How big is the actual underdosage for replanned proton therapy head-and-neck cancer patients?
MO-0225
Abstract
How big is the actual underdosage for replanned proton therapy head-and-neck cancer patients?
Authors: Ihsan Bahij1, Ulrik Vindelev Elstrøm1, Cai Grau1, Stine Sofia Korreman2,1,3
1Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 2Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 3Aarhus University, Department of Clinical Medicine, Aarhus, Denmark
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Purpose or Objective
Need for replanning in proton therapy (PT) for head and neck (HNC) cancer is a major challenge due to the sensitivity of proton beams to interfractional changes.
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.
Material 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.
Conclusion
Two thirds of all HNC patients had a replan, and approximately two thirds of these were for dosimetric reasons. Almost half of patients replanned for dosimetric reasons had underdosage in all 3 CTVs. The largest underdosed volumes were found in CTV3, while the smallest underdosed volumes were seen in CTV1. The underdosed volumes for CTV1 and 2 were generally less than 0,5 cm3 for more than 75% of all cases. These results suggest that the applied replanning strategy was adequate and may form the basis for development of objective replanning thresholds.