Early Follow-Up of Multi-Institutional Trial of Phase I Dose Escalation Using Single Fraction Stereotactic Partial Breast Irradiation (S-PBI) for Early-Stage Breast Cancer.

Abstract

PURPOSE/OBJECTIVE(S)\nWe report on our early experience of a phase I dose escalation study of single fraction stereotactic partial breast irradiation (S-PBI) for early-stage breast cancer after partial mastectomy using a robotic stereotactic radiation system.\n\n\nMATERIALS/METHODS\nPatient eligibility included DCIS or invasive epithelial histologies, AJCC clinical stage 0, I, or II with tumor size < 3 cm, and negative margins. Prior to simulation, 3-4 gold fiducials were placed around the lumpectomy cavity to be used for real-time tracking during treatments. Dose limiting toxicity (DLT) was defined as grade ≥ 3 toxicity by CTCAE (version 4) deemed definitely related to treatment for: skin, ribs/bone (fracture), pulmonary (radiation pneumonitis), or neurological (intercostal or brachial plexus nerves) or any grade 4 or 5 toxicity definitely attributed to therapy. Both patients and physicians completed baseline and subsequent cosmesis outcome questionnaires using a 4-point scale ranging from excellent, good, fair, or poor. Cohort 1 was 22.5 Gy, cohort 2 26.5 Gy, and cohort 3 30 Gy all delivered in a single fraction. Dose escalation was permitted provided that one or fewer of the first seven, or two or fewer of nine patients experienced a DLT within 90 days of treatment within each dose group. If more patients had DLT in a given dose cohort, the maximum tolerable dose (MTD) would have been exceeded.\n\n\nRESULTS\nFrom 4/2016 to 1/2021, 11, 9, and 10 patients were treated on cohorts 1, 2 and 3 respectively. Of these, data from 18 evaluable patients from cohorts 1 and 2 and 6 patients from cohort 3, with at least 12 months of follow up are reported. Median f/u for cohort 1 was 36 m (r 24-48m), cohort 2 was 18 m (r 18-24m), and cohort 3 12 m (r 1-18m). Average age was 66.8 (r 47-84). Histology included 11 DCIS, and 19 invasive carcinomas. Of 11 DCIS, 9 were ER+, and 2 were ER-. All 19 invasive tumors were ER+. 23/29 patients received endocrine therapy, and no patient received chemotherapy. No patients experienced grade 3 or higher treatment related toxicity in the acute period (≤ 90 days), and MTD was not reached. There were two delayed grade 3 toxicities, (dose cohort 1 and 2, one patient had breast pain, and another had mastitis at 12 months and 9 months). No patients experienced grade 4 or 5 toxic events. Five patients developed fat necrosis in all three cohorts. Physicians scored cosmesis excellent or good at last follow-up in (10/11) 90%, (7/8) 87.5%, and (6/6)100%, in cohort 1,2, and 3 respectively, while patients scored excellent or good in (11/11) 100%, 8/8 (100%), and 5/6 (83%) patients. There has been no report of disease recurrences.\n\n\nCONCLUSION\nDose was escalated to 30 Gy in single fraction, and the majority of patients maintained good or excellent cosmetic outcome, without grade 3 or 4 toxicity. Continued analysis of all cohorts are in progress.