Quantifying Patient Setup Error in Total Body Irradiation With Megavoltage Computed Tomography-guided Helical Tomotherapy

BACKGROUND/AIM: In helical tomotherapy (HT)-based total body irradiation (TBI), geometric accuracy is essential because intensity-modulated dose delivery relies on steep dose gradients. However, residual site-specific setup errors (SEs) after routine image guidance have not been fully characterized. This study quantified residual translational SEs after megavoltage computed tomography (MVCT)-guided global bony registration and derived landmark-specific planning target volume (PTV) and planning organ-at-risk volume (PRV) margins. PATIENTS AND METHODS: Twenty consecutive patients who underwent HT-based TBI as conditioning for allogeneic hematopoietic stem cell transplantation were retrospectively analyzed. A total dose of 12.0 Gy in four fractions was delivered using separate head-first and feet-first plans. After routine MVCT-guided registration, residual translational SEs were manually measured in the left-right, superior-inferior, and anterior-posterior directions at predefined anatomical landmarks: mandible, bilateral shoulders, pelvis, bilateral knees, bilateral heels, lung, liver, and bilateral kidneys. Systematic and random errors were calculated for each landmark, and PTV and PRV margins were estimated using modified margin recipes. RESULTS: Eighty fractions were analyzed. The mandible and lung showed high reproducibility, with residual SEs within 5.0 mm in all directions. In contrast, the shoulders showed the largest PTV-related uncertainty, with estimated PTV margins exceeding 10.0 mm in the superior-inferior and anterior-posterior directions. The liver and kidneys showed marked organ-related uncertainty, particularly in the superior-inferior direction; PRV margins reached 9.2 mm for the liver and up to 9.8 mm for the kidneys. CONCLUSION: Residual SEs in MVCT-guided HT-based TBI were strongly site-dependent. Larger uncertainties in the shoulders, liver, and kidneys indicate that site-specific PTV and PRV margins should be considered rather than applying uniform margins across the whole body.