- Email: [email protected]
MOSkin Detectors for On Line Dosimetry in HDR Ultrasound-Guided Prostate Brachytherapy: Rectal Wall (In Vivo) and Urethra (In Phantom) Measurements
S54
Abstracts / Brachytherapy 13 (2014) S15eS126
number of regions where the dose is low or big heterogeneity occurs, in bone, for example. Conclusions: OncentraBrachy CC convolution algorithm marks a significant dosimetry improvement relative to TG43. The assessment of the clinical significance of this accuracy improvement requires further work. The real patient cases and the ALGEBRA system, together with the workflow as outlined in this work, compose a tool set for benchmarking advanced model-based dose calculation algorithm beyond TG43.
Fig. 1. Isodose lines for a prostate case.
Fig. 2. Dose volume histograms.
PD22 Correction Required for Formula Used to Determine ‘‘Correspondence Factor’’ of the Valencia Skin Applicator Sook Kien Ng, PhD, Adam Robinson, BS, Elwood Armour, PhD, Yi Le, PhD. Johns Hopkins University, Baltimore, MD. Purpose/Objective(s): The Valencia applicator set is used with the MicroSelectron HDR system for brachytherapy treatment of superficial skin lesion. It is designed with a fixed flattening filter built in and uses tungsten-alloy shielding to limit irradiation to the required area. Determination of distal reference distance of the applicator prior to clinical use is required to ensure the source is in the correct position, which is in the middle of the applicator shielding. The output factor verification per manufacturer is done by measurement using well chamber and calculating the manufacturer defined ‘‘correspondence factor’’ (CF). The objective of this study was to evaluate the equation recommended by the manufacture for determining the correspondence factor and to suggest correct. Materials/Methods: The measurements were performed using HDR 1000 Plus well chambers and an acrylic insert provided by the manufacturer. The Valencia applicator was placed on top of the acrylic insert designed to mount at the entrance of the well chamber. The measurements were performed with dwell positions varying from 1319 mm to 1325 mm, with increments of 0.5 mm. The dwell position with maximum reading (R) was recorded as the distal reference distance and the current reading at this position the well chamber reading ‘‘R.’’ Well chamber measurements were repeated 3 times at the applicator’s distal reference distance. The correspondence factor (CF) calculation formula defined by the manufacture and referenced from a publication by Perez-Calatayed et al. (Med Phys 33 (1), 2006) was: CF 5 R * 4(p,T)/(f*Sk), where 4(p,T) is the correction factor for the atmospheric conditions, f is the well chamber calibration factor and Sk is the air kerma strength. The f and Sk factors account for the specific response of the well chamber and actual source strength, respectively. The measurements were acquired for two Valencia applicators with inner diameter of 2 cm (H2 applicator) and 3 cm (H3 applicator). The correspondence factor measurements were repeated with 2 well chambers of the model HDR-1000 Plus but calibration factors that varied by 10%. Results: Using the CF formula recommended by the manufacture, the CF values determined using the measurement done with the first well chamber (model HDR 1000 plus REF 90004) deviated by -15.5% (H2 applicator) and -17.0% (H3 applicator) relative to the CF factor specified by the manufacture. However the CF values determined using the second well chamber (model HDR 1000 plus REF 90008) had deviations of 2.5% (H2 applicator) and 0.3% (H3 applicator) relative to the specified value. Repeat measurements and detailed review of the equation revealed that having the well chamber calibration factor as denominator did not account for the specific response of the well chamber properly. After reviewing the recommended equation it was determined that the chamber calibration factor should properly be in the numerator instead of the denominator: CF 5 R * f*4(p,T)/Sk. When this latter equation is used to calculate a ‘‘correspondence factor,’’ the values for our two chambers are different by only 1%. The absolute value of ‘‘correspondence factor’’ using this latter equation is very different from the number suggested by the manufacturer. Conclusions: This evaluation suggests that a review of the Valencia applicator calibration procedure and ‘‘correspondence factor’’ values be performed and that all users of this applicator likewise re-evaluate their commissioning.
Table 1 Dose parameters for the prostate case
Target-V100 Target-V150 Target-V200 Target-D90 Bladder-D10 Rectum-D10
TG43
CC
MC
PD23
96.1% 28.4% 12.3% 104% 35.1% 39.6%
94.5% 25.2% 8.32% 102% 34.9% 36.0%
95.2% 26.3% 9.92% 102% 35.2% 36.7%
MOSkin Detectors for On Line Dosimetry in HDR UltrasoundGuided Prostate Brachytherapy: Rectal Wall (In Vivo) and Urethra (In Phantom) Measurements Mauro Carrara, PhD1, Chiara Tenconi, MS1, Marta Borroni, PhD1, Annamaria Cerrotta, MD1, Carlo Fallai, MD1, Dean Cutajar, PhD2, Marco Petasecca, PhD2, Joseph Bucci, MD3, Grazia Gambarini, PhD4, Anatoly Rosenfeld, PhD2, Emanuele Pignoli, PhD1. 1Diagnostic Imaging
Abstracts / Brachytherapy 13 (2014) S15eS126 and Radiation Oncology, Fondazione IRCCS Istituto Nazionale dei Tumori, Milan, Italy; 2Centre for Medical Radiation Physics, University of Wollongong, Wollongong, Australia; 3St George Cancer Care Centre, Kogarah, Australia; 4Universita degli Studi and INFN, Milan, Italy. Purpose: Due to the many manual steps involved in ultrasound (US)-guided HDR prostate brachytherapy (BRT), treatment accuracy evaluation is crucial and could be performed implementing a systematic and independent patient specific quality assurance. In vivo dosimetry may be an effective tool to perform this task as it is intended to compare planned and delivered doses, improving moreover possible error detection capability. In this work, MOSkin detectors were integrated on a US-probe and their suitability to perform in vivo rectal wall dosimetry was studied. Moreover, feasibility of urethral in vivo dosimetry was studied performing preliminary measurements in a tissue-equivalent phantom reproducing a typical prostate BT implant. Materials and Methods: MOSkins are a specific type of MOSFET dosimeter developed and optimized to measure dose in steep dose gradients. Their sensitive volume, defined by the volume of the gate oxide, is 4.8 x 10-6 mm3. In this study, MOSkin dosimeters were calibrated in a water equivalent phantom and adopted to perform two different kinds of measurements: A) in vivo rectal wall measurements: MOSkin detectors were integrated on the trans rectal ultrasound (TRUS) probe which is adopted for real time treatment planning in Ir-192 HDR prostate BRT. During treatment, TRUS probe is left inside the rectum and MOSkins longitudinal locations are determined with respect to the prostate knowing their distances from the transversal transducer. This dosimetric system was adopted on 6 patients consecutively treated at our department with HDR prostate BRT as monotherapy (i.e., 2x14Gy, with a time interval of 3-4weeks). Measured doses were compared to the doses calculated by means of the treatment planning system (TPS). B) in phantom urethral measurements: a typical prostate implant was realized inside a tissue-equivalent cylindrical gel phantom. CT imaging was performed in order to precisely localize needles and the urethral catheter and to plan a treatment. A series of measurements with the same irradiation set-up were performed with different dose prescriptions according to different developed treatment plans. The cumulative urethral doses measured with the single or dual-MOSkin (i.e., two detectors coupled ‘‘face to face’’) detectors were compared to the doses calculated by the TPS in the same points. Results: The high dose delivered to the dosimeters at each treatment fraction slightly changes their sensitivities (i.e., up to 3% reduction after each treatment). Calibration factors need therefore to be established before each irradiation. A) Calculated and delivered doses to the rectal wall were in accordance on 5 out of 6 patients, taking into account a 1mm MOSkin positioning uncertainty along the antero-posterior direction. In one patient, measured doses resulted to be significantly higher than the calculated ones (i.e., mean DD513%). In this case, analysis of post irradiation images was performed and permitted us to demonstrate a 2mm prostate movement towards the probe in the time interval between image acquisition and treatment delivery. B) The highest discrepancies between calculated and measured doses to the urethra were found to be within 8% and 3.8% for single and dualMOSkin detectors, respectively. The dual-MOSkin configuration provides greater accuracy and is suggested in case the dosimeters are placed inside the irradiated volume (i.e. urethra) as for dual-MOSkin angular isotropy is significantly improved. Conclusions: MOSkin detectors might be an effective tool for independent real time verification of the dose delivered to the rectal wall in HDR prostate BRT. The use of the developed dosimeters integrated onto the US-probe is very convenient and was incorporated in our department into clinical practice. In light of promising preliminary in phantom measurements, urethral in vivo measurements will soon be studied and added to our independent patient specific quality assurance procedure.
S55
PD24 A Dosimetric Update of the Model S700 Xoft Axxent 50 kV Electronic Brachytherapy Source Including a Cloverleaf-Shaped Insert Jessica R. Hiatt, MS1, Mark J. Rivard, PhD2. 1Radiation Oncology, Rhode Island Hospital, Providence, RI; 2Radiation Oncology, Tufts University School of Medicine, Boston, MA. Purpose: The model S700 Xoft Axxent electronic brachytherapy source by Xoft was characterized by Rivard et al. in 2006. The source design was modified in 2006 to include a plastic centering insert at the source tip to more accurately position the anode. The objectives of the current study were to establish an accurate source model for simulation purposes, to dosimetrically characterize the new source and obtain its TG-43 brachytherapy dosimetry parameters, and to determine dose differences between the source with and without the centering insert. Materials and Methods: Design information from photomicroscopy of dissected model S700 sources and from vendor-supplied CAD drawings were used to aid establishment of a modern Monte Carlo source model, which included the complex olefin cloverleaf insert intended to improve centering and axial anode positioning plus promote water flow for cooling the source anode. These data were used to create an MCNP5 input file for subsequent radiation transport simulations of dose distributions in a water phantom. Compared to the pre-2006 simulation geometry, the influence of volume averaging close to the source was substantially reduced through use of a tally space having tighter radial binning. A track-length estimator was used to evaluate collision kerma as a function of radial distances and polar angles for determination of TG-43 dosimetry parameters. Results for the source at 50 kVp were determined every 0.1 cm from 0.3 to 15.0 cm and every 1 from 0 to 180 . Simulations ran for 2 billion histories, resulting in statistical
Abstracts / Brachytherapy 13 (2014) S15eS126
number of regions where the dose is low or big heterogeneity occurs, in bone, for example. Conclusions: OncentraBrachy CC convolution algorithm marks a significant dosimetry improvement relative to TG43. The assessment of the clinical significance of this accuracy improvement requires further work. The real patient cases and the ALGEBRA system, together with the workflow as outlined in this work, compose a tool set for benchmarking advanced model-based dose calculation algorithm beyond TG43.
Fig. 1. Isodose lines for a prostate case.
Fig. 2. Dose volume histograms.
PD22 Correction Required for Formula Used to Determine ‘‘Correspondence Factor’’ of the Valencia Skin Applicator Sook Kien Ng, PhD, Adam Robinson, BS, Elwood Armour, PhD, Yi Le, PhD. Johns Hopkins University, Baltimore, MD. Purpose/Objective(s): The Valencia applicator set is used with the MicroSelectron HDR system for brachytherapy treatment of superficial skin lesion. It is designed with a fixed flattening filter built in and uses tungsten-alloy shielding to limit irradiation to the required area. Determination of distal reference distance of the applicator prior to clinical use is required to ensure the source is in the correct position, which is in the middle of the applicator shielding. The output factor verification per manufacturer is done by measurement using well chamber and calculating the manufacturer defined ‘‘correspondence factor’’ (CF). The objective of this study was to evaluate the equation recommended by the manufacture for determining the correspondence factor and to suggest correct. Materials/Methods: The measurements were performed using HDR 1000 Plus well chambers and an acrylic insert provided by the manufacturer. The Valencia applicator was placed on top of the acrylic insert designed to mount at the entrance of the well chamber. The measurements were performed with dwell positions varying from 1319 mm to 1325 mm, with increments of 0.5 mm. The dwell position with maximum reading (R) was recorded as the distal reference distance and the current reading at this position the well chamber reading ‘‘R.’’ Well chamber measurements were repeated 3 times at the applicator’s distal reference distance. The correspondence factor (CF) calculation formula defined by the manufacture and referenced from a publication by Perez-Calatayed et al. (Med Phys 33 (1), 2006) was: CF 5 R * 4(p,T)/(f*Sk), where 4(p,T) is the correction factor for the atmospheric conditions, f is the well chamber calibration factor and Sk is the air kerma strength. The f and Sk factors account for the specific response of the well chamber and actual source strength, respectively. The measurements were acquired for two Valencia applicators with inner diameter of 2 cm (H2 applicator) and 3 cm (H3 applicator). The correspondence factor measurements were repeated with 2 well chambers of the model HDR-1000 Plus but calibration factors that varied by 10%. Results: Using the CF formula recommended by the manufacture, the CF values determined using the measurement done with the first well chamber (model HDR 1000 plus REF 90004) deviated by -15.5% (H2 applicator) and -17.0% (H3 applicator) relative to the CF factor specified by the manufacture. However the CF values determined using the second well chamber (model HDR 1000 plus REF 90008) had deviations of 2.5% (H2 applicator) and 0.3% (H3 applicator) relative to the specified value. Repeat measurements and detailed review of the equation revealed that having the well chamber calibration factor as denominator did not account for the specific response of the well chamber properly. After reviewing the recommended equation it was determined that the chamber calibration factor should properly be in the numerator instead of the denominator: CF 5 R * f*4(p,T)/Sk. When this latter equation is used to calculate a ‘‘correspondence factor,’’ the values for our two chambers are different by only 1%. The absolute value of ‘‘correspondence factor’’ using this latter equation is very different from the number suggested by the manufacturer. Conclusions: This evaluation suggests that a review of the Valencia applicator calibration procedure and ‘‘correspondence factor’’ values be performed and that all users of this applicator likewise re-evaluate their commissioning.
Table 1 Dose parameters for the prostate case
Target-V100 Target-V150 Target-V200 Target-D90 Bladder-D10 Rectum-D10
TG43
CC
MC
PD23
96.1% 28.4% 12.3% 104% 35.1% 39.6%
94.5% 25.2% 8.32% 102% 34.9% 36.0%
95.2% 26.3% 9.92% 102% 35.2% 36.7%
MOSkin Detectors for On Line Dosimetry in HDR UltrasoundGuided Prostate Brachytherapy: Rectal Wall (In Vivo) and Urethra (In Phantom) Measurements Mauro Carrara, PhD1, Chiara Tenconi, MS1, Marta Borroni, PhD1, Annamaria Cerrotta, MD1, Carlo Fallai, MD1, Dean Cutajar, PhD2, Marco Petasecca, PhD2, Joseph Bucci, MD3, Grazia Gambarini, PhD4, Anatoly Rosenfeld, PhD2, Emanuele Pignoli, PhD1. 1Diagnostic Imaging
Abstracts / Brachytherapy 13 (2014) S15eS126 and Radiation Oncology, Fondazione IRCCS Istituto Nazionale dei Tumori, Milan, Italy; 2Centre for Medical Radiation Physics, University of Wollongong, Wollongong, Australia; 3St George Cancer Care Centre, Kogarah, Australia; 4Universita degli Studi and INFN, Milan, Italy. Purpose: Due to the many manual steps involved in ultrasound (US)-guided HDR prostate brachytherapy (BRT), treatment accuracy evaluation is crucial and could be performed implementing a systematic and independent patient specific quality assurance. In vivo dosimetry may be an effective tool to perform this task as it is intended to compare planned and delivered doses, improving moreover possible error detection capability. In this work, MOSkin detectors were integrated on a US-probe and their suitability to perform in vivo rectal wall dosimetry was studied. Moreover, feasibility of urethral in vivo dosimetry was studied performing preliminary measurements in a tissue-equivalent phantom reproducing a typical prostate BT implant. Materials and Methods: MOSkins are a specific type of MOSFET dosimeter developed and optimized to measure dose in steep dose gradients. Their sensitive volume, defined by the volume of the gate oxide, is 4.8 x 10-6 mm3. In this study, MOSkin dosimeters were calibrated in a water equivalent phantom and adopted to perform two different kinds of measurements: A) in vivo rectal wall measurements: MOSkin detectors were integrated on the trans rectal ultrasound (TRUS) probe which is adopted for real time treatment planning in Ir-192 HDR prostate BRT. During treatment, TRUS probe is left inside the rectum and MOSkins longitudinal locations are determined with respect to the prostate knowing their distances from the transversal transducer. This dosimetric system was adopted on 6 patients consecutively treated at our department with HDR prostate BRT as monotherapy (i.e., 2x14Gy, with a time interval of 3-4weeks). Measured doses were compared to the doses calculated by means of the treatment planning system (TPS). B) in phantom urethral measurements: a typical prostate implant was realized inside a tissue-equivalent cylindrical gel phantom. CT imaging was performed in order to precisely localize needles and the urethral catheter and to plan a treatment. A series of measurements with the same irradiation set-up were performed with different dose prescriptions according to different developed treatment plans. The cumulative urethral doses measured with the single or dual-MOSkin (i.e., two detectors coupled ‘‘face to face’’) detectors were compared to the doses calculated by the TPS in the same points. Results: The high dose delivered to the dosimeters at each treatment fraction slightly changes their sensitivities (i.e., up to 3% reduction after each treatment). Calibration factors need therefore to be established before each irradiation. A) Calculated and delivered doses to the rectal wall were in accordance on 5 out of 6 patients, taking into account a 1mm MOSkin positioning uncertainty along the antero-posterior direction. In one patient, measured doses resulted to be significantly higher than the calculated ones (i.e., mean DD513%). In this case, analysis of post irradiation images was performed and permitted us to demonstrate a 2mm prostate movement towards the probe in the time interval between image acquisition and treatment delivery. B) The highest discrepancies between calculated and measured doses to the urethra were found to be within 8% and 3.8% for single and dualMOSkin detectors, respectively. The dual-MOSkin configuration provides greater accuracy and is suggested in case the dosimeters are placed inside the irradiated volume (i.e. urethra) as for dual-MOSkin angular isotropy is significantly improved. Conclusions: MOSkin detectors might be an effective tool for independent real time verification of the dose delivered to the rectal wall in HDR prostate BRT. The use of the developed dosimeters integrated onto the US-probe is very convenient and was incorporated in our department into clinical practice. In light of promising preliminary in phantom measurements, urethral in vivo measurements will soon be studied and added to our independent patient specific quality assurance procedure.
S55
PD24 A Dosimetric Update of the Model S700 Xoft Axxent 50 kV Electronic Brachytherapy Source Including a Cloverleaf-Shaped Insert Jessica R. Hiatt, MS1, Mark J. Rivard, PhD2. 1Radiation Oncology, Rhode Island Hospital, Providence, RI; 2Radiation Oncology, Tufts University School of Medicine, Boston, MA. Purpose: The model S700 Xoft Axxent electronic brachytherapy source by Xoft was characterized by Rivard et al. in 2006. The source design was modified in 2006 to include a plastic centering insert at the source tip to more accurately position the anode. The objectives of the current study were to establish an accurate source model for simulation purposes, to dosimetrically characterize the new source and obtain its TG-43 brachytherapy dosimetry parameters, and to determine dose differences between the source with and without the centering insert. Materials and Methods: Design information from photomicroscopy of dissected model S700 sources and from vendor-supplied CAD drawings were used to aid establishment of a modern Monte Carlo source model, which included the complex olefin cloverleaf insert intended to improve centering and axial anode positioning plus promote water flow for cooling the source anode. These data were used to create an MCNP5 input file for subsequent radiation transport simulations of dose distributions in a water phantom. Compared to the pre-2006 simulation geometry, the influence of volume averaging close to the source was substantially reduced through use of a tally space having tighter radial binning. A track-length estimator was used to evaluate collision kerma as a function of radial distances and polar angles for determination of TG-43 dosimetry parameters. Results for the source at 50 kVp were determined every 0.1 cm from 0.3 to 15.0 cm and every 1 from 0 to 180 . Simulations ran for 2 billion histories, resulting in statistical