ESTRO 2024 - Abstract Book

S706

Clinical - Breast

ESTRO 2024

Public Health, Dallas, USA. 4 University of Texas Southwestern Medical Center, Radiology, Dallas, USA. 5 Vanderbilt, Radiation Oncology, Nashville, USA. 6 University of Texas Southwestern Medical Center, Medical Oncology, Dallas, USA. 7 University of California San Diego, Radiation Oncology, San Diego, USA. 8 Central Arkansas Radiation Thereapy Institute, Radiation Oncology, Little Rock, USA. 9 University of Texas Southwestern Medical Center, Pathology, Dallas, USA

Purpose/Objective:

We report an interim analysis of an expanded cohort phase I dose escalation study of single fraction ablative (30, 34 or 38Gy) pre-operative stereotactic ablative partial breast irradiation (S-PBI) (NCT04040569) primarily designed to assess the impact of pre-operative dose on toxicity and tumor response for early stage breast cancer.

Material/Methods:

Patient eligibility included invasive epithelial histologies, AJCC clinical stage T1/T2N0 with tumor size < 3 cm, estrogen+/progesterone+ receptor/Her2- that do not require chemotherapy. Fiducials are placed prior to radiation. Patients start endocrine therapy two weeks after completion of radiation and surgery. Surgery is done 2.8-12 months after radiation at discretion of investigators. Patients are treated on either a robotic radiosurgery unit, MR LINAC, or Cobalt stereotactic breast unit. The primary objective is to escalate single fraction ablative S-PBI dose to an ablative dose without exceeding MTD. Secondary objectives include PCR rates, local control, toxicity, cosmesis, and distant disease free survival. In addition to treatment dose, we analyze interval from radiation to time to surgery, and impact on PCR rates using Miller Payne Score (MPS). From 12/2019 to 6/2023 11 and 15 patients were accrued and treated to the 30 Gy and 34Gy dose levels, respectively. At dose level 30Gy, 8 of the 11 patients have undergone surgery with an interval from radiation to surgery of median 4.3 months (range 2.8-5.9 months). In contrast, for the 34 Gy cohort a total of 15/15 patients have had surgery with a median time to surgery of 7.4 months (range 5.9-12 months). At dose level 1, 0/8 patients had a pathologic complete response (PCR) while 3/8 (37.5%) patients had a near complete response defined as RCB 1 and Miller-Payne 4/5, with two patients having only 1 mm of residual disease. At dose level 2 (34Gy), 7/15 patients had a PCR (46.6 %), while 14/15 (93.3%) had either a PCR or near PCR (nPCR). Of the 7 patients that had a nPCR, 50% had residual disease foci of only 1-3mm. ROC curve analysis was conducted to find the value for predicting optimal time to surgery after radiation versus pathologic response rates using MPS. Day 155 (5.2 months) was found to be the optimal value for predicting MPS 4/5 nPCR/PCR, (95% CI:0.556 to 1.000), sensitivity 0.941 and specificity of 0.833. Day 277 (9.23 months) is an optimal cut-off for predicting MPS of 5 (PCR), versus MPS (3/4), (95% CI:0.608-0.998), sensitivity=0.571 and specificity=0.938. Using an ordered logistic regression model, for every day surgery is delayed after radiation, there is an increase in probability of 2% higher MPS 4 or 5 (p=0.005). Results:

Conclusion:

Although the interval between completion of pre-operative radiation and surgery was not a controlled variable in this study, the interim analysis of our trial implies tumor response/PCR rates may be strongly impacted by this