Evaluation of tissue computed tomography number changes and dosimetric shifts after conventional whole-breast irradiation in patients undergoing breast-conserving surgery

The aim of this study was to assess tissue computed tomography (CT) number changes and corresponding dosimetric shifts in repeatedly performed simulation CT (re-sim CT) scans after conventionally fractionated irradiation in breast cancer patients. A total of 28 breast cancer patients who underwent breast-conserving surgery were enrolled in this study. All the patients had received 50.4 Gy of conventional whole-breast irradiation (WBI) and underwent re-sim CT scans for tumor bed boost. For evaluation of dosimetric shifts between initial and re-sim CT scans, electron boost plans in the same field size with the same monitor unit with source-to-skin distance of 100 cm were conducted. Dosimetric parameters (V_105%, V_103%, V_100%, V_98%, V_95%, V_90%: V_x% indicates volumes which receive X% of prescribed doses) between initial and re-sim CT scans were compared. The CT number data (CT_mean, CT_max, CT_min) of the original and irradiated CT (re-sim CT) scans from each representative structure (lung, rib bone, soft tissue, muscle, etc.) were examined and recruited. CT numbers showed highly variable changes. Soft tissue CT_mean and muscle CT_max/CT_min showed statistically and significantly increased values in the CT (re-sim CT) compared to the original CT scans. Rib bone CT_mean/CT_min showed statistically and significantly decreased values in the re-sim CT compared to the original CT scans. Other CT number values showed no statistically significant changes. Among the dosimetric parameters, only V_105% (p = 0.015, mean = 3.07 cc versus 1.63 cc) and V_103% (p = 0.017, mean = 13.8 cc versus 11.9 cc) exhibited statistically increased values in the re-sim CT compared to the original CT scans. CT number changes after conventional WBI were different according to tissue component. For electron boost plans, the implementation of a re-sim CT might be helpfully considered because significant dosimetric factor changes were observed especially in the high-dose areas (hot spots: V_105% and V_103%).