). In the energy range of electronic brachytherapy sources, the photoelectric effect dominates. Dose deposition is strongly dependent on tissue’s atomic number. Dose to ipsilateral breast was reduced as well. Maximum skin dose was reduced by around 10% due to lack of scatter at patient’s boundary. Maximum rib dose increases by 2-3 times in Monte Carlo simulations (as an example, maximum rib dose for patient #2 increases from 100% to 300% of the prescription dose) due to the high average atomic number of the bones. Figure1 shows the DVHs for patient #1 based on TG43 formulism and Monte Carlo calculations seperately
W EDNESDAY, S EPTEMBER 2, 2009
prostate volume were objectively identified by automated tracing of implanted I-125 sources 7 times in the first month after implantation. Materials: Following implantation with stranded I-125 sources, prostates of 20 patients were scanned with C-arm conebeam CT immediately after finishing the procedure (day 0). Consecutive low-dose CT studies were performed on day 1, 2, 6, 14, 20 and 28. A commercially available seed-finder (Variseed 7.2) was used to generate seed positions from the CT-series. Results of the seed finding were verified by an experienced observer. The positions of the implanted sources were registered to the distribution on day 28 using a optimization procedure. This procedure identifies the position of each seed in all distributions and links this position to the position of the seed on day 28. Rotations and translations of the seed distributions are corrected by transformation of the seed distribution of the compared study in 3 dimensions. The linking and transformation of the distribution is shown in the figure for the transversal plane.Because we use stranded seeds in the procedure the distribution of seeds had a shape similar to a cylinder. The prostate was therefore modeled as a cylinder with the seeds deposited along the axis. Changes in radial and axial directions were determined.From the radial and axial position changes volume changes were calculated.
Results: The mean decrease in the radial direction was 6% (SD 3%) from day 0 to day 28. Day 1 20 showed a gradual decrease from 3% to 1% (SD 2%) on average, where the volume on day 28 is defined as 100%. In the axial direction a mean decrease of 4% (SD 4%) was observed from day 0 to day 28. Day 1 20 did not show a clear time trend. On average, the relative size was 1% (SD 2%) larger than the distribution on day 28.On average, taking day 28 as a reference, the volume of the prostate on day 0 was found to be 118% (SD 6%). From day 1 to day 20 the volume reduced from 109% to 103% (SD 4 %). Conclusions: Most of the implanted prostate volume changes occur within the first 24 hours after implantation. From day 1 to day 28 only minor volume changes are observed. 283 oral
Conclusions: For electronic brachytherapy sources, the tissue inhomogeneities and patient boundary will significantly affect the dose distribution. For intracavitary APBI plans, large reduce in target coverage, lower dose to ipsilateral breast, lower skin dose and much higher rib dose were observed. A Monte Carlo based dose verification was suggested to account for the influence of inhomogeneities. 282 oral OBJECTIVE COMPUTER AIDED ANALYSIS OF TIME TRENDS IN IMPLANTED PROSTATE VOLUMES AFTER I-125 BRACHYTHERAPY R. Westendorp1 , R. Kattevilder2 , A. Minken1 , C. Hoekstra2 , J. Immerzeel2 1 2
RISO, Medical Physics, Deventer, Netherlands RISO, Radiotherapy, Deventer, Netherlands
Purpose: Implantation of the prostate with I-125 sources causes edema. Depending on the time trend of this edema, dosimetry and thereby outcome might be influenced.In this study time trends in volume of the implanted
FRICKE GEL LAYER DOSIMETRY FOR IN-PHANTOM ABSOLUTE DOSE MEASUREMENTS IN HDR BRACHYTHERAPY G. Gambarini1 , M. Carrara3 , I. Gambini4 , S. Tomatis3 , A. Negri1 , M. Mariani4 , C. Fallai5 , P. Olmi5 , G. Zonca3 1 U NIVERSITÀ DEGLI S TUDI DI M ILANO, Physics Department, Milano, Italy 2 N ATIONAL I NSTITUTE OF N UCLEAR P HYSICS, Milan, Italy 3 F ONDAZIONE IRCCS - I STITUTO N AZIONALE DEI T UMORI, Medical Physics Unit, Milan, Italy 4 P OLITECNICO DI M ILANO U NIVERSITY, Department of Energy, Milano, Italy 5 F ONDAZIONE IRCCS - I STITUTO N AZIONALE DEI T UMORI, Radiotherapy Unit, Milan, Italy
Purpose: In high dose rate (HDR) brachytherapy, an accurate and efficient system that measures the dose distribution in three dimensions with high spatial resolution would be very useful for the verification of complex treatments as well as the characterisation of some source parameters. Aim of this study was to optimize, characterize and test a Fricke gel formulation with the above cited characteristics which can be adopted for absolute dosimetry in HDR brachytherapy. Materials: Fricke gel-layer dosimeters (FGLD) were produced in laboratory by infusing a ferrous sulphate solution and the metal-ion indicator XylenolOrange (XO) in a tissue-equivalent gel matrix. Their standard composition was: Agarose in the amount of 3% of the final weight, ferrous sulphate solution [1mM Fe(NH4)2(SO4)2H2O], sulphuric acid [25mM H2SO4] and XO [0.165mM C31H27N2Na5O13S]. Exposure to ionizing radiation of FGLD produces a conversion of ferrous ions Fe2+ into ferric ions Fe3+ , with yield proportional to the absorbed dose. Due to the fact that the complex XO-Fe3+ causes visible light absorption around 585nm, the absorbed dose is evaluated
P ROFFERED PAPER
281 oral MONTE CARLO BASED PLAN VERIFICATION FOR ELECTRONIC BRACHYTHERAPY: THE DOSIMETRIC INFLUENCE OF PATIENT BOUNDARY AND TISSUE INHOMOGENEITIES B. Guo1 , N. Papanikolaou1 , C. Shi1 1 T HE U NIVERSITY OF T EXAS H EALTH S CIENCE C ENTER AT S AN A NTONIO, Department of Radiation Oncology, San Antonio, TX, USA
Purpose: TG43 formulism, which is used in most treatment planning systems for brachytherapy, neglects the affects of patient boundary and tissue inhomogeneities in dose distribution. This has been known to cause errors in dose prediction. For low-energy electronic brachytherapy sources, the error may be too large to neglect. In this study, we quantified the influence of inhomogeneities in the dose distribution for accelerated partial breast irradiation (APBI) plans using Xoft Axxent electronic brachytherapy source. Materials: The Monte Carlo code EGSnrc was used to calculation the dose distributions. CT image sets of the patients were segmented in detail by both Hounsfield Unit (HU) numbers and the contours drawn by physicians to create the patient-specifc phantoms used in Monte Carlo simulations. An inhouse Matlab program was developed to analyze the dose distributions and generate DVHs and isodoses. 7 treatment plans for 6 patients underwent intracavitary APBI were studied. For each plan, DVHs to the target and critical structures including breasts, lungs, heart, skin and ribs and the isodose distributions on three orthogonal views were generated and compared with those calculated by TG43 formalism. Results: For all the plans we studied, comparison between Monte Carlo simulations and Calculations by TG43 formulism revealed that target coverage was significantly reduced in Monte Carlo simulation results (as an example, for patient #2, V100 was reduced from 90% by TG43 formulism to only 60% by Monte simulation). The reason is that the breast tissue has a lower average atomic number than water (Zbreast/Zwater=0.85 according to ICRP89 report). In the energy range of electronic brachytherapy sources, the photoelectric effect dominates. Dose deposition is strongly dependent on tissue’s atomic number. Dose to ipsilateral breast was reduced as well. Maximum skin dose was reduced by around 10% due to lack of scatter at patient’s boundary. Maximum rib dose increases by 2-3 times in Monte Carlo simulations (as an example, maximum rib dose for patient #2 increases from 100% to 300% of the prescription dose) due to the high average atomic number of the bones. Figure1 shows the DVHs for patient #1 based on TG43 formulism and Monte Carlo calculations seperately
W EDNESDAY, S EPTEMBER 2, 2009
prostate volume were objectively identified by automated tracing of implanted I-125 sources 7 times in the first month after implantation. Materials: Following implantation with stranded I-125 sources, prostates of 20 patients were scanned with C-arm conebeam CT immediately after finishing the procedure (day 0). Consecutive low-dose CT studies were performed on day 1, 2, 6, 14, 20 and 28. A commercially available seed-finder (Variseed 7.2) was used to generate seed positions from the CT-series. Results of the seed finding were verified by an experienced observer. The positions of the implanted sources were registered to the distribution on day 28 using a optimization procedure. This procedure identifies the position of each seed in all distributions and links this position to the position of the seed on day 28. Rotations and translations of the seed distributions are corrected by transformation of the seed distribution of the compared study in 3 dimensions. The linking and transformation of the distribution is shown in the figure for the transversal plane.Because we use stranded seeds in the procedure the distribution of seeds had a shape similar to a cylinder. The prostate was therefore modeled as a cylinder with the seeds deposited along the axis. Changes in radial and axial directions were determined.From the radial and axial position changes volume changes were calculated.
Results: The mean decrease in the radial direction was 6% (SD 3%) from day 0 to day 28. Day 1 20 showed a gradual decrease from 3% to 1% (SD 2%) on average, where the volume on day 28 is defined as 100%. In the axial direction a mean decrease of 4% (SD 4%) was observed from day 0 to day 28. Day 1 20 did not show a clear time trend. On average, the relative size was 1% (SD 2%) larger than the distribution on day 28.On average, taking day 28 as a reference, the volume of the prostate on day 0 was found to be 118% (SD 6%). From day 1 to day 20 the volume reduced from 109% to 103% (SD 4 %). Conclusions: Most of the implanted prostate volume changes occur within the first 24 hours after implantation. From day 1 to day 28 only minor volume changes are observed. 283 oral
Conclusions: For electronic brachytherapy sources, the tissue inhomogeneities and patient boundary will significantly affect the dose distribution. For intracavitary APBI plans, large reduce in target coverage, lower dose to ipsilateral breast, lower skin dose and much higher rib dose were observed. A Monte Carlo based dose verification was suggested to account for the influence of inhomogeneities. 282 oral OBJECTIVE COMPUTER AIDED ANALYSIS OF TIME TRENDS IN IMPLANTED PROSTATE VOLUMES AFTER I-125 BRACHYTHERAPY R. Westendorp1 , R. Kattevilder2 , A. Minken1 , C. Hoekstra2 , J. Immerzeel2 1 2
RISO, Medical Physics, Deventer, Netherlands RISO, Radiotherapy, Deventer, Netherlands
Purpose: Implantation of the prostate with I-125 sources causes edema. Depending on the time trend of this edema, dosimetry and thereby outcome might be influenced.In this study time trends in volume of the implanted
FRICKE GEL LAYER DOSIMETRY FOR IN-PHANTOM ABSOLUTE DOSE MEASUREMENTS IN HDR BRACHYTHERAPY G. Gambarini1 , M. Carrara3 , I. Gambini4 , S. Tomatis3 , A. Negri1 , M. Mariani4 , C. Fallai5 , P. Olmi5 , G. Zonca3 1 U NIVERSITÀ DEGLI S TUDI DI M ILANO, Physics Department, Milano, Italy 2 N ATIONAL I NSTITUTE OF N UCLEAR P HYSICS, Milan, Italy 3 F ONDAZIONE IRCCS - I STITUTO N AZIONALE DEI T UMORI, Medical Physics Unit, Milan, Italy 4 P OLITECNICO DI M ILANO U NIVERSITY, Department of Energy, Milano, Italy 5 F ONDAZIONE IRCCS - I STITUTO N AZIONALE DEI T UMORI, Radiotherapy Unit, Milan, Italy
Purpose: In high dose rate (HDR) brachytherapy, an accurate and efficient system that measures the dose distribution in three dimensions with high spatial resolution would be very useful for the verification of complex treatments as well as the characterisation of some source parameters. Aim of this study was to optimize, characterize and test a Fricke gel formulation with the above cited characteristics which can be adopted for absolute dosimetry in HDR brachytherapy. Materials: Fricke gel-layer dosimeters (FGLD) were produced in laboratory by infusing a ferrous sulphate solution and the metal-ion indicator XylenolOrange (XO) in a tissue-equivalent gel matrix. Their standard composition was: Agarose in the amount of 3% of the final weight, ferrous sulphate solution [1mM Fe(NH4)2(SO4)2H2O], sulphuric acid [25mM H2SO4] and XO [0.165mM C31H27N2Na5O13S]. Exposure to ionizing radiation of FGLD produces a conversion of ferrous ions Fe2+ into ferric ions Fe3+ , with yield proportional to the absorbed dose. Due to the fact that the complex XO-Fe3+ causes visible light absorption around 585nm, the absorbed dose is evaluated