Purpose/Objective(s) To assess the anatomic conformity of custom thermoplastic boluses generated based on patient-specific 3D-printed molds. Materials/Methods A retrospective review of consecutive cases of 3D printer-assisted custom thermoplastic bolus was performed. For each case, target volume-specific contours (molds) were generated using computed tomography-based (CT) simulation scans in a treatment planning system. Molds were exported into a standard tessellation language (STL) file for rapid manufacturing on a 3D Printer using polylactic acid (PLA) filament. STL files were converted into G-code script and printed on slicer settings of 220°C nozzle temperature, 0.3 mm layer height, and 20% infill. The thickness of the thermoplastic bolus utilized was based on the treating radiation oncologist's prescription and ranged between 5.0 and 10.0 mm. Once heated, thermoplastic boluses were shaped using the 3D printed target volume-specific PLA molds. The quality of 3D printer-assisted thermoplastic boluses was confirmed on CT scanners before clinical use. Anatomic conformity of the boluses was quantified as mean/standard deviation (SD) of bolus-to-body air gap measured at five points equidistant from the center of the target volume. These isocentric points were assessed on first-fraction cone-beam CT scans. Results From September 2018 to January 2022, 13 3D printer-assisted thermoplastic boluses were generated for treatment to the scalp (4), nose (6), anus (2), and vulva (1) with a median dose of 50.0 Gy (range 24-70) and median of 20.0 fx prescribed (range 5-35). All treatments were 6MV/MeV with a median bolus thickness of 5.0 mm (range 5.0-10.0). The mean air gap visualized was 2.0 mm (range 0.0-6.8) ± 0.16 SD. Based on Malone Bolus Fit Classification System (excellent: no gap; good: <5 mm; tolerable: 5-10mm; poor: >10 mm), 12 cases were considered "good fit" with one case categorized as "excellent fit" with no notable air gaps. Conclusion This novel approach in generating 3D printer-assisted thermoplastic boluses allows for rapid offline manufacturing of patient-specific boluses without sacrificing quality in anatomic conformity.

Abstract Purpose: Breast reirradiation (reRT) after breast conserving surgery (BCS) has emerged as a viable alternative to mastectomy for women presenting with recurrent or new primary breast cancer. There are limited data on safety of different fractionation regimens. This multi-institutional study reports safety and efficacy among women treated with repeat BCS and reRT. Methods and Materials: Patients who underwent repeat BCS followed by RT from 2015–2021 at 2 institutions were analyzed. Univariate logistic regression models were used to identify predictors of acute and late toxicities. Kaplan Meier estimates were used to evaluate overall survival (OS), distant metastasis free survival (DMFS) and locoregional recurrence-free survival (LR-RFS). Results: Sixty-six patients were reviewed with median follow-up of 16 months (range: 3–60 months). At time of first recurrence, 41% had invasive carcinoma with a ductal carcinoma in situ (DCIS) component, 41% had invasive carcinoma alone and 18% had DCIS alone. All were clinically node negative. For the reirradiation course, 95% received partial breast irradiation (PBI) (57.5% with 1.5Gy BID; 27% with 1.8Gy daily; 10.5% with hypofractionation), and 5% received whole breast irradiation (1.8-2Gy/fx), all of whom had received PBI for initial course. One patient experienced grade 3 fibrosis, and one patient experienced grade 3 telangiectasia. None had grade 4 or higher late adverse events. We found no association between the fractionation of the second course of RT or the cumulative dose (measured as EQD2) with acute or late toxicity. At 2 years, OS was 100%, DMFS was 91.6%, and LR-RFS was 100%. Conclusion: In this multi-institutional series of patients with recurrent or new primary breast cancer, a second breast conservation surgery followed by reirradiation was effective with no local recurrences and an acceptable toxicity profile across a range of available fractionation regimens.

In this population of pts with unresected nodal disease, boost RT to radiographically positive LN regions can be safely delivered with low rates of grade 3+ toxicity. The majority of failures were distant with no isolated LRFs. Failures were highest in the IMN/axSCV group (∼40%). Further treatment escalation is necessary for these pts.

In this hypothesis-generating retrospective cohort study, patients with oligoprogressive malignancy treated with SBRT have similar freedom from new systemic therapy to patients with oligometastatic malignancy, strengthening the rationale for treating oligoprogressive malignancy with SBRT.