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 patients had received 50.4 Gy of conventionally fractionated irradiation 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, same monitor unit with source-to-skin distance (SSD) 100cm were conducted. Dosimetric parameters (D105%, D103%, D100%, D98%, D95%, D90%: Dx% indicates volumes which receive X% of prescribed doses) between initial and re-sim CT scans were compared. The CT number data (CTmean, CTmax, CTmin) of 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 CTmean and muscle CTmax/CTmin showed statistically and significantly increased values in re-sim CT compared with original CT scans. Rib bone CTmean/CTmin showed statistically and significantly decreased values in re-sim CT compared with original CT scans. Other CT number values showed no statistically significant changes. Among dosimetric parameters, only D105% (p = 0.015, mean 3.07cc versus 1.63cc) and D103% (p = 0.017, mean 13.8cc versus 11.9cc) showed statistically increased values in re-sim CT compared with original CT scans.
CT number changes after conventionally fractionated irradiation were different according to the tissue component. For electron boost plans, implementation of re-sim CT might be considered because significant dosimetric factor changes were observed especially in the high dose areas (hot spots; D105% andD103%).
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CMC IRB Ethics Committee
All authors have declared no conflicts of interest.