Breast boost delineation in breast radiotherapy: surgical clips versus novel technique using post-operative surgical margins versus pre-operative positron emission tomography (PET) scan

Summary: To compare mismatch in tumor volumes obtained from using surgical clips versus post-operative surgical margins defined from modern positron emission tomography (PET) imaging and potential clinical impact Sixteen Patients treated at our institution for whole breast irradiation (WBI) followed by a boost from April 2018 to December 2022 with pre-operative whole body PET-CT and histopathology report were included in the study. The routine practice in our institution is to generate PTVboost by giving isotropic margins of 5mm to surgical clips. In the novel technique, pre-operative PET-CT was fused with radiotherapy planning CT scan using the deformable registration. The tumor bed will be delineated using deformable registration and anisotropic margins were generated using the post-operative surgical margins obtained from postoperative histopathology report. Tumor Volume and Overlap Analysis The PTV volumes generated using the two different techniques were compared. Three PTV volumes were defined: - PTV A: Volume defined by PET and post-operative surgical margins. - PTV B: Volume defined by surgical clips and isotropic margins. -PTV C: Volume defined by PET imaging and uniform margins (isotropic 10mm margins). The volumes were compared based on their overlap in cubic centimeters (cc), and statistical significance evaluated. The extent of overlap between PTV A and PTV B, PTV A and PTV C and PTV B and PTV C, was used to assess the consistency. Boost Dose Distribution The majority of patients received a boost dose of 12.5 Gy/5 fractions (56.2%), followed by 10 Gy/5 fractions (37.5%), and a single patient received 12 Gy/6 fractions (6.3%). PTV Volume Comparisons The PTV volumes generated by the novel PET-based approach with post-operative surgical margins (PTV A) were generally larger than those generated by the traditional clip-based method (PTV B), but smaller than those generated by applying uniform margins around the PET data (PTV C). (Table 1,2) PTV Overlap The overlap between PTV A (PET + surgical margins) and PTV B (surgical clips) was 54.5 cc (95% CI: 28–81), with a p-value of 0.569, indicating no statistically significant difference between the two methods. The overlap between PTV A and PTV C (PET + uniform margins) was 44.6 cc (95% CI: 28.6–61), with a p-value of 0.001, indicating a statistically significant difference between these two techniques. The overlap between PTV B (surgical clips) and PTV C (PET + uniform margins) was PTV C (PET + uniform margins) , with a p-value of 0.001, indicating statistically significant difference between the two methods. The pre-op treatment volumes were much lower than post-op volumes. Large randomized trials are needed to validate our results. Vikas Kumar Pandey (he/him/his), MD (Presenting Author) - Manipal Hospital, Delhi; Anusheel Munshi (he/him/his), MD Radiation Oncology (Co-Author) - IOCI, Fortis Noida