Abstract P1-10-11: Pronevs.supine positioning for optimizing radiation heart exposure during left breast radiotherapy: A simple clinical tool for prediction*

The benefit of reduced radiation heart exposure in the prone vs. supine position, individually differs according to the anatomical features of the patient (Varga Z, Int J Radiat Oncol Biol Phys. 2009;75:94-110). Preferable positioning may be predicted by a statistical model. This validated calculator using the patient9s body mass index (BMI), the median distance of the anterior surface of the left coronary artery (LAD) from the chest wall (Dmed) and the heart area (Aheart) included in the radiation field in a single CT scan at the middle of the heart (median plane, Pmed) provides quantitative estimates of the dose differences to the LAD or heart in the two positions (Varga Z, Acta Oncol. 2014;53:58-64). In this prospective cohort study, the goal was to develop a reliable method to collect data for the calculator without acquiring a full series of CT scans in both positions. The study was approved by the Institutional Review Board of the University of Szeged, and all the enrolled patients gave their written informed consent to participation. Eligible patients needed postoperative left breast radiotherapy. In 100 patients, a single CT slice image representing the middle of the heart (reference plane, Pref) was acquired by using the AP scout view in the supine position for the selection of the transversal plane appropriate for the measurements of Dmed and Aheart. Thereafter, CT series were acquired in both positions, and analyses were performed on 1. the conformance of the Pref with Pmed , 2. the effect of plane miss on the choice of preferable position. 3. Finally the sensitivity and specificity of this simple clinical method was reevaluated based on the dosimetry data (mean LAD dose, V25Gy heart) obtained using the topogram for selecting the position. In 55 cases, Pref was the same as Pmed, while in 27 and 18 cases, Pref and Pmed differed by 2 or more planes, respectively. No difference was found between the values of Dmed or Aheart asmeasured in Prefvs.Pmed, independently whether Pref was identified correctly or not.The suggestion on treatment position per Pref measurements, was correct in 85 cases, was ambiguous in 8 cases and incorrect in 7 cases. Sensitivity of the calculator based on Pref measurements was 90% (both mean LAD dose and V25Gy heart), while specificity was 72% (mean LAD dose) and 68% (V25Gy heart). The simple clinical method was tested in additional 60 left breast radiotherapy cases, by taking a topogram and a CT series in the suggested position only. Mean LAD dose (mean [CI95%] prone vs. supine: 6.5 [5.7-7.3] Gy vs. 7.5 [5.6-9.5] Gy) and V25Gy heart (mean [CI95%] prone vs. supine: 0.87 [0.67-1.11] % vs. 1.13 [0.55-1.71]%) well corresponded to the institutional dose constraints based on dosimetry data achieved with individual positioning in >300 patients with left-sided breast cancer. We consider this simple clinical tool appropriate for assisting individual positioning aiming at maximum heart protection during left breast irradiation. *Supported by the VKSZ 12-1-2013-0012 project. Citation Format: Kahan Z, Gaal S, Cserhati A, Rarosi F, Boda K, Varga Z. Prone vs. supine positioning for optimizing radiation heart exposure during left breast radiotherapy: A simple clinical tool for prediction* [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-10-11.