ESTRO 38 Abstract book

S706 ESTRO 38

To evaluate tolerability of escalating doses of single fraction stereotactic partial breast irradiation (S-PBI) in treating early stage breast cancer after partial mastectomy. We conducted a phase 1 dose escalation trial testing 3 dose levels, 22.5Gy, 26.5Gy and 30 Gy. The primary objective was to escalate the S-PBI single fraction dose utilizing a robotic stereotactic radiation system treating the lumpectomy cavity without exceeding the maximum tolerated dose (MTD) in patients with early stage breast cancer. We report on the first dose cohort. Material and Methods Eligible patients included DCIS or invasive epithelial histologies, AJCC clinical stage 0, I, or II with tumor size <3cm, and negative margins. Prior to simulation 3-4 four gold fiducials were placed around the lumpectomy cavity for real-time respiratory tracking. Dose limiting toxicity (DLT) equaled grade ≥3 toxicity by CTCAE (version 4) deemed definitely related to treatment for: skin, rib 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 four point scale of – excellent, good, fair, poor. Adjuvant endocrine or chemotherapy was permitted. Starting dose was 22.5 Gy in 1 fraction, dose cohort two is 26.5 Gy and third cohort is 30 Gy. Dose was escalated provided 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 MTD would be exceeded. Results 11 patients enrolled on cohort one and had a median age of 66 years. All patients had ER/PR+ and Her2/neu- tumor profile. Median follow-up was 18 months. 80% of patients received endocrine therapy and 0% chemotherapy. MTD was not reached in cohort one and dose was escalated to cohort two. There were 19 grade 1 toxicity events, 2 grade 2 toxicities, and 1 grade ≥3 toxicity (which was breast pain that lasted for two days and resolved after antibiotics). There have been no recurrences or distant metastases. One patient developed fat necrosis and it was diagnosed 12 months after radiation treatment. Physicians scored cosmesis excellent or good in 100%, 100%, 100% and 100% (p=1.00) respectively, at baseline, 3, 6 and 12 months post SBRT, while patients scored the same periods 90.9%, 100%, 100% and 90.0% (p=1.00). Conclusion Dose was escalated to 22.5 Gy in 1 fraction without a DLT and the majority of patient had good or excellent cosmesis. We are currently completing dose escalation for subsequent cohorts of 26.5Gy and 30 Gy in 1 fraction of S-PBI hoping to increase the likelihood of long term tumor control while evaluating toxicity and cosmesis. EP-1290 Impact of patient and treatment factors on heart and lung dose in left-sided breast radiotherapy V.T. Nguyen 1 , T. Finazzi 1 , A. Papachristofilou 1 , F. Zimmermann 1 1 University Hospital Basel, Clinic of Radiotherapy and Radiation Oncology, Basel, Switzerland Purpose or Objective The heart and lung are routinely exposed to incidental irradiation during adjuvant radiotherapy (RT) of breast cancer. We analyzed the impact of patient characteristics and treatment factors on heart and lung radiation dose in patients receiving adjuvant radiotherapy for left-sided breast cancer. Material and Methods We identified patients who were treated with adjuvant RT for left-sided breast cancer in our institution between 2013 and 2018. The mean radiation doses to the heart (mean heart dose; MHD) and left lung (mean lung dose;

Purpose or Objective Radiotherapy (RT) to the breast has a functional impact on the upper extremity, especially when regional lymphatics are irradiated. This may be mediated in part through RT dose to the adjacent musculature. This study set out to compare the muscle doses deposited with protons vs. photons using 3D conformal RT. Material and Methods Five consecutive patients diagnosed with node positive left-sided breast cancer were included. For a prior dosimetric study, plans were created with pencil beam scanning proton RT at free breathing and 3D conformal RT at DIBH. The plans were optimized for cardiac and pulmonary dose. For this study, the muscle volumes for eleven shoulder muscles anatomically located within or near the treatment field were contoured separately. The volume of each muscle receiving at least 30 Gy (V30 Gy) were calculated for each muscle and compared between treatment modalities using the two-tailed paired Student T test. Results

Conclusion Pencil beam scanning proton RT delivers less dose to most of the adjacent musculature as compared to 3D conformal RT during irradiation of the breast and draining lymphatics. The largest absolute differences are in the posterior shoulder muscles in the exit of the nodal field. The smallest differences in dose are noted in the pectoralis muscles which receive high dose in both plans. The reduction in muscle dose associated with protons may have implications for upper extremity function, although additional research is required. EP-1289 Phase I dose escalation trial using single fraction Stereotactic PBI for early stage breast cancer A. Rahimi 1 , N. Kim 2 , A. Spangler 2 , M. Leitch 3 , P. Alluri 2 , X. Gu 2 , C. Ahn 4 , S. Goudreau 5 , S. Seiler 5 , D. Farr 3 , R. Wooldridge 3 , B. Haley 6 , S. Bahrami 2 , D. Chen 2 , R. Rao 7 , R. Timmerman 2 1 University Texas Southwestern Medical Center, Radiation Oncology, Dallas, USA ; 2 University of Texas Southwestern Medical Center, Radiation Oncology, Dallas, USA ; 3 University of Texas Southwestern Medical Center, Surgery, Dallas, USA ; 4 University of Texas Southwestern Medical Center, Biostatistics, Dallas, USA ; 5 University of Texas Southwestern Medical Center, Radiology, Dallas, USA ; 6 University of Texas Southwestern Medical Center, Medical Oncology, Dallas, USA ; 7 Columbia University Medical Center, Breast Surgery, New York City, USA

Purpose or Objective