Room: Exhibit Hall | Forum 5
Purpose: To test the feasibility of fractional dose accumulation on the new Halcyon IGRT system, assisting a more informed clinical decision on the optimal timing for the Adaptive Radiotherapy (ART) re-planning, based on patient-specific and dose-guided indicators.
Methods: A hypopharyngeal carcinoma patient treated on Halcyon with noticeable anatomic change was retrospectively selected. Dose1 (D1) of Plan1 on CT1 was delivered for 20 fractions before D2 of Plan2 on CT2 was executed for the remaining 13 fractions per physician's observation and experience. After geometric and dosimetric validations of Velocity DIR based on Halcyon images, CT1/CT2 was deformed to fractional MV CBCT (before/after fraction#20), yielding dCT1/dCT2 respectively. Reconstructed doses (rD1/rD2) of Plan1/Plan2 on dCT1/dCT2 were accumulated incrementally before/after fraction#20 respectively. An in-house gamma analysis code using 3mm/3% global/10% threshold criteria, and the target dose-volume-histograms (DVHs) were used to compare the planned and reconstructed dose. The target volume varieties were assessed quantitatively on MV CBCT images.
Results: Based on the anatomic similarity between CT2 and MV CBCT#21, the dosimetric agreement between rD2 and D2 was 91.01%. The gamma passing rates between accumulated D1 and rD1 increased with fluctuation before reaching a maximum of 97.52% at fraction#11, and fell monotonically to a minimum of 91.49% at fraction#33 without re-planning. Physician-ordered ART since fraction#21 maintained the agreement above 95.42% till end, close to the timing when the rate started below 95% at fraction#24 (94.99%), which could serve as a dose-guided indicator of re-planning opportunity and frequency. Comparatively, the anatomy-based decision was less reliable, as suggested by correlation uncertainties between the dosimetric and geometric varieties. Furthermore, DVH disagreement could be another dose-guided indicator of re-planning necessity by evaluating potential risk of target under-dose.
Conclusion: This study demonstrated the feasibility of ART for Halcyon MV CBCT-guided radiotherapy system, based on dose-guided and patient-specific timing and frequency indicators.
Funding Support, Disclosures, and Conflict of Interest: This work was jointly supported by Capital's Funds for Health Improvement and Research (2018-4-1027), Beijing Natural Science Foundation (7172048, 1174016 and 1184014).