Dosimetric advantages and clinical feasibility of a novel lateral decubitus position for left breast cancer patients undergoing PORT

Radiation-induced cardiac toxicity remains a significant concern in breast cancer radiotherapy. Our institution developed a novel lateral decubitus position (nLDP) technique for patients unable to maintain deep inspiration breath-hold (DIBH) during postoperative radiotherapy (PORT). This study quantitatively evaluated the cardiac dosimetry benefits and setup errors of nLDP and compared its heart-sparing effects to DIBH within the low-risk patients (free breathing mean heart dose FB–MHD <4 Gy). Patients with left-sided breast cancer indicated for PORT were enrolled and stratified into three cohorts based on positioning and breathing techniques: Cohort 1 (nLDP in FB, nLDP-FB), Cohort 2 (supine position in FB, SP-FB), and Cohort 3 (SP in DIBH, SP-DIBH). The planning target volume (PTV) encompassed the whole breast/chest wall ± regional lymph nodes, and organs at risk (OARs) were delineated according to institutional protocols. While all patients were clinically treated without internal mammary node irradiation (IMNI), supplementary IMN-inclusive plans were generated specifically for Cohort 1 for dosimetric comparison. All plans utilized 6 MV photon intensity-modulated radiation therapy (IMRT). Dosimetric parameters, normal tissue complication probability (NTCP), anatomical heart-PTV distances, and setup errors were assessed. For the low-risk patients, cardiac dose reductions in nLDP were compared with DIBH. A total of 76 patients were analyzed: Cohort 1 (n = 28, median age 51.5 years, 75% underwent modified radical mastectomy MRM), Cohort 2 (n = 28, median age 50.5 years, 61% MRM), and Cohort 3 (n = 20, median age 45 years, 55% MRM). In Cohort 1, nLDP significantly reduced the mean doses and V5 Gy–V30 Gy to the whole heart and cardiac substructures compared to SP, regardless of IMNI inclusion, without compromising PTV coverage. Furthermore, nLDP significantly increased the heart–PTV distance and lowered the NTCP for cardiac mortality and contralateral breast secondary malignancy. The setup errors for nLDP remained below 0.5 cm, with no significant difference compared to SP. In patients with FB-MHD < 4 Gy, nLDP achieved heart dose reductions comparable to DIBH. The nLDP offers an effective cardioprotective alternative to DIBH by significantly reducing cardiac doses without compromising PTV coverage and setup stability.