Dosimetric Variation Due to CT Inter-Slice Spacing in Four-Dimensional Carbon Beam Lung Therapy

Purpose: When CT data with a thick slice thickness are used in treatment planning, geometrical uncertainty may induce dosimetric errors. We evaluated carbon ion dose variations due to different CT slice thicknesses using a four-dimensional (4D) carbon ion beam dose calculation, and compared results between ungated and gated respiratory strategies. Material and Methods: Seven lung patients were scanned in 4D mode with a 0.5-mm slice thickness using a 256-multi-slice CT scanner. CT images were averaged with various numbers of images to simulate reconstructed images with various slice thicknesses (0.5 mm-5.0 mm). Two scenarios were studied (respiratory-ungated and -gated strategies). Range compensators were designed for each of the CT volumes with coarse inter-slice spacing to cover the internal target volume (ITV), as defined from 4DCT. Carbon ion dose distribution was computed for each resulting ITV on the 0.5-mm slice 4DCT data. The accumulated dose distribution was then calculated using deformable registration for 4D dose assessment. Results: The magnitude of over- and under-dosage was found to be larger with the use of range compensators designed with a coarser inter-slice spacing than those obtained with a 0.5-mm slice thickness. Although no under dosage was observed within the clinical target volume (CTV) region, D95 remained at over 97% of the prescribed dose for the ungated strategy and 95% for the gated strategy for all slice thickness. Inter-slice spacing of less than 3 mm may be able minimize dose variation between the ungated and gated strategies. Conclusions: Although volumes with increased inter-slice spacing may reduce geometrical accuracy at a certain respiratory phase, this does not significantly affect delivery of the accumulated dose to the target during the treatment course.