SU-F-T-454: Dose-Mass-Histogram Sensitivity to Anatomical Changes During Radiotherapy for HNSCC.

PURPOSE: To determine the sensitivity of dose-mass-histogram (DMH) due to anatomical changes of head-and-neck squamous cell carcinoma (HNSCC) radiotherapy (RT).

METHODS: Eight patients undergoing RT treatment for HNSCC were scanned during the third and sixth week of RT. These second (CT2) and third (CT3) CTs were co-registered to the planning CT (CT1). Contours were propagated via deformable registration from CT1 and doses were re-calculated. DMHs were extracted for each CT set. DMH sensitivity was assessed by dose-mass indices (DMIs), which represent the dose delivered to a certain mass of and anatomical structure. DMIs included: dose to 98%, 95% and 2% of the target masses (PTV1, PTV2, and PTV3) and organs-at-risk (OARs): cord DMI2%, brainstem DMI2%, left_ and right_parotid DMI2% and DMI50%, and mandible DMI2%. A two-tailed paired t-test was used to compare changes to DMIs in CT2 and CT3 with respect to CT1 (CT2/CT1 and CT3/CT1).

RESULTS: Changes to DMHs were found for all OARs and PTVs, but they were significant only for the PTVs. Maximum dose to PTVs increased significantly for CT2/CT1 in all three PTVs, but CT3/CT1 changes were only significantly different for PTV1 and PTV2. Dose coverage to the three PTVs was also significantly different, DMI98% was lower for both CT2/CT1 and CT3/CT1. DMI95% was significantly lower for PTV1 for CT2/CT1, PTV2 for CT2/CT1 and CT3/CT1, and PTV3 for CT3/CT1.

CONCLUSION: Changes in anatomy significantly change dose-mass coverage for the planning targets, making it necessary to re-plan in order to maintain the therapeutic goals. Maximum dose to the PTVs increase significantly as RT progresses, which may not be problematic as long as the high dose remains in the gross tumor volume. Doses to OARs were minimally affected and the differences were not significant.