Impact of Minimum Segment Width on VMAT Plan Quality for Nasopharyngeal Carcinoma

This study aimed to investigate the impact of the minimum segment width (MSW) on the plan quality of volumetric modulated arc therapy (VMAT) for nasopharyngeal carcinoma (NPC), seeking to identify the optimal MSW and provide clinical references for VMAT plan design in NPC.

The planning CT images of 40 NPC patients treated in our institution between January 2019 and March 2019 were retrospectively selected, including 8 cases of T1, 11 of T2, 12 of T3, and 9 of T4 stage. Target volumes and organs at risk (OARs) were delineated according to ICRU Reports 50, 62, and 83. To ensure consistency in plan optimization, a priority-classified optimization model was utilized to classify all targets and OARs into four priority levels: the brainstem, spinal cord, optic chiasm, and optic nerves were assigned Level I; the planning target volume (PTV) and temporal lobes were Level II; the eyes, lenses, and pituitary gland were Level III; and the parotid glands, mandible, temporomandibular joints (TMJs), thyroid, inner ears, and oral cavity were Level IV. During the optimization process, Level I OARs were strictly prioritized first, followed sequentially by Levels II to IV. Four VMAT plans were generated for each patient using the Monaco 6.00.11 treatment planning system (TPS), with the MSW parameter set to 0.5 cm, 0.8 cm, 1.0 cm, and 1.5 cm, respectively, while keeping other physical parameters identical. Dosimetric differences in PTVs and OARs were compared among the four groups to evaluate the effect of varying MSWs on plan quality.

All VMAT plans achieved successful optimization while strictly meeting the prescription dose constraints for Level I OARs. For patients with T1-2 stage, the prescription dose coverage for all PTVs exceeded 95% across the four groups. As the MSW increased, the D2, D50, D95, and homogeneity index (HI) of the targets significantly increased (P < 0.05), whereas the D98 and conformity index (CI) significantly decreased (P < 0.05). The parotid gland doses were lower in the MSW 0.5 cm and MSW 0.8 cm groups. Additionally, the monitor units (MUs) exhibited a decreasing trend with the increase of MSW. For patients with T3-4 stage, the prescription dose coverage of PTVnx increased as the MSW decreased across the four groups; however, none of the plans reached the 95% coverage threshold for PTVnx. The MSW 0.5 cm group exhibited lower doses to the left parotid glands, right TMJ, and oral mucosa compared to the MSW 1.5 cm group, with no substantial differences observed for other OARs. MUs also decreased with increasing MSW in these patients.

The MSW significantly affects the quality of VMAT plans for NPC. For T1-2 patients, all four MSW groups achieved >95% PTV coverage; while MSWs of 0.5 cm and 0.8 cm provided better sparing of the parotid glands, the 0.8 cm MSW required a shorter treatment time. Therefore, an MSW of 0.8 cm is recommended for T1-2 patients. For T3-4 patients, none of the plans achieved 95% PTV coverage; thus, advanced higher-precision radiotherapy modalities are recommended. However, if VMAT is clinically selected, an MSW of 0.5 cm is recommended for plan design to maximize target coverage.