Clinical Evaluation of a Rotational Set-up Correction Device for Multiple Brain Metastases Using Single-Isocenter Stereotactic Radiation Therapy

Abstract

Stereotactic radiation therapy (SRT) using a single plan with one isocenter by helical tomotherapy (HT) can simultaneously treat multiple brain metastases with highly conformal dose distributions. This may reduce treatment time and intra-fractional motion. However, rotational set-up errors in the pitch and yaw angles cannot be corrected in the HT system. Therefore, increases in rotational set-up error and distance of the target-to-rotational-center may be associated with significant dosimetric uncertainties with this irradiation technique. Previous research shows that when rotational errors are increased to 2.0°, the prescription doses which cover 95% of the planning target volume were only 63% of all tested cases. Here, we developed a new head supporting device that allows for a highly-accurate correction of rotational errors during image-guided radiation therapy. In this study, we investigated the amounts of rotational correction required prior to irradiation, and evaluated the dosimetric efficacy of rotational correction for SRT, using HT unit to treat multiple brain metastases. A novel rotational set-up correction device was constructed from carbon material and conjected with the commonly-used head shell system on the top of our device. We were able to rotate in 0.1° increments for pitch and yaw angles using two screws. We retrospectively analyzed data from three patients irradiated by single-isocenter SRT for multiple brain metastases using the HT unit. The SRT plans were created with 30 Gy in three fractions prescribed for D99% to clinical target volumes. The required rotational correction values were determined by mega-voltage computed tomography (MVCT) before irradiation. Residual correction values were quantified after correction by the developed device. Moreover, dosimetric efficacy was evaluated using the calculated dose distribution with MVCT after rotational correction. Dose distribution was compared between 4D (three horizontal axes + roll) and 6D correction. We clarified efficacy of rotational correction depending on the distance of the target-to-rotational-center. The rotational correction for pitch was 1.2° ± 1.7°, roll: 0.9° ± 1.2°, and yaw: 1.3° ± 1.6°. The maximum correction value in pitch and yaw was 3.0°. The residual correction value after rotational correction was 0.2° ± 0.3° in both pitch and yaw angles. The difference in target dose between 4D and 6D corrections was improved by 2.5% in coverage index, and 1.0% on D99%. Dosimetric efficacy of rotational correction did not depend on the distance of the target-to-rotational-center. This suggests that irradiated doses for brain metastases in any location can be improved. Novel rotational set-up correction device can be corrected to within 0.5° in pitch and yaw angle error, and benefited rotational correction and irradiated dose distribution for single-isocenter SRT for multiple brain metastases.