Purpose: This study tested the Pearson correlation coefficient (PCC) and the change in beam path length (BPL) on daily cone beam (CBCT) images as adaptive planning indicators in proton beam therapy (PBT) for head-and-neck (HN) tumors. PCC gives information about tumor morphological changes, whereas BPL change gives information about changes in tissue.
Methods: We retrospectively analyzed CBCT images of 247 HN patients for the measurements of BPL change and analyzed (BPL group). Each beam was ray traced from the isocenter to the skin surface along the central axis using the daily CBCT images. 148 CBCT images of 20 HN patients who were adaptively replanned during the course of treatment for PCC (PCC group). The target volumes were mapped on the CBCT images and the PCC values were obtained between first and the successive CBCT images.
Results: Total 91 patients among BPL group were replanned. 87% of them demonstrated the change of BPL > 3 mm. The sensitivity of the BPL threshold 3mm is 90%. The average and the median PCC values for PCC group were 0.45 and 0.50, respectively, on the day when a QACT was triggered. The PCC threshold for replan is determined to be 0.5 so that half of the replanning group belongs to this criterion. Among the PCC group, 16 demonstrated the BPL change >3mm, which suggest replan per the BPL threshold and 4 showed less. Two of 4 cases showed PCC less than 0.5. By combining both of the BPL and the PCC thresholds over all sensitivity is 95%.
Conclusion: The Pearson correlation coefficient (PCC) of tumor morphology and/or beam path length (BPL) change from isocenter to the skin surface along beam central axis on CBCT images can be used as an adaptive plan indicator for proton beam therapy (PBT) HN patients with 95 % sensitivity.
TH- External Beam- Particle/high LET therapy: Proton therapy - adaptive replanning