Dosimetric evaluation of target and peri-cavity by replacing CT values for cavities within the target in volumetric modulated arc therapy.

Volumetric-modulated arc therapy (VMAT) enables advanced dose modulation and is effective for treating tumors with complex shapes. However, for targets that contain low-density regions such as cavities, issues arise with dose distribution heterogeneity caused by high-fluence beams and excessive doses to the surrounding tissues owing to positional shifts during the treatment period. This study aimed to evaluate a treatment planning method in VMAT that replaces the computed tomography (CT) values of cavities within the target to suppress excessive peribronchial doses caused by interfractional motion. We created multiple VMAT plans using virtual phantom and chest CT images, replacing the CT values of the bronchial regions within the planning target volume (PTV) with different values and excluding the bronchial regions from the PTV. Additionally, interfractional motion was simulated by shifting iso-centers (IC) by ± 0.2 cm and ± 0.4 cm, and the error rate of D0.1cc around the bronchial area, as well as the homogeneity index (HI), mean dose, D2, and D98% of the PTV were compared. In the virtual phantom study, the error rate of D0.1 cc in the PTV containing the bronchus increased by 8.8%, however replacing the CT values within the cavity with -146 HU reduced the error rate to 1.39%. Although the HI of the PTV worsened slightly with the replacement of the CT value, the mean dose was maintained. Similarly, in chest CT images, replacing the CT values of the bronchial region reduced the D0.1cc error rate by approximately 0.7%. In this study, our findings demonstrated that replacing the CT values in low-density regions within the target of -146 HU, which represented the relative electron density value of 0.836, could suppress excessive dose around the bronchial area.