Image-Guided Adaptive Radiotherapy For Left Breast Irradiation After Breast-Conserving Therapy Based On Deformable Registration

Objective: Through comparing the dosimetry differences of the image-guided adaptive radiotherapy plan based on deformable registration and conventional plan of breast neoplasms patients undergoing whole breast and tumor bed simultaneous integrated boost intensitymodulated radiotherapy(SIB-IMRT) after segmental mastectomy, to evaluate the merits of the adaptive radiotherapy.Methods: Thirteen patients with breast neoplasms in stage 0-II after segmental mastectomy treated with whole breast and tumor bed SIB-IMRT were included in this study.To scan each patient with kilovolt cone-beam CT(CBCT) every four fractions of radiotherapy. Each patient need to be scanned seven times in the whole course of treatment. We acquired two sets of CBCT images in each time of scan. Then stitched the two sets together to include the whole breast and lung. Using the deformable image registration software to register the stitched CBCT sets and planning CT images with contours. Then the fusion images with new contours including the anatomical information of tissues and organs at present were propagated. All the new contours were transferred to the planning CT images in Varian Eclipse Treatment Planning System. Then the adaptive plans were designed with these new contours. All the adaptive plans of each patient were added to produce the DA plan. Then the dosimetry differences between the DA plans and the conventional CT plans were evaluated.Results: The maximum dose of the whole breast PTV and tumor bed PTV in DA plans were less than the CT plans. Whereas the minimum dose of the whole breast PTV and tumor bed PTV in DA plans were greater than in CT plans. All the differences were statistically significant(P<0.05). The differences of homogeneity index of the whole breast PTV and tumor bed PTV between DA plans and CT plans were statistically significant(P<0.05). The homogeneity index in DA plans were better than the CT plans. The mean dose of the left lung, total lung, heart and the contralateral breast, the volume of radiation 5, 10, 20, 30, 40 and 50Gy of the left lung and heart, the volume of radiation 5, 10, 20, 30 and 40 Gy of the left lung, and the volume of radiation 5 and 10 Gy of the contralateral breast in the DA plans were less than in CT plans respectively. All the differences were statistically significant(P<0.05). The mean dose, conformity index and D95% of the whole breast PTV and tumor bed PTV and the volume of radiation 50 Gy of the total lung in DA plans were as well as in the CT plans. There were no statistically differences(P<0.05).Conclusion: The image-guided adaptive radiotherapy plan based on deformable registration of breast neoplasms patients undergoing whole breast and tumor bed SIB-IMRT after segmental mastectomy has better PTV dosimetry uniformity and lower OARs irradiation than the conventional CT plan. The dose of organs at risk in the adaptive plan is less than the CT plan. With the use of image-guided adaptive radiotherapy, radiation will be delivered in precision with further healthy tissue sparing.