Purpose: Image-guided adaptive intensity-modulated proton therapy [IMPT] is expected to have clinical benefits for moving tumours. The therefore required time-resolved IMPT dose calculations were performed on day-of-treatment scatter-corrected 4DCBCTs [4DCBCT(cor)] of 5 lung cancer patients to evaluate the dosimetric accuracy of 4DCBCT(cor). For comparison, dose calculations were performed on corresponding planning 4DCTs and day-of-treatment 4D virtual CTs [4DvCT].
Methods: Day-of-treatment free-breathing CBCT projections and planning 4DCT images were used as input to a CBCT correction workflow previously validated using phantom data, generating 4DvCT [CT-to-CBCT deformable registration] and 4DCBCT(cor) images [projections-based correction using prior 4DvCT] with 10 phase bins. Robust IMPT plans administering 60 Gy in 8 fractions with a 3-field arrangement were created on a physician-contoured free-breathing CT [pCT] using the treatment planning system RayStation. A density override of the internal target volume [ITV] was performed. The Monte Carlo dose engine and robustness settings of 3% range and 6 mm setup uncertainty were applied.The dose was recalculated on every phase of planning 4DCT, day-of-treatment 4DvCT, and 4DCBCT(cor), so that accumulated doses, using the same vector fields that were used to generate 4DvCT and 4DCBCT(cor), could be computed on the corresponding mid-position images. For evaluation, the contours from the pCT were deformably registered to (midp)CT and (midp)vCT. The latter were copied to the (midp)CBCT(cor). Comparison was performed by means of dose-volume-histogram [DVH] parameters.
Results: 4DvCT and 4DCBCT(cor) showed quality enhancements compared to 4DCBCT. The median DVH differences between 4DvCT and 4DCBCT(cor) for bronchi D(2%), mean lung dose, and ITV D(98%) were 1%, 2%, and 2%, respectively. Deviations were larger for the 4DCT due to inter-scan changes.
Conclusion: This study suggests that daily time-resolved proton dose calculation for lung tumour patients on 4DvCT and 4DCBCT(cor) is feasible. Analysis of additional patients is ongoing to validate this result.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the German Research Foundation (DFG): project number 399148265, graduate college GRK2274.