Author: Chia-Lung Chien, Wen C. Hsi, Romy Megahed, Kayla Schneider 👨🔬
Affiliation: Department of Radiation Oncology, University of Arkansas for Medical Sciences (UAMS), Department of Radiation Oncology, University of Arkansas for Medical Sciences 🌍
Purpose:
The dosimetric variations induced by inter-fractional anatomical changes in intensity-modulated proton therapy could impact the clinic outcomes for head-and-neck (HN) cancer patients. This study focused on the quantitative evaluation of dosimetric variations from anatomical changes observed in repeated CT (QACT) scans.
Methods:
We selected 8 out of 55 HN cancer patients treated during the first-year operation of our proton beamline. The treatment plan of each patient was constructed with a 3mm-3.5% robustness optimization to have 100% prescription-dose covering 97-99% volume of the clinical-treatment-volume (CTV) on a planning CT (PCT). The planned beam-set on PCT was rigidly mapped to recalculate on QACT then was deformably transported back to the PCT. The Dose-Volume-Histogram (DVH) for the CTV and organ-at-risk (OAR) contours were obtained for both PCT and 2 QACTs per patient. The dosimetric variation of CTV coverage was assessed by conducting a one-way ANOVA on conformity index (CI) values between the DVH of PCT and the series of QACTs. The dosimetric variations of OAR were obtained by evaluating dose against known constraints for PCT and each QACT dose.
Results:
The 3mm-3.5% robustness often placed the prescription isodose outside the CTV, with CI > 1.0 for 6 of 8 patients in both PCT and QACT plans, while CI < 1.0 occurred due to multiple OARs tightly abutting the CTV. No statistically significant differences were observed in CI values between PCT and QACT plans. The constraint for the ipsilateral parotid was violated by both for PCT and QACT plans. We did not observe any instances where constraints met in the PCT were violated in the QACT.
Conclusion:
The negligible variations on CI-values and consistent dose constraints of OARs between PCT and QACT plans indicate that the anatomical changes have no significant impact on our planning strategy with the 3mm-3.5% robustness.