Clinical Implications of Geometric and Dosimetric Uncertainties Due to Inter- and Intra-Fractional Movement During Volumetric Modulated Arc Therapy for Breast Cancer Patients

Abstract

Compared to conventional radiotherapy (RT) techniques, modern RT with fewer fractions and smaller target volumes requires higher accuracy. This study evaluated the necessity of pre-treatment and intra-fractional cone-beam computed tomography (CBCT) by analyzing inter- and intra-fractional CBCT images of breast cancer patients receiving RT.From 57 patients, 1,206 pre-treatment CBCT and 1,067 intra-fractional CBCT images were collected. After positioning the patient according to the three skin markers drawn during simulation, pre-treatment CBCT images were acquired. During partial-arc volumetric modulated arc therapy, a simultaneous intra-fractional CBCT scan was taken as the gantry rotated around the patient. The change in position between simulation and the actual position of the patient before and during RT was assessed according to the differences in x-, y-, and z-axes. After the difference in each axis was measured, the absolute distance between two CT images was measured. Changes in dosimetric parameters were evaluated in selected patients with extreme intra-fractional movement.For right-sided breast cancer patients, left-sided breast cancer patients treated using deep-inspiration breath hold, and left-sided breast cancer patients treated using continuous positive airway pressure, median inter-fractional deviations were 0.53 (range 0.06-2.98) cm, 0.66 (range 0.08-4.41) cm, and 0.69 (range 0.04-3.80) cm and median intra-fractional deviations were 0.14 (range 0.00-0.62) cm, 0.23 (range 0.02-0.96) cm, and 0.24 (0.00-1.15) cm, respectively. Modified plans reflecting large changes in intra-fractional position in 10 selected cases revealed insufficient target coverage in seven cases and more than 20-fold increase in the volume of heart receiving more than 25 Gy of radiation in two cases.Even though the intra-fractional ROM was minimal in most cases, few cases showed huge variations that could significantly affect the goal of RT; therefore, intra-fractional verification might be necessary as well as inter-fractional verification.