Selection of head and neck cancer patients for adaptive replanning of radiation treatment using kV-CBCT

Abstract

Purpose; To develop a CBCT-based method for selection of head and neck cancer (HNC) patients that may require adaptive replanning. Materials and Methods; We retrospectively analyzed 11 HNC patients with different degree of weight loss during the 7 weeks of IMRT treatment. Weight loss related dosimetric parameters were determined using CBCT and verified against new CT scans acquired during adaptive replanning process. The planning CT body surface in the head and neck region was modified using the CBCT body surface. The target volumes (PTV) and organs at risks (OAR) were modified where necessary following the body shrinkage. Anatomical and dosimetric changes were studied using the resulting modified CT. Dose calculation was performed using the parameters of the original treatment plan with the modified CT structure set using Eclipse treatment planning system. Action levels for adaptive replanning were developed evaluating dosimetric changes in all structures. Results; The largest volume loss occurred in the lateral neck, the cheeks and chin regions. The correlation % dose versus % volume changes is quasi-linear and patient/structure-specific. A 10 mm loss in the neck average radius corresponded to an increment in the dose to the PTVs/OARs of up to 4%. Deep PTV’s maintain good coverage (V100 > 95%) during the treatment but PTV coverage is reduced in shallow regions. Conclusion; The estimated dosimetric changes obtained for CBCT and replanning CT are consistent. The anatomical and dosimetric changes in HNC patients derived from CBCT can be used to guide decision-making as to when during RT treatment replanning is necessary or optional.