Positioning accuracy and daily dose assessment for prostate cancer treatment using in‐room CT image guidance at a proton therapy facility

Purpose To evaluate the effectiveness of CT image‐guided proton radiotherapy for prostate cancer by analyzing the positioning uncertainty and assessing daily dose change due to anatomical variations. Materials and methods Patients with prostate cancer were treated by opposed lateral proton beams based on a passive scattering method using an in‐room CT image‐guided system. The system employs a single couch for both CT scanning and beam delivery. The patient was positioned by matching the boundary between the prostate and the rectum's anterior region identified in the CT images to the corresponding boundary in the simulator images after bone matching. We acquired orthogonal kV x‐ray images after couch movement and confirmed the body position by referring to the bony structure prior to treatment. In offline analyses, we contoured the targeted anatomical structures on 375 sets of daily in‐room CT images for 10 patients. The uncertainty of the image‐matching procedure was evaluated using the prostate contours and actual couch corrections. We also performed dose calculations using the same set of CT images, and evaluated daily change of dose–volume histograms (DVHs) to compare the effectiveness of the treatment using prostate matching to the bone‐matching procedure. Results The isocenter shifts by prostate matching after bone matching were 0.5 ± 1.8 and −0.8 ± 2.6 mm along the superior–inferior (SI) and anterior–posterior (AP) directions, respectively. The body movement errors (σ) after couch movement were 0.7, 0.5, and 0.3 mm along the lateral, SI and AP direction, respectively, for 30 patients. The estimated errors (σ) in the prostate matching were 1.0 and 1.3 mm, and, in conjunction with the movement errors, the total positioning uncertainty was estimated to be 1.0 and 1.4 mm along the SI and AP directions, respectively. Daily DVH analyses showed that in the prostate matching, 98.7% and 86.1% of the total 375 irradiations maintained a dose condition of V95% > 95% for the prostate and a dose constraint of V77%