S. Koch, F. Ong, and J. Dortmans; Medisch Spectrum Twente, Enschede, Netherlands
Purpose/Objective(s): A concern in the treatment of head and neck patients is that anatomical changes may develop (e.g. tumor shrinkage), leading to deviations from the intended dose distribution. Various adaptive strategies have been introduced, including daily online replanning, but the potential benefit needs to be weighed against the additional workload. For this purpose we present a long-term retrospective evaluation of dose delivered to head and neck patients in a conventional IGRT protocol. The dose distributions were calculated from treatment data to assess whether clinical objectives were actually met. Materials/
Methods: Starting in 2020 on a new ring gantry linac, 83 head and neck treatments (16 to 35 fractions) were performed in four years. Photon dose plans used a V95%>99% objective for target volumes, and organ at risk constraints according to the QUANTEC review. Up to three dose levels were delineated, with 3 or 5 mm PTV margin. Daily CBCT imaging was used for 3D patient alignment. For head and neck IGRT cases specifically, the treatment management system performs a calculation of delivered dose for every treated fraction. It then accumulates the fraction doses on the frame of reference of the planning CT, to enable monitoring the total dose delivered. Both previous steps employ a deformable registration between CBCT and planning CT. Registration was visually verified on all CBCTs by means of the propagated CTV contours. The algorithm also supports repeat planning CTs, which were acquired once in two separate cases. Data evaluation focused on: •Volume of the CTV before treatment and at the last fraction •Accumulated CTV dose coverage V95% •Any unplanned OAR dose constraint violations.
Results: After checking for errors in deformable registration or incomplete data, 65 cases were deemed suitable for evaluation (114 CTVs). Mean CTV volume decrease was 2% (SD 7%). The trend towards shrinkage was weak but significant (p<0.05 in a paired t-test). In 55 out of 65 cases, the accumulated dose for each CTV met the coverage objective. The remaining cases each presented a CTV V95% between 93% and 99%. Half of these were already underdosed in the plan, mostly due to the presence of air. Low dose level CTVs, if present, always reached V95%>99%. Unplanned constraint violations were observed 7 times, for 7 different types of OAR in 6 patients. The excess dose range was 1.6% to 23% (median 5.4%). Conclusion: Dose distributions delivered in IGRT mode to head and neck patients were reconstructed from their treatment data, using a built-in dedicated algorithm. Although not available for every treated patient, our results do indicate a good overall agreement between planned and delivered doses. Most patients would have little benefit from daily plan adaptation, at least with the current margins and fractionation. In other cases the dose accumulation could assist in early assessment of deviations, which will be a topic of further research.
