Single-fraction stereotactic radiotherapy for Partial Breast Irradiation using the Gammapod system
PD-0234
Abstract
Single-fraction stereotactic radiotherapy for Partial Breast Irradiation using the Gammapod system
Authors: Marco Trovo1, Chiara Reverberi1, Agnese Prisco1, Luca Seriau2, Serena Bertozzi2, Marta Bonotto3, Mauro Mansutti3, Maria Orsaria4, Enrico Pegolo4, Marika Guarnieri5, Daniela Marfisi5, Vito Gagliardi5, Eugenia Moretti5, Paolo Scalchi5, Chiara Zuiani6, Carla Cedolini2
1Azienda Sanitaria Universitaria Udine, Radiation Oncology, Udine, Italy; 2Azienda Sanitaria Universitaria Udine, Breast Surgery, Udine, Italy; 3Azienda Sanitaria Universitaria Udine, Medical Oncology, Udine, Italy; 4Azienda Sanitaria Universitaria Udine, Pathology, Udine, Italy; 5Azienda Sanitaria Universitaria Udine, Medical Physics, Udine, Italy; 6Azienda Sanitaria Universitaria Udine, Radiology, Udine, Italy
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Purpose or Objective
We are conducting a phase II clinical trial to investigate the safety of single fraction Partial Breast Irradiation (PBI) post breast conserving surgery using the GammaPod system, a novel technology designed to deliver a highly focused dose to the breast under stereotactic localization. The preliminary results of the first 40 patients, corresponding to the completion of the first stage of the trial, are reported.
Material and Methods
The study was designed using the 2-stage Simon method. In the first stage, 40 patients are enrolled. If there are more than 6 Grade 2 or higher toxicity events during the first stage of the study, the study will be closed due to excess toxicity; otherwise, accrual is to continue until 148 patients are enrolled. Eligibilities include ≥50 years of age with early stage (pT1-T2 pN0) invasive ductal carcinoma, and ≥70 years of age with lobular carcinoma or pN1a invasive ductal carcinoma. From February to September 2022, 40 patients were enrolled. CTV was based on NSABP B-39/RTOG 0413 guidelines; PTV was obtained by a 3 mm isotropic expansion of the CTV to sufficiently account for the stereotactic geometric uncertainty of the device. A single dose of 17.5 Gy was delivered to the 95% of the PTV using the GammaPod system, with imaging, planning and dose delivery mostly completed within a 1 hour session. Toxicity was graded according to the Common Toxicity Criteria (v4.0), including pain, erythema, hyperpigmentation and fibrosis. Toxicity was evaluated by a radiation oncologist at 1, 3, and 6 months.
Results
With a median follow-up of 5 months (range, 1-8 months), no case of Grade 2 or higher acute toxicity was found, only three patients (7.5%) showed Grade 1 skin toxicity. The median age was 70 years (range, 50-88 years). The tumor phenotypes were as follows: 37 (93%) ER/PgR positive-Her2 negative, and 3 (7%) ER/PgR positive-Her2 positive. Seven (18%) patients had a Ki-67>20%. Five (13%) were T1a, 16 (40%) were T1b, and 19 (47%) T1c tumors. Thirty-seven (93%) were N0. Thirty-four (85%) patients received hormonal therapy, 1 (2%) received chemotherapy, trastuzumab and hormonal therapy, and 5 (13%) received no adjuvant systemic therapy. The mean PTV was 50 cc (range, 13-107 cc). The median mean radiation dose delivered to the PTV was 18.5 Gy. The median D2 (maximum dose) to the skin was 12 Gy.
Conclusion
The highly focused dose distribution with sharp dose fall-off combined with a small PTV margin due to stereotactic localization allowed a single dose of 17.5 Gy to be delivered safely with a very low acute toxicity profile. To confirm these preliminary results, we will continue accrual up to 148 patients and following-up to assess long-term toxicity and local-regional control.