Purpose: Traditional craniospinal irradiation (CSI) treats the entire subarachnoid space, using long fields with multiple isocenters and junction shifts. Multi-isocentric VMAT approaches have recently been implemented, requiring precise image-guidance and delivery to avoid hot or colds spots in the spine. This work characterizes the dosimetric impact of patient setup uncertainty by quantifying the impact of translational shifts on PTV coverage, recording image-guided patient setup errors during treatment, and using in-vivo dosimetry to measure delivered dose.
Methods: This study analyzed multi-isocentric VMAT CSI irradiations for two patients receiving 3600 cGy and 3960 cGy in 180 cGy fractions to the craniospinal axis. Eclipse™ treatment planning software was used to quantify changes in PTV coverage from simulated plan shifts of 3 mm and 5 mm in the vertical, longitudinal, and lateral directions. Image-guidance was used for each fraction (N=42). An in-house application determined an optimal couch shift with preferential weighting of the head isocenter, and longitudinal offsets determined from the plan; residuals at all isocenters were calculated. EPID-based transit dosimetry using PerFRACTION evaluated treatment delivery at gamma thresholds of 3%/3mm, 3%/5mm, and 5%/5mm.
Results: CSI PTVs showed V100% decreases from 99.8% to 97.2% and 93.6% for 3 mm and 5 mm composite shifts, respectively. Couch residual magnitudes averaged across translational directions for patient setup were 0.1 mm, 1.8 mm, and 0.7 mm at the head, chest, and abdomen isocenters; in-vivo transit dosimetry results showed at each isocenter, an average pass rate (min, max) of 99.5% (96.6%, 100%), 96.2% (86.7%, 99.3%), and 91.6% (77.4%, 98.6%) at gamma thresholds of 3%/3mm.
Conclusion: This work aimed to characterize CSI dosimetry using modelled translational shifts, residuals, and transit dosimetry. Transit dosimetry failures led to setup and planning interventions, including the addition of a 2 mm shim under the head, and additional dose calculations due to weight loss.