Dosimetric Comparison of Proton Therapy Plans Calculated Using Fast Kv Switch Dual Energy CT Versus Single Energy CT
Abstract
Purpose
Dual energy CT (DECT) has been demonstrated to more accurately predict relative stopping power (RSP) for proton therapy planning over the conventional method of HU-RSP conversion using single energy CT (SECT). In this study, treatment plans are calculated in phantom using DECT and SECT and a dosimetric comparison of accuracy is evaluated.
Methods
A SECT and DECT scan were taken of a phantom with known composition, and several treatment plans are calculated on both scans. The plans are then recalculated without re-optimization onto a scan with CT numbers forced to the known mass density of the phantom and inserts to create a reference plan. The difference in dose and range is calculated from the nominal plan to the reference plan to evaluate the accuracy of RSP calculation using DECT and SECT.
Results
The D95 for the PTV resulted in an average percent relative error of -0.56% ± 0.19% in DECT and -0.79% ± 0.39% in SECT when calculated on the reference CT. DECT average dose to the OARs 1 mm distal and proximal to the PTV was more accurate than SECT for all plans except for the plan ranging through breast and adipose. The average difference in range when calculated on the reference image is 0.20 mm ± 0.75 mm compared to an average range change of 1.01 mm ± 1.10 mm planned with SECT. The positive value indicates over-ranging in the reference scan into distal structures.
Conclusion
Plans calculated on DECT showed decreased range error into proximal and distal structures compared to the plans calculated on the SECT scan. The average dose difference from the DECT to the reference plan was less than in SECT. This suggests DECT as a more accurate means of calculating RSP for proton therapy treatment planning over the conventional SECT.