ESTRO 36 Abstract Book

S164 ESTRO 36 _______________________________________________________________________________________________

Acknowledgement: BVO was supported by The Danish Cancer Society and CIRRO OC-0316 Single dose external beam preoperative radiotherapy in breast cancer: experience and guidelines K.R. Charaghvandi 1 , S. Yoo 2 , B. Van Asselen 1 , M.D. Den Hartogh 1 , H.J.G.D. Van den Bongard 1 , J.K. Horton 2 1 University Medical Center Utrrecht, Radiation Oncology Department, Utrecht, The Netherlands 2 Duke Cancer Center, Department of Radiation Oncology, Durham, USA Purpose or Objective In patients treated with breast-conserving surgery, suboptimal cosmetic results have been reported following post-operative 3D-conformal accelerated partial breast irradiation (APBI) [1]. The delivery of radiation (RT) preoperatively to the intact tumor enables better target visualization and reduction in treatment volumes, and thus, the potential for less normal tissue toxicity [2]. We compiled our clinical experience and dosimetric data to develop practical guidelines for this new treatment approach. Material and Methods Dosimetry and toxicity data were pooled from 2 preoperative single dose APBI cohorts and 1 planning-study [table 1]. In an American dose escalation trial, patients ≥55 years, with non-lobular cT1N0Mx or DCIS received a single dose of 15, 18 or 21Gy (1.5cm CTV margin) using IMRT in the prone position (n=32) followed by lumpectomy within 10 days [fig 1A]. In the Netherlands, the feasibility of VMAT-based supine APBI (20 Gy to tumor, 15Gy to 20mm clinical target volume(CTV)) was evaluated initially in a planning study (n=20) and is currently ongoing in women ≥50 years with cT1-2(max. 3 cm)N0Mx non-lobular carcinoma (n=10) [fig. 1]. In this trial, lumpectomy is planned 6 months after RT. Acute toxicity was scored according to CTCAE version 4.03. Single dose APBI constraints were derived from pooled OAR estimates. The limit for optimal OAR dosimetry was set at the 75 th percentile, whereas the 90 th percentile represented non- optimal but acceptable dosimetry. Replanning of the US cohort was performed to check the adherence of these constraints for uniform single dose 21Gy APBI with a 20mm CTV margin (n=32). Results In the dose escalation cohort (n=32), grade 1 local radiation dermatitis, fibrosis and fatigue developed in 39%, 23% and 6% of the cases within 90 days after RT. At a median follow-up of 57 days in the 15/20Gy simultaneous boost cohort (n=10) grade 1 fatigue and local fibrosis was encountered in 90% and 80% of patients, without any radiation dermatitis. In both cohorts no acute ≥grade 3 toxicity developed. Based on pooled OAR estimates, optimal dosimetry was defined as a maximum value of 0.7Gy for the mean heart dose, 1.5Gy Dmax for contralateral breast, 1.3Gy for mean ipsilateral lung dose, and 12.3Gy as D20cc dose to chest wall [table 1]. All replanning plans adhered to these constraints. At least acceptable mean ipsilateral breast dose (max. 5.5Gy) was achieved in 97% of cases. Optimal skin dosimetry was set at Dmax ≤100% prescription dose, D1cc ≤13.2Gy and D10cc ≤10.0Gy.

Conclusion Acute toxicity following single dose IMRT-based preoperative APBI appears to be mild. Clinically feasible optimal and acceptable OAR constraints were developed for future studies. Long-term follow-up is required for validation and the skin dosimetry constraints, in particular, may need additional modification based on long-term outcomes. Chronic toxicity will be available at the 36 th ESTRO conference. [1] Olivotto et al. 2013 [2] van der Leij et al. 2014 OC-0317 MR radiomics and fractal dimension in cervical cancer predicting pathological complete response N. Dinapoli 1 , A.L. Valentini 2 , A. Pesce 1 , R . Gatta 1 , C. Masciocchi 1 , V. Ninivaggi 2 , B. Gui 2 , G. Matt ana 1 , M.A.