- Email: [email protected]
OC-0264: Impact of inhomogeneity corrected dose calculation on dosimetric quality parameters in gynaecological cases
ESTRO 33, 2014
S101
institutions. In addition, vaginal reference length (VRL) was measured, defined by the distance from the centre of the vaginal sources and PIBS. Results: 153 patients from seven institutions were included. The method was applicable in all institutions. Large dose variations were seen between the patients, in particular at the top at the left and right vaginal wall (median dose 194Gy (61-947) / 178Gy (61-980), respectively). At 5mm depth, doses were respectively 98 Gy (55-212)/ 91 Gy (54-227) left/right, and 71Gy (51-145)/ 67 Gy (49-189) anterior/posterior. The median total EQD2 doses at PIBS and ± 2cm were respectively 41 Gy (3-81), 53 Gy (32-109) and 5 Gy (1-51). Median VRL was 5.4 cm (1.0-8.3 cm). The dose at PIBS+2cm (mid vagina) level was inverse correlated with the VRL and dose variation at this level was coming from BT contribution. The variation of vaginal dose at PIBS-2cm (introitus) level was mainly dependent on the location of the EBRT field border. Conclusions: This novel method allows comparison of dose to the vagina between centres, independently of used applicator and protocol. Due to steep dose gradients and limitations of the EQD2 model, the dose assessment at the vaginal surface may be prone to considerable uncertainties. The 5mm depth dose may give a better and more robust representation of the vaginal dose to the top than as the vaginal surface dose. The dose at the level of PIBS±2cm represents the dose throughout the vagina and is mainly dependent on VRL and the location of the EBRT field border. OC-0263 Brachytherapy vaginal dose de-escalation in locally advanced cervical cancer S. Mohamed1, J.C. Lindegaard1, A. De Leeuw2, I.M. Jürgenliemk-Schulz2, R. Pötter3, C. Kirisits3, K. Tanderup4 1 Aarhus University Hospital, Oncology Department, Aarhus C, Denmark 2 University Medical Center Utrecht, Radiation Oncology Department, Utrecht, The Netherlands 3 Medical University of Vienna, Radiotherapy Department, Vienna, Austria 4 Washington University, Radiation Oncology Department, Saint Louis, USA Purpose/Objective: Vaginal mucosa is exposed to radiotherapy induced damage to variable degrees. This planning study explores the possibilities of brachytherapy (BT) vaginal dose de-escalation. We propose a planning strategy which aims to decrease the vaginal loading utilizing vaginal dose reporting points. Materials and Methods: Fifty consecutive patients were included from 3 institutions which used different applicators and dose rates. Patients with lower or middle vaginal involvement at diagnosis were not included. Tandem-ring applicator was used in 30 pts, and tandem-ovoids in 20 pts. Interstitial needles were added in 13 patients. Four HDR fractions were delivered in 21 patients and 2 PDR fractions in 29 patients. EBRT doses were 45–50Gy/25–30fx. 24% were staged IB, 4% IIA, 56% IIB, 14% IIIB and 2% IVB. The range of clinically measured tumour width at the first BT fraction (BT1) was 10–70mm with a median of 35mm at BT1 and median of 50mm at diagnosis. Lateral vaginal dose reporting points were inserted at the vault level at the mucosal surface (0mm) and at 5mm depth, and anterior and posterior vaginal points were at 5mm depth. We created a vaginal dose de-escalation (VDD) plan by decreasing dwell times in ovoid/ring and increasing in tandem/needles. VDD plan was to deliver similar high risk clinical target volume (HR CTV) D90 as the optimised delivered plan and the 2nd priority was to keep the dose to the vaginal mucosal points lower than of 140% of the physical fractional dose corresponding to 85Gy EQD2. We compared the DVH parameters of the delivered optimised BT plan with the VDD plan. Doses are reported in total Gy EQD2 using the LQ model (α/β=3Gy for OARs, α/β=10Gy for tumour, T½=1.5h). Results: Table 1 shows a summary for doses in total Gy EQD2. HR CTV D90 and D98 in VDD plans were not statistically different from the optimised delivered plans, while doses to vagina, bladder, and rectum were significantly reduced. Doses from the delivered plans to the vaginal vault mucosa varied widely and ranged from 102–782Gy and at 5mm depth the range was 67–459Gy. The doses to the vaginal points and the vaginal TRAK were significantly lower with the VDD plan. The average ratio between vaginal and total TRAK was 0.51 for delivered plans and 0.33 for VDD plans. The average of the vaginal mucosal doses achieved by the VDD plan was 145 ±34 % of the physical fractional dose corresponding to 85Gy EQD2.
Conclusions: It is possible to manipulate the source loading pattern to de-escalate doses to the vagina and still obtain comparable doses to the target volumes. The VDD strategy also decreased bladder and rectum dose. Reporting the doses to the vagina using the vaginal points is feasible and encouraged. OC-0264 Impact of inhomogeneity corrected dose calculation on dosimetric quality parameters in gynaecological cases J. Hofbauer1, N. Nesvacil2, A. Sturdza2, R. Pötter2, C. Kirisits3 1 Krankenhaus Hietzing, Department of Medical Physics, Vienna, Austria 2 Medical University of Vienna/AKH Vienna, Department of Radiation Oncology Comprehensive Cancer Center, Vienna, Austria 3 Medical University of Vienna/AKH Vienna, Department of Radiation Oncology & Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria Purpose/Objective: The aim of this study was to show the variation of dosimetric quality parameters in 3D image guided Brachytherapy (BT) planning by using the Acuros BV™ Dose Calculation Tool (Varian Medical Systems) instead of common TG43. Acuros is based on the Boltzmann Transfer Equation (BTE) which is taking into account inhomogeneities such as anatomical structures and applicator material since it uses electron density information from CT. Materials and Methods: We analysed dose volume histogram (DVH) parameters from CT-based treatment plans of 9 randomly selected cervix cancer patients treated with HDR-BT by plastic tandem-ring applicators (Nucletron), 5 of which with interstitial technique by additional needles implanted. The treatment planning system Brachy Vision v10.0 (Varian Medical Systems) was used to create plans based both on standard TG43 and Acuros v1.4.0 algorithms. HR-CTV as well as organs at risk (OAR) such as bladder, rectum and sigmoid were contoured as regions of interest. The corresponding DVH parameters were D50, D90, D98 and V100 for the HR-CTV and D0.1cm³ and D2cm³ for the OAR respectively. In addition a phantom study was done to show the impact of applicator material.Tandem-ring applicators made of titanium (Varian Medical Systems) as well as of the clinically used plastic were CT-scanned in a water phantom. Both plain and with titanium needles inserted plastic applicator were used. A region near the applicator surface was created as a surrogate for an OAR as well as a region around the applicator itself as surrogate for a target volume. Corresponding dosimetric quality parameters were investigated. Results: Inhomogeneity corrected dose calculation did not show significant impact on dosimetric quality parameters for the target volume in evaluated gynaecological cases, as we found differences (mean±1SD) in the HR-CTV of -0.10%±0.47% for D50, -0.47%±0.33% for D90, -0.35%±0.59% for D98 and -0.17%±0.16% for V100 respectively, negative values indicating Acuros showing less dosage than TG43. For the OAR we found differences of -0.63%±0.27% for D0.1cm³, and 0.87%±0.25%for D2cm³ of the bladder, -1.42%±0.72% for D0.1cm³, and 2.14%±0.63%for D2cm³ of the rectum and -1.43%±0.65% for D0.1cm³, and 1.69%±0.81 for D2cm³ of the sigmoid. Regarding the phantom study the difference in DVH-parameters for the titanium applicator showed an overestimation of dosage by TG43 of about 1% for the target surrogate and about 2% for the OAR surrogate. For the plastic applicator regardless with or without needles the differences were found negligible below 1%. Conclusions: The comparison in dose calculation did show a difference in DVH parameters for gynaecological plans between the calculation algorithms of TG43 and BTE-based Acuros for the OAR up to 2%. Differences in the coverage of the target volumes were found insignificant. The phantom study obviously showed a shielding effect of applicator material of higher density such as the titanium applicator but could be neglected for the plastic.
S101
institutions. In addition, vaginal reference length (VRL) was measured, defined by the distance from the centre of the vaginal sources and PIBS. Results: 153 patients from seven institutions were included. The method was applicable in all institutions. Large dose variations were seen between the patients, in particular at the top at the left and right vaginal wall (median dose 194Gy (61-947) / 178Gy (61-980), respectively). At 5mm depth, doses were respectively 98 Gy (55-212)/ 91 Gy (54-227) left/right, and 71Gy (51-145)/ 67 Gy (49-189) anterior/posterior. The median total EQD2 doses at PIBS and ± 2cm were respectively 41 Gy (3-81), 53 Gy (32-109) and 5 Gy (1-51). Median VRL was 5.4 cm (1.0-8.3 cm). The dose at PIBS+2cm (mid vagina) level was inverse correlated with the VRL and dose variation at this level was coming from BT contribution. The variation of vaginal dose at PIBS-2cm (introitus) level was mainly dependent on the location of the EBRT field border. Conclusions: This novel method allows comparison of dose to the vagina between centres, independently of used applicator and protocol. Due to steep dose gradients and limitations of the EQD2 model, the dose assessment at the vaginal surface may be prone to considerable uncertainties. The 5mm depth dose may give a better and more robust representation of the vaginal dose to the top than as the vaginal surface dose. The dose at the level of PIBS±2cm represents the dose throughout the vagina and is mainly dependent on VRL and the location of the EBRT field border. OC-0263 Brachytherapy vaginal dose de-escalation in locally advanced cervical cancer S. Mohamed1, J.C. Lindegaard1, A. De Leeuw2, I.M. Jürgenliemk-Schulz2, R. Pötter3, C. Kirisits3, K. Tanderup4 1 Aarhus University Hospital, Oncology Department, Aarhus C, Denmark 2 University Medical Center Utrecht, Radiation Oncology Department, Utrecht, The Netherlands 3 Medical University of Vienna, Radiotherapy Department, Vienna, Austria 4 Washington University, Radiation Oncology Department, Saint Louis, USA Purpose/Objective: Vaginal mucosa is exposed to radiotherapy induced damage to variable degrees. This planning study explores the possibilities of brachytherapy (BT) vaginal dose de-escalation. We propose a planning strategy which aims to decrease the vaginal loading utilizing vaginal dose reporting points. Materials and Methods: Fifty consecutive patients were included from 3 institutions which used different applicators and dose rates. Patients with lower or middle vaginal involvement at diagnosis were not included. Tandem-ring applicator was used in 30 pts, and tandem-ovoids in 20 pts. Interstitial needles were added in 13 patients. Four HDR fractions were delivered in 21 patients and 2 PDR fractions in 29 patients. EBRT doses were 45–50Gy/25–30fx. 24% were staged IB, 4% IIA, 56% IIB, 14% IIIB and 2% IVB. The range of clinically measured tumour width at the first BT fraction (BT1) was 10–70mm with a median of 35mm at BT1 and median of 50mm at diagnosis. Lateral vaginal dose reporting points were inserted at the vault level at the mucosal surface (0mm) and at 5mm depth, and anterior and posterior vaginal points were at 5mm depth. We created a vaginal dose de-escalation (VDD) plan by decreasing dwell times in ovoid/ring and increasing in tandem/needles. VDD plan was to deliver similar high risk clinical target volume (HR CTV) D90 as the optimised delivered plan and the 2nd priority was to keep the dose to the vaginal mucosal points lower than of 140% of the physical fractional dose corresponding to 85Gy EQD2. We compared the DVH parameters of the delivered optimised BT plan with the VDD plan. Doses are reported in total Gy EQD2 using the LQ model (α/β=3Gy for OARs, α/β=10Gy for tumour, T½=1.5h). Results: Table 1 shows a summary for doses in total Gy EQD2. HR CTV D90 and D98 in VDD plans were not statistically different from the optimised delivered plans, while doses to vagina, bladder, and rectum were significantly reduced. Doses from the delivered plans to the vaginal vault mucosa varied widely and ranged from 102–782Gy and at 5mm depth the range was 67–459Gy. The doses to the vaginal points and the vaginal TRAK were significantly lower with the VDD plan. The average ratio between vaginal and total TRAK was 0.51 for delivered plans and 0.33 for VDD plans. The average of the vaginal mucosal doses achieved by the VDD plan was 145 ±34 % of the physical fractional dose corresponding to 85Gy EQD2.
Conclusions: It is possible to manipulate the source loading pattern to de-escalate doses to the vagina and still obtain comparable doses to the target volumes. The VDD strategy also decreased bladder and rectum dose. Reporting the doses to the vagina using the vaginal points is feasible and encouraged. OC-0264 Impact of inhomogeneity corrected dose calculation on dosimetric quality parameters in gynaecological cases J. Hofbauer1, N. Nesvacil2, A. Sturdza2, R. Pötter2, C. Kirisits3 1 Krankenhaus Hietzing, Department of Medical Physics, Vienna, Austria 2 Medical University of Vienna/AKH Vienna, Department of Radiation Oncology Comprehensive Cancer Center, Vienna, Austria 3 Medical University of Vienna/AKH Vienna, Department of Radiation Oncology & Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria Purpose/Objective: The aim of this study was to show the variation of dosimetric quality parameters in 3D image guided Brachytherapy (BT) planning by using the Acuros BV™ Dose Calculation Tool (Varian Medical Systems) instead of common TG43. Acuros is based on the Boltzmann Transfer Equation (BTE) which is taking into account inhomogeneities such as anatomical structures and applicator material since it uses electron density information from CT. Materials and Methods: We analysed dose volume histogram (DVH) parameters from CT-based treatment plans of 9 randomly selected cervix cancer patients treated with HDR-BT by plastic tandem-ring applicators (Nucletron), 5 of which with interstitial technique by additional needles implanted. The treatment planning system Brachy Vision v10.0 (Varian Medical Systems) was used to create plans based both on standard TG43 and Acuros v1.4.0 algorithms. HR-CTV as well as organs at risk (OAR) such as bladder, rectum and sigmoid were contoured as regions of interest. The corresponding DVH parameters were D50, D90, D98 and V100 for the HR-CTV and D0.1cm³ and D2cm³ for the OAR respectively. In addition a phantom study was done to show the impact of applicator material.Tandem-ring applicators made of titanium (Varian Medical Systems) as well as of the clinically used plastic were CT-scanned in a water phantom. Both plain and with titanium needles inserted plastic applicator were used. A region near the applicator surface was created as a surrogate for an OAR as well as a region around the applicator itself as surrogate for a target volume. Corresponding dosimetric quality parameters were investigated. Results: Inhomogeneity corrected dose calculation did not show significant impact on dosimetric quality parameters for the target volume in evaluated gynaecological cases, as we found differences (mean±1SD) in the HR-CTV of -0.10%±0.47% for D50, -0.47%±0.33% for D90, -0.35%±0.59% for D98 and -0.17%±0.16% for V100 respectively, negative values indicating Acuros showing less dosage than TG43. For the OAR we found differences of -0.63%±0.27% for D0.1cm³, and 0.87%±0.25%for D2cm³ of the bladder, -1.42%±0.72% for D0.1cm³, and 2.14%±0.63%for D2cm³ of the rectum and -1.43%±0.65% for D0.1cm³, and 1.69%±0.81 for D2cm³ of the sigmoid. Regarding the phantom study the difference in DVH-parameters for the titanium applicator showed an overestimation of dosage by TG43 of about 1% for the target surrogate and about 2% for the OAR surrogate. For the plastic applicator regardless with or without needles the differences were found negligible below 1%. Conclusions: The comparison in dose calculation did show a difference in DVH parameters for gynaecological plans between the calculation algorithms of TG43 and BTE-based Acuros for the OAR up to 2%. Differences in the coverage of the target volumes were found insignificant. The phantom study obviously showed a shielding effect of applicator material of higher density such as the titanium applicator but could be neglected for the plastic.