Dosimetric effects of the leaf positioning error of the halcyon(2.0) dual-layer multileaf collimator (MLC) on the rectal cancer radiotherapy.

Abstract

To study dosimetric effects of leaf positioning errors (LPEs) of the Halcyon(2.0) dual-layer MLC on the long-course chemoradiotherapy (LCCRT) with 45∼50.4 Gy in 25∼28 fractions for rectal cancer. Nine Halcyon(2.0)-based LCCRT plans of rectal cancer were retrospectively involved. Four types of LPEs were introduced: (1) Uniformly distributed dual-layer random LPEs (Dual-R); (2) Proximal-layer systemic LPEs (P-S); (3) Distal-layer systemic LPEs (D-S); (4) Dual-layer systemic LPEs (Dual-S). The sensitivities of D_98%, D_2% and the Equivalent Uniform Dose (EUD) of PTV to various LPEs were investigated as well as varying ranges of EUDs of OARs. The sensitivities of D_98% and EUD of PTV to Dual-R was -0.65%/mm and -0.38%/mm; the sensitivities of both indices to the P-S and D-S were similar to each other, ranging from 1.92%/mm to 2.87%/mm; both indices were more sensitive to the Dual-S and values were 4.97%/mm and 3.84%/mm respectively. The EUD changes of the bladder, and left and right femoral heads were from -13.23% to 14.82%. Single-side systemic LPEs lower than 0.7 mm are acceptable for Halcyon(2.0)-based LCCRT, while dual-layer systemic LPEs lower than 0.4 mm are acceptable, considering relative changes of 2% for D_98% of PTV as threshold.