Comprehensive evaluation of dosimetric impact against position errors in accelerator-based BNCT under different treatment parameter settings

Background Patients who undergo accelerator-based (AB) boron neutron capture therapy (BNCT) for head and neck cancer in the sitting position are generally uncomfortably immobilized, and patient motion during this treatment may be greater than that in other radiotherapy techniques. Furthermore, the treatment time of BNCT is relatively long (up to approximately 1 h), which increases the possibility of patient movement during treatment. As most BNCT irradiations are performed in a single fraction, the dosimetric error due to patient motion is of greater consequence and needs to be evaluated and accounted for. Several treatment parameters are required for BNCT dose calculation. Purpose To investigate the dosimetric impacts (DIs) against position errors using a simple cylindrical phantom for an AB-BNCT system under different treatment parameter settings. Methods The treatment plans were created in RayStation and the dose calculation was performed using the NeuCure (R) dose engine. A cylindrical phantom (16 cm diameter x 20 cm height) made of soft tissue was modeled. Dummy tumors in the form of a 3-cm-diameter sphere were arranged at depths of 2.5 and 6.5 cm (denoted by T-2.5 and T-6.5, respectively). Reference plans were created by setting the following parameters: collimator size = 10, 12, or 15 cm in diameter, collimator-to-surface distance (CSD) = 4.0 or 8.0 cm, tumor-to-blood ratio (T/B ratio) using F-18-fluoro-borono-phenylalanine = 2.5 or 5.0, and B-10 concentration in blood = 20, 25, or 30 ppm. The prescribed dose was D-95% >= 20 Gy-eq for both T-2.5 and T-6.5. Based on the reference plans, phantom-shifted plans were created in 26 directions [all combinations of left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions) and three distances (1.0, 2.0, and 3.0 cm). The DIs were evaluated at D-80% of the tumors. The shift direction dependency of the DI in the LR, AP, and SI directions was evaluated by conducting a multiple regression analysis (MRA) and other analyses where required. Results The coefficients of the MRA of the DIs for LR, AP, and SI shifts were -0.08, 2.16, and -0.04 (p-values = 0.084, <0.01, and 0.334) for T-2.5 and -0.05, 2.08, and 0.15 (p-values = 0.526, <0.01, and 0.065) for T-6.5, respectively. The analysis of variance showed that DIs due to the AP shift were significantly greater for smaller collimator sizes on T-2.5 and smaller CSD on T-6.5. Dose reduction due to SI or LR (lateral) shifts was significantly greater for smaller collimator sizes on both T-2.5 and T-6.5 and smaller CSD on T-2.5, according to the Student's t-test. There were no significant differences in the DIs against both the AP shift and the lateral shift between the different T/B ratios and B-10 concentrations. Conclusion The DIs were largely affected by the shift in the AP direction and were influenced by the different treatment parameters.