Evaluation of internal margins for prostate for step and shoot intensity‐modulated radiation therapy and volumetric modulated arc therapy using different margin formulas

PURPOSE: This feasibility study evaluated the intra‐fractional prostate motion using an ultrasound image‐guided system during step and shoot intensity‐modulated radiation therapy (SS‐IMRT) and volumetric modulated arc therapy (VMAT). Moreover, the internal margins (IMs) using different margin formulas were calculated. METHODS: Fourteen consecutive patients with prostate cancer who underwent SS‐IMRT (n = 5) or VMAT (n = 9) between March 2019 and April 2020 were considered. The intra‐fractional prostate motion was observed in the superior–inferior (SI), anterior–posterior (AP), and left–right (LR) directions. The displacement of the prostate was defined as the displacement from the initial position at the scanning start time, which was evaluated using the mean ± standard deviation (SD). IMs were calculated using the van Herk and restricted maximum likelihood (REML) formulas for SS‐IMRT and VMAT. RESULTS: For SS‐IMRT, the maximum displacements of the prostate motion were 0.17 ± 0.18, 0.56 ± 0.86, and 0.18 ± 0.59 mm in the SI, AP, and LR directions, respectively. For VMAT, the maximum displacements of the prostate motion were 0.19 ± 0.64, 0.22 ± 0.35, and 0.14 ± 0.37 mm in the SI, AP, and LR directions, respectively. The IMs obtained for SS‐IMRT and VMAT were within 2.3 mm and 1.2 mm using the van Herk formula and within 1.2 mm and 0.8 mm using the REML formula. CONCLUSIONS: This feasibility study confirmed that intra‐fractional prostate motion was observed with SS‐IMRT and VMAT using different margin formulas. The IMs should be determined according to each irradiation technique using the REML margin.