Dosimetric comparison of intensity-modulated proton radiotherapy versus intensity-modulated photon-based radiotherapy for breast cancer

Purpose : This study aims to compare the dosimetric differences in intensity-modulated proton therapy (IMPT) using pencil beam scanning technology and intensity-modulated photon-based radiotherapy (IMRT) in hypofractionated whole-breast irradiation (HF-WBI) and find out the more beneficial technique.Methods and Materials : Eight breast cancer (BC) patients with pathological stage T1 ~ 2N0M0 were immobilized and underwent 4D-CT scanning used deep inspiration breath-hold (DIBH) technology. The IMPT and IMRT plans were designed for each patient. The IMPT plans used two en-face beam angles. IMRT plans were designed using the field in field technique. The optimization constraints of the two types of plans were identical. Prescription dose and regimen was 40.05 Gy (relative biological effect [RBE])/15 fx with a 10 Gy (RBE)/5 fx boost, five fractions a week. A dose of 95% of the target volume should not be less than the prescribed dose. The target coverage was evaluated using D1, D2, D50, D95, D98, and D99. The target dose distribution and conformity were evaluated using the Conformity index (CI) and the homogeneity index (HI). The Organs at risk (OARs) were evaluated using mean dose (Dmean) and maximum dose (Dmax). Ipsilateral Lung and Contralateral Lung were evaluated additionally using V5, V10, V20, V30.Results : The mean dose (Dmean) of the Heart (P  = 0.012), Ipsilateral Lung (P  = 0.036), Contralateral Lung (P  = 0.012), and Spinal Cord (P  = 0.012) were significantly reduced in IMPT plans. The IMPT also showed a tendency to reduce the V20 (P  = 0.05) and V30 (P  = 0.05) of the Ipsilateral Lung. But there was no significant difference in target coverage, homogeneity, and conformity between the IMRT and IMPT plans.Conclusion : Compared to IMRT, the IMPT using pencil beam scanning technology can spare OARs without compromising target coverage in BC patients undergoing HF-WBI, which potentially reduce the incidence of radiation-related adverse effects and thus may positively impact long-term survival.