- Email: [email protected]
681 poster COMPARISON BETWEEN ONCENTRA MASTERPLAN AND PLATO BPS TREATMENT PLANNING SYSTEMS
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B RACHYTHERAPY: P HYSICS
screen in the CCD camera field of view and firmly hold in place. Whenever the source is in the CCD field of view and the room light is turned off, a fluorescent halo due to the source radiation is clearly visible on the screen and recorded by the camera. Source dwell position and time as well as source transit speed are then deducted from that halo by means of an automated source position recognition algorithm. As results of the analysis, QuoVadis returns the estimated source transit speed, measured dwell positions, measured dwell times, differences between measured and reference values and their comparison with tolerance values.In this study, brachytherapy irradiations were performed by means of a 18 channels Microselectron HDR remote afterloading device. To validate QuoVadis, the inter-frames reproducibility of the automated source position recognition algorithm was evaluated and QA measurements were performed and compared to reference values of all the 18 channels. Results: Mean and median differences of the location of a standing source over 891 images were both of 0.02±0.01mm, respectively, resulting in a very high reproducibility of the source dwell position recognition on each image frame. The reported figure shows the planar distribution (in millimetres) of all the obtained positions of the standing source.
5 patients were treated with a conformal plan (CP) CP used CT images to define the Gross Tumour Volume (GTV entire circumference of involved tumour bearing region of the oesophagus). Margins of 2 to 3 cm were added superiorly and inferiorly by extending the volume of the GTV manually to define the PTV. 8Gy was prescribed to the isodose providing the best PTV coverage. We compared the dosimetry of SP with CP retrospectively for the first 5 patients and prospectively for the last 5 patients. Results: Mean V100 PTV (volume that received 100% of the prescribed dose) coverage for CP were 80.7% (range 53.6 - 96.9%) and 55.8% (range 22.1 - 79.1%) for SP. Mean V200 PTV ( mucosal dose) was 33.06% (range 23.04 42.36%) and 15.34% (range 6.43 24.31) for CP and SP respectively.Mean D90PTV (dose to 90% of the PTV) for the CP cohort was 6.65 Gy (range 3.72 9.6 Gy) when compared with SP = 4.55 Gy (range 1.83-6.9). The improvement in the mean D90PTV in the CP cohort was 2.1Gy (range 1.06-3.19). The maximum value of dose to 0.1cc of cord (D0.1cc) were CP= 2.77Gy and SP=2.03Gy.No acute grade 3 or 4 toxicity was observed. 2 patients have required stents for local progression following brachytherapy. Conclusions: This small series demonstrates the advantage of conformal planning and is now routinely used in our centre. 680 poster THE EFFICACY OF HYPERBARIC OXYGEN THERAPY IN THE TREATMENT OF MEDICALLY REFRACTORY SOFT TISSUE NECROSIS AFTER PENILE BRACHYTHERAPY A. Gomez-Iturriaga1 , J. Crook2 , W. Evans3 , S. Elantholi Parameswaran4 , J. Jezioranski5 1 H OSPITAL DE C RUCES, Radiation Oncology, Baracaldo-Vizcaya, Spain 2 BC C ANCER AGENCY - C ENTRE FOR THE S OUTHERN I NTERIO, Radiation Oncology, Kelowna B.C., Canada 3 TORONTO G ENERAL H OSPITAL, Hyperbaric Medicine, Toronto, Canada 4 P RINCESS M ARGARET H OSPITAL, Radiation Oncology, Toronto, Canada 5 P RINCESS M ARGARET H OSPITAL, Physics of Radiation, Toronto, Canada
Regarding QA measurements, mean and median differences between reference and measured dwell positions were both of 0.06±0.16mm, whereas mean and median differences between reference and measured dwell times were both of -0.03±0.03s. Setting tolerance values of 0.5mm and 0.1s, no measured value was outside these tolerances. Conclusions: QuoVadis represents an accurate reliable and affordable tool for the determination of some important non-dosimetric brachytherapy source parameters recommended by the AAPM TG-56 protocol. 679 poster SINGLE CENTRE EXPERIENCE USING 3D CT CONFORMAL PLANNING FOR PATIENTS TREATED WITH INTRALUMINAL BRACHYTHERAPY (ILBT) IN OESOPHAGEAL CANCER G. Radhakrishna1 , P. Bownes1 , C. Richardson1 , K. Harris1 , G. Roe1 , E. Brearley1 , D. Sebag-Montefiore1 , A. M. Crellin1 1 S T J AMES I NSTITUTE OF O NCOLOGY T HE L EEDS T EACHING H OSPITALS NHS T RUST, Leeds, United Kingdom Purpose: Preserving swallowing function in patients with advanced oesophageal cancer is important for palliation. Intraluminal brachytherapy can be used to maintain swallowing function. We compared the use of 3D conformal planning and 2D planning for patients treated with intraluminal brachytherapy in our centre. Materials: 10 patients were treated in the St. James’ Institute of Oncology from April to December 2010. All patients were treated with afterloading HDR Ir192 (Nucletron) single line source treatment. Naso gastric loading tubes were inserted under direct fluoroscopic guidance by a consultant radiologist. 6 patients have been treated with 30 Gy in 10 fractions 3 D Conformal External Beam Radiotherapy followed by a single 8 Gy ILBT boost, 2 patients had brachytherapy as salvage following local relapse after previous external beam radiation (30 GY in 10 fractions) but were able to manage a soft diet and 2 patients had brachytherapy following local relapse after external beam radiation and failed stent insertions (due to stent migration).A standard prescription (SP) to a defined length was a single 8Gy dose prescribed to 1 cm from the source axis on orthogonal radiographs for the first 5 patients treated.The last
Purpose: Soft tissue necrosis is reported in up to 26% of patients undergoing radiotherapy for penile cancer. Management options include local irrigation, wound debridement, antibiotics, local anti-inflammatories and analgesics. Refractory lesions may be treated with partial penectomy. Hyperbaric Oxygen Therapy (HBO) has a well-defined role in the treatment of late radiation toxicities. We present our experience with HBO for medically refractory soft tissue necrosis after penile brachytherapy. Materials: From November 2001 to January 2009, 7 men of 43 treated with penile brachytherapy for squamous carcinoma developed medically refractory soft tissue necrosis and were treated with HBO. All had received a prescribed dose of 60Gy through interstitial Pulse Dose Rate (PDR) brachytherapy using Paris system guidelines. All had failed conservative medical therapies for soft tissue necrosis. Results: Median age was 55 years (range: 35-72). Co-morbidities potentially effecting wound healing included hypertension in 2 patients but none had diabetes mellitus, or peripheral vascular disease. Five were current smokers and one a former smoker. Median time between completion of brachytherapy and appearance of soft tissue necrosis was 13 months (range 9-24) and the median interval before starting HBO was 7.5 months (3-13). The median number of "dives" per HBO course was 40 (30-53). All 7 experienced an excellent response with healing of the necrosis and resolution of symptoms, although 3 required an additional course, two for consolidation of healing and one for a relapse 4 months later. No patient was submitted to penectomy. Conclusions: Consistent with the evidence of its efficacy in other organ systems, HBO should be considered whenever possible as a management option in patients with medically refractory soft tissue ulceration of the penis following radiation therapy.
Brachytherapy: Physics 681 poster COMPARISON BETWEEN ONCENTRA MASTERPLAN AND PLATO BPS TREATMENT PLANNING SYSTEMS M. Piergentili1 , C. D’Ambrosio1 , L. Mondini1 , S. Garelli2 , S. Agostinelli2 , F. Foppiano1 1
ASL 5 "S PEZZINO ", S.C. Fisica Sanitaria, La Spezia, Italy N ATIONAL I NSTITUTE FOR C ANCER R ESEARCH, S.C. Fisica Medica, Genova, Italy
2
Purpose: During the commissioning of the Brachytherapy module, version 3.2, of the Oncentra MasterPlan (OM) treatment plan system from Nucletron we noticed different dwell time in respect of those obtained with Plato system, also from Nucletron. Our purpose is describe these differences and the resulting differences in the given doses
B RACHYTHERAPY: P HYSICS Materials: The commissioning of the Brachytherapy module of OM treatment plan system was performed taking Plato as the standard TPS used in clinical practice. Commissioning tests consisted in comparison of dwell times in simple catheter configurations, varying the length and the diameter of the cylinder applicator used in clinical practice for endovaginal intracavitary gynaecological implants and the prescription dose. The source step was 2.5mm.Using the option axis points, a set of dose points were created at 5mm external to the applicator surface. The dose was optimized at these points.We used Gafchromic EBT2 films to compare the isodoses obtained with the two treatment plannings. Results: The better agreement between the dwell times of the two treatment planning systems was obtained with DTGR 0.452 in OM and 0.5 used as default in Plato BPS, as previously indicated by C. Alves de Oliveira et al. Using these parameters the total dwell time the agreement was under 1%.The differences obtained regarding dwell time in the central part of catheter reached the 50% in the case of cylinder applicator with diameter 35 mm and length 4 cm length; the prescribed dose was 500cGy.
Conclusions: We assumed the differences in the dwell times at the extremities of the catheter are due to the new anisotropy modelling. Moreover we supposed the differences in the dwell times in the central positions of the catheters are due to the optimization process that works in a different and unexpected way in the two TPS. 682 poster DOSE TO MEDIUM OR DOSE TO A WATER CAVITY EMBEDDED IN MEDIUM? S. Abbasinejad Enger1 , A. Ahnesjö2 , L. Beaulieu1 1
C ENTRE H OSPITALIER U NIVERSITAIRE DE Q UÉBEC, L’H ÔTEL -D IEU DE Q UÉBEC, Département de Radio-Oncologie, Quebec, Canada 2 U PPSALA U NIVERSITY, Department of Oncology, Radiology and Clinical Immunology, Uppsala, Sweden Purpose: Traditionally in radiation therapy the absorbed dose has been computed and reported as dose to a Bragg-Gray water cavity enclosed in water, Dw. In more modern dose calculation algorithms the transport and energy deposition occur in the local medium and dose to local medium Dm,M or the dose to a water cavity embedded in the medium, Dw,M is reported. The question of which quantity should be adopted for comparison purposes is under debate. The target for radiotherapy is to sterilize cells by imposing damage to their DNA content. Cell nucleus with its DNA content is biologically the most relevant dose target. In brachytherapy the photon energies are sufficiently low that the ranges of secondary electrons may be close to the dimensions of cell nuclei. Aim of this study was to investigate, by means of Monte Carlo (MC), the relations of Dm,M , Dw,M to Dc,M - the dose to a cell in medium, Dn,M - the dose to a cell nucleus in the medium for different energies used in brachytherapy and different cavity sizes. Materials: Geant4.9.3.patch2 MC code is used to calculate Dm,M , Dw,M, Dc,M and Dn,M. A parallel mono-energetic photon beam is let in across one side of a cubic phantom (5x5x5 mm3 ). The phantom consists of different tissue material. The highest photon energy is 350 keV to represent the higher energy end in brachytherapy i.e.192 Ir. The lowest energy is 20 keV to represent the lower energy end 103 Pd or 125 I. Spherical cavities representing cell (range 4-20 mm) and nuclei (2-10 mm) are placed inside the phantom at different depth depending on the energy of the photon beam to ensure CPE conditions. Scatter is eliminated. Absorbed dose and electron fluence inside the cell and the cell nucleus is scored. Absorbed dose is also calculated with the large cavity theory (LCT) and smal cavity theory (SCT) and compared to the MC calculated absorbed dose to investigate when to expect dominant LCT or large LCT behavior, and for which photon energies we have intermediate conditions. Restricted and unrestricted stopping power values were obtained by EGSnrc MC code. Results: The lower end of brachytherapy energies is more sensitive to chemical composition of the cell and nucleus. The difference between cell lines
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with different water content is significant. LCT for the lowest engeries is valid down to 3 mm. For 3 mm nucleus MC calculated dose deviates from the dose calculated with LCT. Conclusions: There is a large variation between different Dn,M values depending on the cell line and the water content. Conversion factors used to coververt Dm,M into Dw,M are not valid for all cell, nucleus size and bracytherapy energies. 683 poster DOSIMETRIC ANALYSIS OF 30 INDIVIDUAL INTRACAVITARY BRACHYTHERAPY TREATMENT PLANS GENERATED WITH THREE BRACHY TREATMENT PLANNING SYSTEMS A. B. Mohamed Yoosuf1 , G. Workman1 , M. O’toole1 , M. straney1 1 B ELFAST C ITY H OSPITAL, Radiotherapy Medical Physics Service, Belfast, United Kingdom Purpose: To compare geometric applicator reconstruction; prescription isodose volumes; organ at risk DVHs and inter-observer variability of High Dose Rate intracavitary brachytherapy (ICBT) plans produced by three different Treatment Planning modules. Materials: 30 HDR intracavitary radiation therapy fractions were included in this study. Orthogonal images using Nucletron integrated brachytherapy unit ®(IBU) were acquired with applicators in situ. Subsequently, CT scanning was carried out. 2D plans were produced on Nucletron Plato ®(N2DP) using the IBU images. The CT data sets were imported into Nucletron Oncentra Masterplan ®(N3DO) and relevant OARs (rectum, bladder & bowel) were contoured. 3D plans were produced with Nucletron 3D Oncentra Masterplan ®, the image set with contoured structures were exported and also planned using Nucletron 3D Plato ®(N3DP). Doses to 2cc volumes (D2cc) of OARs (rectum, bladder, and bowel) were calculated for both 3D planning modules. The implant volume enclosed by the prescription isodose was recorded for all plans. The applicator reconstruction variability between the treatment planning modules was evaluated by defining three parameters: the linear distance between point A and plane of ovoid sources (l); separation of ovoids (s) and the total treatment time (t). Each 3D plan was carried out by two independent planners to assess inter-observer variability. Results: A total of 150 plans were generated and evaluated in this study. The mean implant volume enclosed by prescription isodose (100%) was measured to be 107.5cc (±18.2), 107.5cc (±18.4) & 110.4cc (±18.2) for N2DP, N3DP & N3DO respectively. The geometric applicator reconstruction as defined by three parameters (l, s & t) agreed to within 1% for the three modules. The mean doses recorded with N3DP to D2cc volume were 5.3Gy (±1.4), 3.7Gy (±1.1) & 4.5Gy (±1.3) for bladder, rectum & bowel respectively, while N3DO, results were 5.5Gy (±1.5), 3.4Gy (±1.1) & 4.4Gy (±1.3) respectively. The mean difference in D2cc between N3DP & N3DO was found to be 3.2%, 6.7% & 3.6% for bladder, rectum & bowel respectively. Our interoperator analysis between N3DP & N3DO showed that the mean difference between two operators was 0.4% (±0.4) & 0.3% (±0.03) for geometric applicator reconstruction, 0.4% (±3.2) & 0.6% (±3.7) for volume enclosed by 100% isodose and 1.3% (±1.8) & 0.2% (±0.5) for dose-volume analysis of OARs respectively. Conclusions: The results demonstrate that for three treatment planning modules the geometric applicator reconstruction is very reproducible which gave confidence with reconstruction technique and assisted in the implementation of clinical 3D planning. The differences observed with D2cc in OARs is most likely due to the proximity of the high dose region within the OAR to dwell positions in the applicators and any slight variation in applicator localization can result in relatively large differences in dose. 684 poster DOSIMETRIC VALIDATION OF HDR TREATMENT PLANNING SYSTEM USING DIODES AND SMALL VOLUME IONISATION CHAMBERS E. Hogg1 , S. Misson-Yates2 , S. Aldridge2 1 K ING ’ S C OLLEGE H OSPITAL, Medical Engineering and Physics, London, United Kingdom 2 G UY ’ S & S T. T HOMAS ’ NHS F OUNDATION T RUST, Medical Physics, London, United Kingdom
Purpose: The Flexiplan v.2.6 TPS calculates the radiation dose distribution exposed to human tissue (water) in 3D for an 192 Ir HDR source (Flexisource). At commissioning extensive validation of the Flexiplan TPS was conducted by manual calculations and film dosimetry with reference to the Monte Carlo (MC) data from the Granero paper and TG-43.The aim of this study is to experimentally verify the isodose data produced by the TPS, in multiple planes, through comparison with both ion chamber and diode measurements in a PTW MP3 water tank. Materials: Tank QC was performed, based on advice given in IPEM 81, to establish the accuracy of tank movements. A daily QC procedure was developed to ensure repeatability of both the Flexisource position within the applicator and the source-chamber set-up.A titanium Rotterdam IUT applica-
B RACHYTHERAPY: P HYSICS
screen in the CCD camera field of view and firmly hold in place. Whenever the source is in the CCD field of view and the room light is turned off, a fluorescent halo due to the source radiation is clearly visible on the screen and recorded by the camera. Source dwell position and time as well as source transit speed are then deducted from that halo by means of an automated source position recognition algorithm. As results of the analysis, QuoVadis returns the estimated source transit speed, measured dwell positions, measured dwell times, differences between measured and reference values and their comparison with tolerance values.In this study, brachytherapy irradiations were performed by means of a 18 channels Microselectron HDR remote afterloading device. To validate QuoVadis, the inter-frames reproducibility of the automated source position recognition algorithm was evaluated and QA measurements were performed and compared to reference values of all the 18 channels. Results: Mean and median differences of the location of a standing source over 891 images were both of 0.02±0.01mm, respectively, resulting in a very high reproducibility of the source dwell position recognition on each image frame. The reported figure shows the planar distribution (in millimetres) of all the obtained positions of the standing source.
5 patients were treated with a conformal plan (CP) CP used CT images to define the Gross Tumour Volume (GTV entire circumference of involved tumour bearing region of the oesophagus). Margins of 2 to 3 cm were added superiorly and inferiorly by extending the volume of the GTV manually to define the PTV. 8Gy was prescribed to the isodose providing the best PTV coverage. We compared the dosimetry of SP with CP retrospectively for the first 5 patients and prospectively for the last 5 patients. Results: Mean V100 PTV (volume that received 100% of the prescribed dose) coverage for CP were 80.7% (range 53.6 - 96.9%) and 55.8% (range 22.1 - 79.1%) for SP. Mean V200 PTV ( mucosal dose) was 33.06% (range 23.04 42.36%) and 15.34% (range 6.43 24.31) for CP and SP respectively.Mean D90PTV (dose to 90% of the PTV) for the CP cohort was 6.65 Gy (range 3.72 9.6 Gy) when compared with SP = 4.55 Gy (range 1.83-6.9). The improvement in the mean D90PTV in the CP cohort was 2.1Gy (range 1.06-3.19). The maximum value of dose to 0.1cc of cord (D0.1cc) were CP= 2.77Gy and SP=2.03Gy.No acute grade 3 or 4 toxicity was observed. 2 patients have required stents for local progression following brachytherapy. Conclusions: This small series demonstrates the advantage of conformal planning and is now routinely used in our centre. 680 poster THE EFFICACY OF HYPERBARIC OXYGEN THERAPY IN THE TREATMENT OF MEDICALLY REFRACTORY SOFT TISSUE NECROSIS AFTER PENILE BRACHYTHERAPY A. Gomez-Iturriaga1 , J. Crook2 , W. Evans3 , S. Elantholi Parameswaran4 , J. Jezioranski5 1 H OSPITAL DE C RUCES, Radiation Oncology, Baracaldo-Vizcaya, Spain 2 BC C ANCER AGENCY - C ENTRE FOR THE S OUTHERN I NTERIO, Radiation Oncology, Kelowna B.C., Canada 3 TORONTO G ENERAL H OSPITAL, Hyperbaric Medicine, Toronto, Canada 4 P RINCESS M ARGARET H OSPITAL, Radiation Oncology, Toronto, Canada 5 P RINCESS M ARGARET H OSPITAL, Physics of Radiation, Toronto, Canada
Regarding QA measurements, mean and median differences between reference and measured dwell positions were both of 0.06±0.16mm, whereas mean and median differences between reference and measured dwell times were both of -0.03±0.03s. Setting tolerance values of 0.5mm and 0.1s, no measured value was outside these tolerances. Conclusions: QuoVadis represents an accurate reliable and affordable tool for the determination of some important non-dosimetric brachytherapy source parameters recommended by the AAPM TG-56 protocol. 679 poster SINGLE CENTRE EXPERIENCE USING 3D CT CONFORMAL PLANNING FOR PATIENTS TREATED WITH INTRALUMINAL BRACHYTHERAPY (ILBT) IN OESOPHAGEAL CANCER G. Radhakrishna1 , P. Bownes1 , C. Richardson1 , K. Harris1 , G. Roe1 , E. Brearley1 , D. Sebag-Montefiore1 , A. M. Crellin1 1 S T J AMES I NSTITUTE OF O NCOLOGY T HE L EEDS T EACHING H OSPITALS NHS T RUST, Leeds, United Kingdom Purpose: Preserving swallowing function in patients with advanced oesophageal cancer is important for palliation. Intraluminal brachytherapy can be used to maintain swallowing function. We compared the use of 3D conformal planning and 2D planning for patients treated with intraluminal brachytherapy in our centre. Materials: 10 patients were treated in the St. James’ Institute of Oncology from April to December 2010. All patients were treated with afterloading HDR Ir192 (Nucletron) single line source treatment. Naso gastric loading tubes were inserted under direct fluoroscopic guidance by a consultant radiologist. 6 patients have been treated with 30 Gy in 10 fractions 3 D Conformal External Beam Radiotherapy followed by a single 8 Gy ILBT boost, 2 patients had brachytherapy as salvage following local relapse after previous external beam radiation (30 GY in 10 fractions) but were able to manage a soft diet and 2 patients had brachytherapy following local relapse after external beam radiation and failed stent insertions (due to stent migration).A standard prescription (SP) to a defined length was a single 8Gy dose prescribed to 1 cm from the source axis on orthogonal radiographs for the first 5 patients treated.The last
Purpose: Soft tissue necrosis is reported in up to 26% of patients undergoing radiotherapy for penile cancer. Management options include local irrigation, wound debridement, antibiotics, local anti-inflammatories and analgesics. Refractory lesions may be treated with partial penectomy. Hyperbaric Oxygen Therapy (HBO) has a well-defined role in the treatment of late radiation toxicities. We present our experience with HBO for medically refractory soft tissue necrosis after penile brachytherapy. Materials: From November 2001 to January 2009, 7 men of 43 treated with penile brachytherapy for squamous carcinoma developed medically refractory soft tissue necrosis and were treated with HBO. All had received a prescribed dose of 60Gy through interstitial Pulse Dose Rate (PDR) brachytherapy using Paris system guidelines. All had failed conservative medical therapies for soft tissue necrosis. Results: Median age was 55 years (range: 35-72). Co-morbidities potentially effecting wound healing included hypertension in 2 patients but none had diabetes mellitus, or peripheral vascular disease. Five were current smokers and one a former smoker. Median time between completion of brachytherapy and appearance of soft tissue necrosis was 13 months (range 9-24) and the median interval before starting HBO was 7.5 months (3-13). The median number of "dives" per HBO course was 40 (30-53). All 7 experienced an excellent response with healing of the necrosis and resolution of symptoms, although 3 required an additional course, two for consolidation of healing and one for a relapse 4 months later. No patient was submitted to penectomy. Conclusions: Consistent with the evidence of its efficacy in other organ systems, HBO should be considered whenever possible as a management option in patients with medically refractory soft tissue ulceration of the penis following radiation therapy.
Brachytherapy: Physics 681 poster COMPARISON BETWEEN ONCENTRA MASTERPLAN AND PLATO BPS TREATMENT PLANNING SYSTEMS M. Piergentili1 , C. D’Ambrosio1 , L. Mondini1 , S. Garelli2 , S. Agostinelli2 , F. Foppiano1 1
ASL 5 "S PEZZINO ", S.C. Fisica Sanitaria, La Spezia, Italy N ATIONAL I NSTITUTE FOR C ANCER R ESEARCH, S.C. Fisica Medica, Genova, Italy
2
Purpose: During the commissioning of the Brachytherapy module, version 3.2, of the Oncentra MasterPlan (OM) treatment plan system from Nucletron we noticed different dwell time in respect of those obtained with Plato system, also from Nucletron. Our purpose is describe these differences and the resulting differences in the given doses
B RACHYTHERAPY: P HYSICS Materials: The commissioning of the Brachytherapy module of OM treatment plan system was performed taking Plato as the standard TPS used in clinical practice. Commissioning tests consisted in comparison of dwell times in simple catheter configurations, varying the length and the diameter of the cylinder applicator used in clinical practice for endovaginal intracavitary gynaecological implants and the prescription dose. The source step was 2.5mm.Using the option axis points, a set of dose points were created at 5mm external to the applicator surface. The dose was optimized at these points.We used Gafchromic EBT2 films to compare the isodoses obtained with the two treatment plannings. Results: The better agreement between the dwell times of the two treatment planning systems was obtained with DTGR 0.452 in OM and 0.5 used as default in Plato BPS, as previously indicated by C. Alves de Oliveira et al. Using these parameters the total dwell time the agreement was under 1%.The differences obtained regarding dwell time in the central part of catheter reached the 50% in the case of cylinder applicator with diameter 35 mm and length 4 cm length; the prescribed dose was 500cGy.
Conclusions: We assumed the differences in the dwell times at the extremities of the catheter are due to the new anisotropy modelling. Moreover we supposed the differences in the dwell times in the central positions of the catheters are due to the optimization process that works in a different and unexpected way in the two TPS. 682 poster DOSE TO MEDIUM OR DOSE TO A WATER CAVITY EMBEDDED IN MEDIUM? S. Abbasinejad Enger1 , A. Ahnesjö2 , L. Beaulieu1 1
C ENTRE H OSPITALIER U NIVERSITAIRE DE Q UÉBEC, L’H ÔTEL -D IEU DE Q UÉBEC, Département de Radio-Oncologie, Quebec, Canada 2 U PPSALA U NIVERSITY, Department of Oncology, Radiology and Clinical Immunology, Uppsala, Sweden Purpose: Traditionally in radiation therapy the absorbed dose has been computed and reported as dose to a Bragg-Gray water cavity enclosed in water, Dw. In more modern dose calculation algorithms the transport and energy deposition occur in the local medium and dose to local medium Dm,M or the dose to a water cavity embedded in the medium, Dw,M is reported. The question of which quantity should be adopted for comparison purposes is under debate. The target for radiotherapy is to sterilize cells by imposing damage to their DNA content. Cell nucleus with its DNA content is biologically the most relevant dose target. In brachytherapy the photon energies are sufficiently low that the ranges of secondary electrons may be close to the dimensions of cell nuclei. Aim of this study was to investigate, by means of Monte Carlo (MC), the relations of Dm,M , Dw,M to Dc,M - the dose to a cell in medium, Dn,M - the dose to a cell nucleus in the medium for different energies used in brachytherapy and different cavity sizes. Materials: Geant4.9.3.patch2 MC code is used to calculate Dm,M , Dw,M, Dc,M and Dn,M. A parallel mono-energetic photon beam is let in across one side of a cubic phantom (5x5x5 mm3 ). The phantom consists of different tissue material. The highest photon energy is 350 keV to represent the higher energy end in brachytherapy i.e.192 Ir. The lowest energy is 20 keV to represent the lower energy end 103 Pd or 125 I. Spherical cavities representing cell (range 4-20 mm) and nuclei (2-10 mm) are placed inside the phantom at different depth depending on the energy of the photon beam to ensure CPE conditions. Scatter is eliminated. Absorbed dose and electron fluence inside the cell and the cell nucleus is scored. Absorbed dose is also calculated with the large cavity theory (LCT) and smal cavity theory (SCT) and compared to the MC calculated absorbed dose to investigate when to expect dominant LCT or large LCT behavior, and for which photon energies we have intermediate conditions. Restricted and unrestricted stopping power values were obtained by EGSnrc MC code. Results: The lower end of brachytherapy energies is more sensitive to chemical composition of the cell and nucleus. The difference between cell lines
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with different water content is significant. LCT for the lowest engeries is valid down to 3 mm. For 3 mm nucleus MC calculated dose deviates from the dose calculated with LCT. Conclusions: There is a large variation between different Dn,M values depending on the cell line and the water content. Conversion factors used to coververt Dm,M into Dw,M are not valid for all cell, nucleus size and bracytherapy energies. 683 poster DOSIMETRIC ANALYSIS OF 30 INDIVIDUAL INTRACAVITARY BRACHYTHERAPY TREATMENT PLANS GENERATED WITH THREE BRACHY TREATMENT PLANNING SYSTEMS A. B. Mohamed Yoosuf1 , G. Workman1 , M. O’toole1 , M. straney1 1 B ELFAST C ITY H OSPITAL, Radiotherapy Medical Physics Service, Belfast, United Kingdom Purpose: To compare geometric applicator reconstruction; prescription isodose volumes; organ at risk DVHs and inter-observer variability of High Dose Rate intracavitary brachytherapy (ICBT) plans produced by three different Treatment Planning modules. Materials: 30 HDR intracavitary radiation therapy fractions were included in this study. Orthogonal images using Nucletron integrated brachytherapy unit ®(IBU) were acquired with applicators in situ. Subsequently, CT scanning was carried out. 2D plans were produced on Nucletron Plato ®(N2DP) using the IBU images. The CT data sets were imported into Nucletron Oncentra Masterplan ®(N3DO) and relevant OARs (rectum, bladder & bowel) were contoured. 3D plans were produced with Nucletron 3D Oncentra Masterplan ®, the image set with contoured structures were exported and also planned using Nucletron 3D Plato ®(N3DP). Doses to 2cc volumes (D2cc) of OARs (rectum, bladder, and bowel) were calculated for both 3D planning modules. The implant volume enclosed by the prescription isodose was recorded for all plans. The applicator reconstruction variability between the treatment planning modules was evaluated by defining three parameters: the linear distance between point A and plane of ovoid sources (l); separation of ovoids (s) and the total treatment time (t). Each 3D plan was carried out by two independent planners to assess inter-observer variability. Results: A total of 150 plans were generated and evaluated in this study. The mean implant volume enclosed by prescription isodose (100%) was measured to be 107.5cc (±18.2), 107.5cc (±18.4) & 110.4cc (±18.2) for N2DP, N3DP & N3DO respectively. The geometric applicator reconstruction as defined by three parameters (l, s & t) agreed to within 1% for the three modules. The mean doses recorded with N3DP to D2cc volume were 5.3Gy (±1.4), 3.7Gy (±1.1) & 4.5Gy (±1.3) for bladder, rectum & bowel respectively, while N3DO, results were 5.5Gy (±1.5), 3.4Gy (±1.1) & 4.4Gy (±1.3) respectively. The mean difference in D2cc between N3DP & N3DO was found to be 3.2%, 6.7% & 3.6% for bladder, rectum & bowel respectively. Our interoperator analysis between N3DP & N3DO showed that the mean difference between two operators was 0.4% (±0.4) & 0.3% (±0.03) for geometric applicator reconstruction, 0.4% (±3.2) & 0.6% (±3.7) for volume enclosed by 100% isodose and 1.3% (±1.8) & 0.2% (±0.5) for dose-volume analysis of OARs respectively. Conclusions: The results demonstrate that for three treatment planning modules the geometric applicator reconstruction is very reproducible which gave confidence with reconstruction technique and assisted in the implementation of clinical 3D planning. The differences observed with D2cc in OARs is most likely due to the proximity of the high dose region within the OAR to dwell positions in the applicators and any slight variation in applicator localization can result in relatively large differences in dose. 684 poster DOSIMETRIC VALIDATION OF HDR TREATMENT PLANNING SYSTEM USING DIODES AND SMALL VOLUME IONISATION CHAMBERS E. Hogg1 , S. Misson-Yates2 , S. Aldridge2 1 K ING ’ S C OLLEGE H OSPITAL, Medical Engineering and Physics, London, United Kingdom 2 G UY ’ S & S T. T HOMAS ’ NHS F OUNDATION T RUST, Medical Physics, London, United Kingdom
Purpose: The Flexiplan v.2.6 TPS calculates the radiation dose distribution exposed to human tissue (water) in 3D for an 192 Ir HDR source (Flexisource). At commissioning extensive validation of the Flexiplan TPS was conducted by manual calculations and film dosimetry with reference to the Monte Carlo (MC) data from the Granero paper and TG-43.The aim of this study is to experimentally verify the isodose data produced by the TPS, in multiple planes, through comparison with both ion chamber and diode measurements in a PTW MP3 water tank. Materials: Tank QC was performed, based on advice given in IPEM 81, to establish the accuracy of tank movements. A daily QC procedure was developed to ensure repeatability of both the Flexisource position within the applicator and the source-chamber set-up.A titanium Rotterdam IUT applica-