ESTRO 2023 - Abstract Book

S159

Saturday 13 May

ESTRO 2023

The aim of this study was to evaluate the efficacy of clinical replanning thresholds, by quantifying actual underdosages in clinical target volumes (CTVs) for all HNC patients undergoing replanning in our clinic over a two-year period. Materials and Methods This retrospective study included all HNC patients treated at one proton therapy center from January 2019 to January 2021. Patients were identified who had a replan due to expected lack of dose coverage for one of the CTVs (according to DAHANCA guidelines); CTV1: Primary tumor, involved nodes with an isotropic margin of 5 mm. CTV2: CTV1 and surrounding volume with high risk of subclinical tumor extension. Gross tumor volume with an isotropic margin of 10 mm. CTV3: CTV2 and regional elective lymph nodes without margin. Assessment of replanning need in the clinic is based on weekly control CT (cCT) in which the original treatment plan is recalculated. A physician assesses whether the plan still meets constraints. If necessary, a new plan is created to recover expected underdosage. We quantified the s of volumes receiving less than 95% of prescription dose within CTV1, CTV2 and CTV3, and the minimum doses in these volumes, in the cCTs corresponding to the replan decision time point. Only volumes ≥ 0.03cm3 were included, and adjacent volumes that were within a distance of 1 cm in any cardinal direction were merged as one volume. Results 58 out of 87 patients had at least 1 replan, whereof 40 were due to expected lack of dose coverage for at least one of the CTVs. 33 out of 40 patients had 1 replan, and 7 patients had 2 replans. Out of the total 47 replans, 16 exhibited underdosage in all three CTVs. 18/47 had lack of dose in two CTVs, 11/47 had lack of dose in one CTV and 2/47 had no lack of dose to any CTV. Figure 1 shows the distribution of s of the largest underdosed volumes as well as the total underdosed volumes. The largest underdosed single volume for all 3 CTVs was 5.32 cm3 in CTV2. CTV3 generally had largest underdosed volumes. Figure 2 shows the four largest underdosed volumes in the 3 CTVs for all patients, as well as minimum doses in these volumes relative to prescribed dose. The figure illustrates that CTV3 had more cold volumes than CTV2, and CTV2 had more than CTV1.