Dose and normal tissue complication probability analysis of various radiotherapy regimens for thymomas

This study aimed to compare dosimetric differences and normal tissue complication probability (NTCP) among coplanar intensity-modulated radiation therapy (CO-IMRT), non-coplanar IMRT (NONCO-IMRT), coplanar volumetric-modulated arc therapy (CO-VMAT), and non-coplanar VMAT (NONCO-VMAT) in thymoma radiotherapy and clarify the value of non-coplanar plans.

Forty-eight post-thymoma surgery patients were enrolled in this study. Four radiotherapy plans (50 Gy/25 fractions) were made for each patient. The target conformity index (CI), homogeneity index (HI), and organ-at-risk (OAR) (lungs, heart, etc.) dose parameters were then evaluated. NTCP was calculated using the Lyman-Kutcher-Burman (LKB) model. Statistical analysis was performed using the Student-Newman-Keuls (SNK-q) test (p < 0.05 for significance).

There were no significant clinical differences in target volume coverage and homogeneity among the four plans. The lung V₂₀, heart V₃₀, spinal cord D_max, and esophagus D_max of the VMAT techniques were all lower than those of IMRT (p < 0.001). The breast dose of CO-VMAT was significantly higher, while the breast parameters of NONCO-VMAT were close to those of IMRT. NTCP results were consistent with dosimetric findings: VMAT plans had lower NTCP for the heart, spinal cord, and esophagus; moreover, the breast NTCP of CO-VMAT was significantly increased (1.75%), while that of NONCO-VMAT decreased to 0.28%, which was similar to that of IMRT.

All four plans meet the thymoma target dose requirement. However, NONCO-VMAT is preferred for its advantages in multi-organ protection and balanced breast dose. Additionally, CO-VMAT should be used cautiously in female patients due to the higher breast dose.