- Email: [email protected]
Effect of Heterogeneity on Dose Deposited by a Flat HDR Surface Applicator
S98
Abstracts / Brachytherapy 13 (2014) S15eS126
Results: At the radial distances within a few millimeters from the center of the seed, the doses from the 0.5 mm deep slot were about 3%, 4%, and 6% lower than the 1.4 mm deep slot in XZ, YZ, and XY planes, respectively (see figure). However, higher dose was noticed at the off axis within few millimeters from the source surface and, therefore, the collimation effect was less pronounced with 0.5 mm deep slot. Figure on the left column shows the dose distribution with 0.5 mm deep slot and figure on the right column shows the dose distribution with 1.4 mm deep slot. It was noticed that the dose to the normal tissues and critical structure such as sclera would be problematic with smaller depth slots. Conclusions: For the USC #9 plaque, there were significant differences in scleral and peripheral doses in the eye amongst the various simulation parameters, with large differences observed between the reported geometry by Astrahan et al. and those measured from plaques taken from our clinical inventory. It appears necessary to measure these plaques upon receipt preceding clinical use to ensure that the specified design matches that which is used in the clinical treatment planning system. Otherwise, significant dose errors may result.
PO27
- R&V check: the patient name, fractionation scheme and plan properties (such as TRAK value) in the plan printout are compared to the R&V system. TRAK tolerance is 1%. Three documents are generated: a summary sheet of all the checks being performed and their results, the results of the secondary calculation and a worksheet summarizing the plan information to be checked during treatment. Since its introduction into clinical use in September 2013, BrachyVerifier has been used to check 51 HDR plans. Mean physics check time for tandem and ring cases was 1217 seconds with BrachyVerifier (n 5 23) and 1403 seconds without (n 5 20; p 5 0.045). Conclusions: BrachyVerifier has standardized the physics checks of HDR plans and produced a significant reduction in plan check time. The integration of BrachyVerifier into our clinical practice automated the checks on channel assignments, catheter lengths and prescription, thus reducing the possibility of missing errors that occurred during planning. BrachyVerifier has not eliminated the need for plan checking by a physicist. Moreover, checks such as correct attachment of transfer tubes and other safety-related checks cannot be automated via software. Although the exact implementation of BrachyVerifier is specific to our clinical practices and equipment, the general architecture is portable from one clinic to another.
Brachyverifier: An Automated System for Plan Quality Assurance in High-Dose-Rate Brachytherapy Antonio L. Damato, PhD, Christina Molodowitch, MS, Jorgen L. Hansen, MS, Robert A. Cormack, PhD, Desmond A. O’Farrell, MS, Scott A. Friesen, MS, Mandar S. Bhagwat, PhD, Ivan Buzurovic, PhD, Larissa J. Lee, MD, Akila N. Viswanathan, MD, Phillip M. Devlin, MD, Joseph Killoran, PhD. Radiation Oncology, Dana Farber Cancer Institute /Brigham and Women’s Hospital, Boston, MA. Purpose: To improve the quality assurance (QA) process of high-dose-rate brachytherapy (HDR) by automating the physics plan check process. Materials and Methods: BrachyVerifier is an application written in Java for the automation of plan QA in HDR. The program extracts information from the record-and-verify (R&V) data stored in an ARIA database (Varian Medical Systems, Inc., Palo Alto, CA) and from portable document format (PDF) files of (i) the plan printout from the Oncentra Brachytherapy planning system (Nucletron, an Elekta company, Elekta AB, Stockholm, Sweden), (ii) the written directive, and (iii) the catheter-length measurement report. Checks are customized to the type of plan, which is detected from the plan printout. If more than one type of plan is compatible with the plan printout, the user is prompted to choose the plan type. Check speed was assessed through a review of the time elapsed between plan approval and final plan check in the 43 tandem-and-ring fractions that occurred between July 2013 and November 2013. Statistical significance (p ! 0.05) was assessed using an unpaired Student T-test. Results: Checks specific to a given type of plan have been developed for the following types: tandem and ring, tandem and ring þ interstitial needles, tandem and ovoids, tandem and ovoids þ interstitial needles, tandem þ interstitial needles, interstitial needles only (gynecologic and nongynecologic), straight line cylinder, graphical cylinder and surface applicator. Other types of plans are processed under an ‘‘other’’ category, with no type-specific checks. BrachyVerifier was written and tested to perform the following checks: - Secondary calculation: the point doses in the plan printout are compared to the dose resulting from a 1D TG43 calculation. Tolerance is 5%. - Channel assignments: the channel assigned to each catheter is compared to our clinical practice for a given plan type (e.g., tandem to channel 3). Match must be exact. - Catheter lengths: the catheter lengths in the plan printout are compared to either a) the measurement report or b) the lengths that are expected for a given plan type (e.g., 1500 for tandem). Match must be exact. - Written directive compliance: the fractionation scheme, number of catheters and step size indicated in the written directive are compared to the plan printout. Match must be exact.
Figure. Screenshot of the summary page of BrachyVerifier for a straight line cylinder plan.
PO28 Effect of Heterogeneity on Dose Deposited by a Flat HDR Surface Applicator Mandar S. Bhagwat, PhD1,2, Ivan Buzurovic, PhD1,2, Desmond A. O’Farrell, MSc1,2, Scott A. Friesen, MS1,2, Antonio L. Damato, PhD1,2, Jorgen L. Hansen, MS1,2, Phillip M. Devlin, MD, FACR, FASTRO, FFRRCSI (Hon)1,2, Robert A. Cormack, PhD1,2. 1Dana Farber Brigham and Women’s Cancer Center, Boston, MA; 2Harvard Medical School, Boston, MA. Purpose: Most HDR brachytherapy treatment planning systems (TPS) use TG-43 formalism to calculate dose without including heterogeneity corrections. We investigate two scenarios where heterogeneity corrections could potentially create disagreement between calculated and delivered dose (a) under cortical bone and (b) in the presence of air gaps. Methods and Materials: A plan was generated using Oncentra Brachy TPS (Version 4.3.0.410, Nucletron BV) on a CT scan of a 9-catheter Harrison-Anderson-Mick (HAM) applicator (Mick Radio-Nuclear
Abstracts / Brachytherapy 13 (2014) S15eS126 Instruments, Inc.) laid flat on the MatriXX (IBA Dosimetry GmbH) detector. This detector is an array of 1020 parallel plate ion chambers. All 9 catheters were digitized and dwells within a central square region of the applicator were activated. Each catheter had 19 dwells in increments of 0.5cm. The plan was normalized to our usual prescription depth for Cutaneous T-Cell Lymphoma (CTCL) treatments of 3mm in tissue. The plan was delivered multiple times with the following set-ups: (1) HAM applicator positioned flat on the MatriXX array, (2) Freiburg applicator (Nucletron BV) was placed similarly and (3) HAM applicator resting on combinations of cortical bone equivalent material (CB) and solid water (SW) adding to a fixed depth of 13mm, e.g. 3mm SW and 10mm CB or 13mm SW and 0mm CB, atop MatriXX. 10mm CB closely resembles the thickness of human cranial bone. The uniform HAM applicator does not create air pockets in setup (1). Freiburg applicator has a ball-to-ball construction. This introduces air pockets between the applicator and treatment surface in setup (2). The MatriXX array was operated to capture dosimetric snaps every 0.5s and also yielded an integral dose at the end of treatment. Results: A comparison of integral dose for setup (1) and (2) showed no systematic variation in dose due to the presence of air pockets in (2). The jagged topography of the dose seen in figs (1a) and (1b), and the irregular profile in fig (1c) is due to the non-uniform response of individual ion chambers. Radiation dose would be lower by 7% if 10mm of tissue was replaced by 10mm of cortical bone as seen in fig (1d) from the comparative profiles obtained through setup (3). Thus, TG-43 based TPS calculation for surface applicators treating a patient’s scalp predicts a 7% higher dose to the brain tissue near the cranial bone than what it actually receives during treatment. Conclusion: TG-43 calculation compares well with experimentally measured dose in the presence of air pockets, demonstrating that HAM applicators and Freiburg applicators are dosimetrically equivalent for the treatment of skin lesions, but in the presence of bone a heterogeneity correcting algorithm is required for an improved dose prediction.
S99
It is well known in the literature that the position of the markers does not correspond to those of the source for ring applicators. To overcome this limitation applicator models are provided in Oncentra Brachy (Nucletron). Each model is supplied with standard source paths. Therefore it is extremely important that the applicator model being used reflects as accurately as possible the positions of the source inside the specific ring applicators for the specifc afterloader used in the clinic. Purpose: The purpose of the present study is to quantitatively measure the differences in position between not only the markers and the position of the source; but more importantly between the source path in the currently used Oncentra applicator library and our in-house measured source path. Materials and Methods: Oncentra Brachy v. 4.3 and its Applicator Library module v.2.0 were used for this study. Measurements of the source path for six different CT/MR ring applicators were performed. CT-Scouts were taken for the position of the source inside the ring every 2mm. A Flexitron (Nucletron) afterloader was used and CT-Scouts were taken at 0 and 90 . An in-house digitizing tool was created using Matlab to precisely obtain the position of the source path in X, Y and Z. The maximum distance between the position of the markers inside the ring and the position of the source was measured. The difference in distance between the X, Y, and Z coordinates of the source obtained using the in-house digitizing tool were also compared to those currently in use by the Oncentra applicator models. Results: A maximum difference in distance of 3mm was found between the center of the source and the center of the marker with the position of the source being always before that of the marker. This discrepancy was more pronounced closer to the ring end and the difference gradually decreased as the source got closer to the connector end (0.33mm). It is well known in the literature that these results are caused by friction between the source and the different segments in the ring tube. An average difference of 45% was observed between the values of the Oncentra source path library model in the Z coordinate and those using our in-house model. These results emphasize the importance of creating an in-house model with measurements of the source path not only in X and Y but also in the Z coordinate. These measurements should be done during commissioning of ring applicators. Finally, a 2 mm difference in distance was also found between the Oncentra and our in-house measured source paths. Conclusions: Our results demonstrate that a complete examination of the position of the source path in relation to the position of the markers and to the position of the source path in the Oncentra applicator library is necessary. We conclude that a blind use of either the markers or the Oncentra applicator library should be avoided. Further investigation should be done to measure the impact of these results in brachytherapy dose distribution.
PO30
Figures. (1a) and (1b) show dose distribution for setup (1) and (2) respectively. The profiles in Figure (1c) illustrate dosimetric equivalence of Freiburg and HAM applicator, while Figure (Id) draws attention to the attenuation by bone.
PO29 Importance on the Development of a Clinic-Specific Source Path Position Library while Commissioning Ring Applicators when Compared to the Position of Applicator Markers and to the Ring Source Path Position Library Model Currently in Use by Oncentra Treatment Planning Fabiola Vallejo, MSc, PhD, MCCPM, Bruno Carozza, MSc, Fadi Hobeila, MSc, MCCPM. Service de Physique Medicale, CICL, Centre Integre de Cancerologie de Laval, Laval, QC, Canada.
Backscatter and Transmission through a Lead Shield Used in Surface and Interstitial HDR Brachytherapy Cristian Candela-Juan, Msc1, Domingo Granero, PhD2, Javier Vijande, PhD3, Facundo Ballester, PhD3, Jose Perez-Calatayud, PhD4, Mark J. Rivard, PhD5. 1Radioprotection, La Fe University and Polytechnic Hospital, Valencia, Spain; 2Radiation Physics, ERESA, Hospital General Universitario, Valencia, Spain; 3Atomic, Molecular and Nuclear Physics, University of Valencia, Burjassot, Spain; 4Radiotherapy, La Fe University and Polytechnic Hospital, Valencia, Spain; 5Radiation Oncology, Tufts University School of Medicine, Boston, MA. Purpose: In surface and interstitial high-dose-rate (HDR) brachytherapy, some superficial radiosensitive organs such as the eye lens, thyroid, or gonads may be exposed to relatively high absorbed doses. In order to protect them, lead shields may be placed on the patient surface above the implant. Besides attenuation across the shield, there is a backscatter dose enhancement. The aim of this study is to evaluate the dosimetric perturbations (transmission and backscatter) of these shields in surface and interstitial HDR brachytherapy using 60Co, 192Ir, or 169Yb sources, and to provide the required bolus thicknesses to minimize surface overdoses. Materials and Methods: Monte Carlo simulations in Geant4 were performed for a source containing one of the three radionuclides placed at
Abstracts / Brachytherapy 13 (2014) S15eS126
Results: At the radial distances within a few millimeters from the center of the seed, the doses from the 0.5 mm deep slot were about 3%, 4%, and 6% lower than the 1.4 mm deep slot in XZ, YZ, and XY planes, respectively (see figure). However, higher dose was noticed at the off axis within few millimeters from the source surface and, therefore, the collimation effect was less pronounced with 0.5 mm deep slot. Figure on the left column shows the dose distribution with 0.5 mm deep slot and figure on the right column shows the dose distribution with 1.4 mm deep slot. It was noticed that the dose to the normal tissues and critical structure such as sclera would be problematic with smaller depth slots. Conclusions: For the USC #9 plaque, there were significant differences in scleral and peripheral doses in the eye amongst the various simulation parameters, with large differences observed between the reported geometry by Astrahan et al. and those measured from plaques taken from our clinical inventory. It appears necessary to measure these plaques upon receipt preceding clinical use to ensure that the specified design matches that which is used in the clinical treatment planning system. Otherwise, significant dose errors may result.
PO27
- R&V check: the patient name, fractionation scheme and plan properties (such as TRAK value) in the plan printout are compared to the R&V system. TRAK tolerance is 1%. Three documents are generated: a summary sheet of all the checks being performed and their results, the results of the secondary calculation and a worksheet summarizing the plan information to be checked during treatment. Since its introduction into clinical use in September 2013, BrachyVerifier has been used to check 51 HDR plans. Mean physics check time for tandem and ring cases was 1217 seconds with BrachyVerifier (n 5 23) and 1403 seconds without (n 5 20; p 5 0.045). Conclusions: BrachyVerifier has standardized the physics checks of HDR plans and produced a significant reduction in plan check time. The integration of BrachyVerifier into our clinical practice automated the checks on channel assignments, catheter lengths and prescription, thus reducing the possibility of missing errors that occurred during planning. BrachyVerifier has not eliminated the need for plan checking by a physicist. Moreover, checks such as correct attachment of transfer tubes and other safety-related checks cannot be automated via software. Although the exact implementation of BrachyVerifier is specific to our clinical practices and equipment, the general architecture is portable from one clinic to another.
Brachyverifier: An Automated System for Plan Quality Assurance in High-Dose-Rate Brachytherapy Antonio L. Damato, PhD, Christina Molodowitch, MS, Jorgen L. Hansen, MS, Robert A. Cormack, PhD, Desmond A. O’Farrell, MS, Scott A. Friesen, MS, Mandar S. Bhagwat, PhD, Ivan Buzurovic, PhD, Larissa J. Lee, MD, Akila N. Viswanathan, MD, Phillip M. Devlin, MD, Joseph Killoran, PhD. Radiation Oncology, Dana Farber Cancer Institute /Brigham and Women’s Hospital, Boston, MA. Purpose: To improve the quality assurance (QA) process of high-dose-rate brachytherapy (HDR) by automating the physics plan check process. Materials and Methods: BrachyVerifier is an application written in Java for the automation of plan QA in HDR. The program extracts information from the record-and-verify (R&V) data stored in an ARIA database (Varian Medical Systems, Inc., Palo Alto, CA) and from portable document format (PDF) files of (i) the plan printout from the Oncentra Brachytherapy planning system (Nucletron, an Elekta company, Elekta AB, Stockholm, Sweden), (ii) the written directive, and (iii) the catheter-length measurement report. Checks are customized to the type of plan, which is detected from the plan printout. If more than one type of plan is compatible with the plan printout, the user is prompted to choose the plan type. Check speed was assessed through a review of the time elapsed between plan approval and final plan check in the 43 tandem-and-ring fractions that occurred between July 2013 and November 2013. Statistical significance (p ! 0.05) was assessed using an unpaired Student T-test. Results: Checks specific to a given type of plan have been developed for the following types: tandem and ring, tandem and ring þ interstitial needles, tandem and ovoids, tandem and ovoids þ interstitial needles, tandem þ interstitial needles, interstitial needles only (gynecologic and nongynecologic), straight line cylinder, graphical cylinder and surface applicator. Other types of plans are processed under an ‘‘other’’ category, with no type-specific checks. BrachyVerifier was written and tested to perform the following checks: - Secondary calculation: the point doses in the plan printout are compared to the dose resulting from a 1D TG43 calculation. Tolerance is 5%. - Channel assignments: the channel assigned to each catheter is compared to our clinical practice for a given plan type (e.g., tandem to channel 3). Match must be exact. - Catheter lengths: the catheter lengths in the plan printout are compared to either a) the measurement report or b) the lengths that are expected for a given plan type (e.g., 1500 for tandem). Match must be exact. - Written directive compliance: the fractionation scheme, number of catheters and step size indicated in the written directive are compared to the plan printout. Match must be exact.
Figure. Screenshot of the summary page of BrachyVerifier for a straight line cylinder plan.
PO28 Effect of Heterogeneity on Dose Deposited by a Flat HDR Surface Applicator Mandar S. Bhagwat, PhD1,2, Ivan Buzurovic, PhD1,2, Desmond A. O’Farrell, MSc1,2, Scott A. Friesen, MS1,2, Antonio L. Damato, PhD1,2, Jorgen L. Hansen, MS1,2, Phillip M. Devlin, MD, FACR, FASTRO, FFRRCSI (Hon)1,2, Robert A. Cormack, PhD1,2. 1Dana Farber Brigham and Women’s Cancer Center, Boston, MA; 2Harvard Medical School, Boston, MA. Purpose: Most HDR brachytherapy treatment planning systems (TPS) use TG-43 formalism to calculate dose without including heterogeneity corrections. We investigate two scenarios where heterogeneity corrections could potentially create disagreement between calculated and delivered dose (a) under cortical bone and (b) in the presence of air gaps. Methods and Materials: A plan was generated using Oncentra Brachy TPS (Version 4.3.0.410, Nucletron BV) on a CT scan of a 9-catheter Harrison-Anderson-Mick (HAM) applicator (Mick Radio-Nuclear
Abstracts / Brachytherapy 13 (2014) S15eS126 Instruments, Inc.) laid flat on the MatriXX (IBA Dosimetry GmbH) detector. This detector is an array of 1020 parallel plate ion chambers. All 9 catheters were digitized and dwells within a central square region of the applicator were activated. Each catheter had 19 dwells in increments of 0.5cm. The plan was normalized to our usual prescription depth for Cutaneous T-Cell Lymphoma (CTCL) treatments of 3mm in tissue. The plan was delivered multiple times with the following set-ups: (1) HAM applicator positioned flat on the MatriXX array, (2) Freiburg applicator (Nucletron BV) was placed similarly and (3) HAM applicator resting on combinations of cortical bone equivalent material (CB) and solid water (SW) adding to a fixed depth of 13mm, e.g. 3mm SW and 10mm CB or 13mm SW and 0mm CB, atop MatriXX. 10mm CB closely resembles the thickness of human cranial bone. The uniform HAM applicator does not create air pockets in setup (1). Freiburg applicator has a ball-to-ball construction. This introduces air pockets between the applicator and treatment surface in setup (2). The MatriXX array was operated to capture dosimetric snaps every 0.5s and also yielded an integral dose at the end of treatment. Results: A comparison of integral dose for setup (1) and (2) showed no systematic variation in dose due to the presence of air pockets in (2). The jagged topography of the dose seen in figs (1a) and (1b), and the irregular profile in fig (1c) is due to the non-uniform response of individual ion chambers. Radiation dose would be lower by 7% if 10mm of tissue was replaced by 10mm of cortical bone as seen in fig (1d) from the comparative profiles obtained through setup (3). Thus, TG-43 based TPS calculation for surface applicators treating a patient’s scalp predicts a 7% higher dose to the brain tissue near the cranial bone than what it actually receives during treatment. Conclusion: TG-43 calculation compares well with experimentally measured dose in the presence of air pockets, demonstrating that HAM applicators and Freiburg applicators are dosimetrically equivalent for the treatment of skin lesions, but in the presence of bone a heterogeneity correcting algorithm is required for an improved dose prediction.
S99
It is well known in the literature that the position of the markers does not correspond to those of the source for ring applicators. To overcome this limitation applicator models are provided in Oncentra Brachy (Nucletron). Each model is supplied with standard source paths. Therefore it is extremely important that the applicator model being used reflects as accurately as possible the positions of the source inside the specific ring applicators for the specifc afterloader used in the clinic. Purpose: The purpose of the present study is to quantitatively measure the differences in position between not only the markers and the position of the source; but more importantly between the source path in the currently used Oncentra applicator library and our in-house measured source path. Materials and Methods: Oncentra Brachy v. 4.3 and its Applicator Library module v.2.0 were used for this study. Measurements of the source path for six different CT/MR ring applicators were performed. CT-Scouts were taken for the position of the source inside the ring every 2mm. A Flexitron (Nucletron) afterloader was used and CT-Scouts were taken at 0 and 90 . An in-house digitizing tool was created using Matlab to precisely obtain the position of the source path in X, Y and Z. The maximum distance between the position of the markers inside the ring and the position of the source was measured. The difference in distance between the X, Y, and Z coordinates of the source obtained using the in-house digitizing tool were also compared to those currently in use by the Oncentra applicator models. Results: A maximum difference in distance of 3mm was found between the center of the source and the center of the marker with the position of the source being always before that of the marker. This discrepancy was more pronounced closer to the ring end and the difference gradually decreased as the source got closer to the connector end (0.33mm). It is well known in the literature that these results are caused by friction between the source and the different segments in the ring tube. An average difference of 45% was observed between the values of the Oncentra source path library model in the Z coordinate and those using our in-house model. These results emphasize the importance of creating an in-house model with measurements of the source path not only in X and Y but also in the Z coordinate. These measurements should be done during commissioning of ring applicators. Finally, a 2 mm difference in distance was also found between the Oncentra and our in-house measured source paths. Conclusions: Our results demonstrate that a complete examination of the position of the source path in relation to the position of the markers and to the position of the source path in the Oncentra applicator library is necessary. We conclude that a blind use of either the markers or the Oncentra applicator library should be avoided. Further investigation should be done to measure the impact of these results in brachytherapy dose distribution.
PO30
Figures. (1a) and (1b) show dose distribution for setup (1) and (2) respectively. The profiles in Figure (1c) illustrate dosimetric equivalence of Freiburg and HAM applicator, while Figure (Id) draws attention to the attenuation by bone.
PO29 Importance on the Development of a Clinic-Specific Source Path Position Library while Commissioning Ring Applicators when Compared to the Position of Applicator Markers and to the Ring Source Path Position Library Model Currently in Use by Oncentra Treatment Planning Fabiola Vallejo, MSc, PhD, MCCPM, Bruno Carozza, MSc, Fadi Hobeila, MSc, MCCPM. Service de Physique Medicale, CICL, Centre Integre de Cancerologie de Laval, Laval, QC, Canada.
Backscatter and Transmission through a Lead Shield Used in Surface and Interstitial HDR Brachytherapy Cristian Candela-Juan, Msc1, Domingo Granero, PhD2, Javier Vijande, PhD3, Facundo Ballester, PhD3, Jose Perez-Calatayud, PhD4, Mark J. Rivard, PhD5. 1Radioprotection, La Fe University and Polytechnic Hospital, Valencia, Spain; 2Radiation Physics, ERESA, Hospital General Universitario, Valencia, Spain; 3Atomic, Molecular and Nuclear Physics, University of Valencia, Burjassot, Spain; 4Radiotherapy, La Fe University and Polytechnic Hospital, Valencia, Spain; 5Radiation Oncology, Tufts University School of Medicine, Boston, MA. Purpose: In surface and interstitial high-dose-rate (HDR) brachytherapy, some superficial radiosensitive organs such as the eye lens, thyroid, or gonads may be exposed to relatively high absorbed doses. In order to protect them, lead shields may be placed on the patient surface above the implant. Besides attenuation across the shield, there is a backscatter dose enhancement. The aim of this study is to evaluate the dosimetric perturbations (transmission and backscatter) of these shields in surface and interstitial HDR brachytherapy using 60Co, 192Ir, or 169Yb sources, and to provide the required bolus thicknesses to minimize surface overdoses. Materials and Methods: Monte Carlo simulations in Geant4 were performed for a source containing one of the three radionuclides placed at