SU‐D‐108‐06: Dosimetry of 108 Strut‐Based Accelerated Partial Breast Irradiation (APBI) Treatments With Applicator Distance Less Than 7 Mm From Both the Skin and Ribs

Purpose: To report on the dosimetry of 108 strut‐based APBI treatments where the applicators were less than 7 mm from both the skin and the ribs Methods: Data from 108 patients whose applicators were sandwiched between the skin and ribs were analyzed. The patients were divided into three cohorts: those where the applicator was less than 7 mm from the skin and ribs (n=67), those where the distance was less than 5 mm (n=33), and those with less than 3 mm between both the skin and ribs (n=8). Dosimetric parameters analyzed include PTVEVAL volume (cm3), V90 (%), V95 (%), V100 (%), V150 (cm3), V200 (cm3), and the maximum doses to the skin and ribs. Results: Over all three cohorts the PTV_EVAL volumes ranged from 20.5 cm3 to 125.7 cm3 with a median value of 55.5 cm3. In each of the three groups, the V90 averaged between 94% and 95%, the V95 averaged between 90% and 92%, and the V100 averaged between 85% and 87%. The average V150 ranged between 21.8 cm3 and 26.6 cm3, and the average V200 ranged from 10.9 cm3 to 13.1 cm3. Across the three cohorts, the maximum skin dose ranged from 8.4% of the prescription dose to 117% of the prescription dose (median values were between 85% and 87%) and the maximum rib dose fell between 1.8% of the prescription dose and 198% of the prescription dose with the median values falling between 100% and 107%. Conclusion: This study demonstrates the dosimetric versatility of the strut‐based brachytherapy device. It has been used in patients where the proximity of the skin and chest wall precludes the use of other types of single‐entry brachytherapy devices. It allows for optimized dose coverage of the target while providing dose sparing to the adjacent critical structures. This project was partially funded by Cianna Medical, Inc.