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Comparison between forward and inverse planning for breast IMRT
S360
I. J. Radiation Oncology
2026
● Biology ● Physics
Volume 57, Number 2, Supplement, 2003
Comparison Between Forward and Inverse Planning for Breast IMRT
A.M. Mihai,2,1 K.E. Sixel,2,1 M. Ruschin,1 M. Aznar,1 V. Benk,1,2 E. Rakovitch,2,1 J. Pignol1,2 Radiation Oncology, Toronto Sunnybrook Regional Cancer Centre, Toronto, ON, Canada, 2Radiation Oncology, University of Toronto, Toronto, ON, Canada
1
Purpose/Objective: Intensity-modulated radiation therapy has been shown to improve the dose distribution homogeneity within the breast, compared to conventional wedged irradiation technique. From literature review, 2 methods of dose optimisation have been reported. For both, a part of the dose is delivered using open tangential fields, while the remaining is delivered via small field in filed segments.The small MLC shaped segments realize a 3D compensation for missing tissue. With the forward planned aproach (FPT), segment weight is pre-selected at fixed values. The inverse optimisation algorithm (IPT) automatically determines the appropriate weights of segments to create more uniform dose distribution throught the target. Although both methods decrease the dose distribution inhomogenities,it is not known which one achieves the best dose distribution. Several parameters have been use to evaluate the dose distribution homogeneity: the peak of the differential dose-volume histogram (DVH), the mean and the maximum doses, the volume receiving 95–105% and the volumes receiving more than 105% (V105) and 110% (V110) of the prescribed dose. The aim of the study is to compare the homogeneity of breast IMRT dose distribution using forward versus inverse segment weight optimisation. Materials/Methods: A dosimetric study was performed on 24 patients referred to TSRCC for adjuvant breast irradiation. Following CT simulation, the skin was delineated on each axial CT slice and an open non-wedged tangential plan was generated. Using an in-house module, MLC segment shapes were automatically determined by tracing 5% isodose increments (from 80% to 120%), calculated in the beam sagittal plane from the open field distribution. For each patient, 2 plans were generated following MLC-shape definition. In the FPT scenario, each segment delivered 5% of the prescribed dose, with the remaining dose delivered by equally weighted open beams. The final segment weights were determined by manual iteration. For the IPT, segment weights were automatically optimized using in-house developed software, the endpoint constraint being the dose homogeneity among 50 points selected within the breast. In both cases, the dose distribution was calculated using the Theraplan Plus treatment planning system. Dose was prescribed at a point at mid-separation, 2/3 of the distance between the skin and the base of the tangential fields. The FPT and IPT were evaluated using the clinically significant maximum dose (as defined by ICRU50), the peak of the differential dose volume histogram (in Gy) and the relative irradiated volumes receiving more than 105% (V105) and 110% (V110), respectively, of the prescribed dose. The results are reported as mean values, standard deviation (SD) and ranges. Results: For the entire group, the mean breast separation was 23.51 cm (range 19 to 34 cm), with a mean breast elevation of 6.21 cm (range 3.4 to 12 cm). In table 1, IMRT constantly achieved excellent dose homogeneity throughout the target volume with both optimization schemes. While it is not statistically significant, the IPT results in less tissue receiving doses over 105% or 110%. Looking more carefully to plan, it appears that not only IPT reduces the hot spots on the infra-mammary fold, but also reduces dose distribution inhomogeneities generated on the axilla tail. V105 and V110 results are reported in relative volume and are small, though the absolute volume could be important in large breast patients. For example, in a large breast patient, a relative volume of 0.1% could represent an absolute volume of 32.78 cc. Conclusions: Both optimization approaches produce homogeneous dose distributions. However, the inverse planning IMRT improves the dose homogeneity within the entire 3D-breast volume, while the forward technique improves mainly the dose distribution on the infra-mammary fold.
I. J. Radiation Oncology
2026
● Biology ● Physics
Volume 57, Number 2, Supplement, 2003
Comparison Between Forward and Inverse Planning for Breast IMRT
A.M. Mihai,2,1 K.E. Sixel,2,1 M. Ruschin,1 M. Aznar,1 V. Benk,1,2 E. Rakovitch,2,1 J. Pignol1,2 Radiation Oncology, Toronto Sunnybrook Regional Cancer Centre, Toronto, ON, Canada, 2Radiation Oncology, University of Toronto, Toronto, ON, Canada
1
Purpose/Objective: Intensity-modulated radiation therapy has been shown to improve the dose distribution homogeneity within the breast, compared to conventional wedged irradiation technique. From literature review, 2 methods of dose optimisation have been reported. For both, a part of the dose is delivered using open tangential fields, while the remaining is delivered via small field in filed segments.The small MLC shaped segments realize a 3D compensation for missing tissue. With the forward planned aproach (FPT), segment weight is pre-selected at fixed values. The inverse optimisation algorithm (IPT) automatically determines the appropriate weights of segments to create more uniform dose distribution throught the target. Although both methods decrease the dose distribution inhomogenities,it is not known which one achieves the best dose distribution. Several parameters have been use to evaluate the dose distribution homogeneity: the peak of the differential dose-volume histogram (DVH), the mean and the maximum doses, the volume receiving 95–105% and the volumes receiving more than 105% (V105) and 110% (V110) of the prescribed dose. The aim of the study is to compare the homogeneity of breast IMRT dose distribution using forward versus inverse segment weight optimisation. Materials/Methods: A dosimetric study was performed on 24 patients referred to TSRCC for adjuvant breast irradiation. Following CT simulation, the skin was delineated on each axial CT slice and an open non-wedged tangential plan was generated. Using an in-house module, MLC segment shapes were automatically determined by tracing 5% isodose increments (from 80% to 120%), calculated in the beam sagittal plane from the open field distribution. For each patient, 2 plans were generated following MLC-shape definition. In the FPT scenario, each segment delivered 5% of the prescribed dose, with the remaining dose delivered by equally weighted open beams. The final segment weights were determined by manual iteration. For the IPT, segment weights were automatically optimized using in-house developed software, the endpoint constraint being the dose homogeneity among 50 points selected within the breast. In both cases, the dose distribution was calculated using the Theraplan Plus treatment planning system. Dose was prescribed at a point at mid-separation, 2/3 of the distance between the skin and the base of the tangential fields. The FPT and IPT were evaluated using the clinically significant maximum dose (as defined by ICRU50), the peak of the differential dose volume histogram (in Gy) and the relative irradiated volumes receiving more than 105% (V105) and 110% (V110), respectively, of the prescribed dose. The results are reported as mean values, standard deviation (SD) and ranges. Results: For the entire group, the mean breast separation was 23.51 cm (range 19 to 34 cm), with a mean breast elevation of 6.21 cm (range 3.4 to 12 cm). In table 1, IMRT constantly achieved excellent dose homogeneity throughout the target volume with both optimization schemes. While it is not statistically significant, the IPT results in less tissue receiving doses over 105% or 110%. Looking more carefully to plan, it appears that not only IPT reduces the hot spots on the infra-mammary fold, but also reduces dose distribution inhomogeneities generated on the axilla tail. V105 and V110 results are reported in relative volume and are small, though the absolute volume could be important in large breast patients. For example, in a large breast patient, a relative volume of 0.1% could represent an absolute volume of 32.78 cc. Conclusions: Both optimization approaches produce homogeneous dose distributions. However, the inverse planning IMRT improves the dose homogeneity within the entire 3D-breast volume, while the forward technique improves mainly the dose distribution on the infra-mammary fold.