Dosimetric impact of vacuum bag vs. abdominal compression fixation on PTV margins in IMRT for hepatocellular carcinoma with lymph node metastasis

To compare setup errors between vacuum-bag with thermoplastic shell (VB) and abdominal compression with foam (AC) immobilization techniques in radiotherapy for hepatocellular carcinoma (HCC) with lymph node metastases (LNM), and evaluate their impacts on planning target volume (PTV) margins and dosimetry. This retrospective cohort study of 34 HCC patients with LNM (VB group: n = 17; AC group: n = 17) was performed. Setup errors were verified by cone-beam CT (CBCT), and PTV margins were calculated using the van Herk formula (MPTV = 2.5Σ+0.7σ). Dosimetric parameters (conformity index, CI; homogeneity index, HI) and organ-at-risk (OAR) doses were compared. The VB group demonstrated significantly smaller calculated PTV margins in the superior-inferior (Y-axis) direction (0.32 cm vs. 0.57 cm, p = 0.001). Systematic errors (Σ) were lower for VB in the X and Y axes, while random errors (σ) were lower in the Z-axis. Dosimetrically, the VB group achieved significantly better dose conformity (CI: 0.24 ± 1.09 % vs. −0.76 ± 1.50 %, p = 0.04). No statistically significant differences were observed in other PTV parameters (Dmax, Dmean, TC, HI) or in the majority of OAR dose parameters, although a trend favoring VB was noted in several instances (e.g., Liver V30, p = 0.04). For HCC patients with LNM undergoing IMRT, vacuum bag fixation significantly reduces PTV margins along the superior-inferior direction and improves dose conformity compared to abdominal compression, offering a potential for optimized treatment planning.