Abstract
BACKGROUND: To investigate the anatomical and dosimetric differences in target areas and organs at risk (OARs) after adjusting the radiotherapy schedule for nasopharyngeal carcinoma (NPC). METHODS: In total, 34 patients who underwent radiotherapy for NPC were selected. A new localization computed tomography (CT) scan was performed after 25th and before 26th fraction. The target area and organs at risk were redrawn based on the new CT, and the radiotherapy plan was redesigned for the seven remaining rounds of radiotherapy. Plan(1) consists of the old image and old plan, Plan(2) consists of the new image and new plan, and Plan(1-2) consists of the new image and old plan. Data on the volume and dose of target areas and organs at risk were collected for two schedule sets. RESULTS: In the re-scan CT, the volume of Planning Target Volume for Nodal Region Right (PTVnd (R)), Planning Target Volume for Nodal Region Left (PTVnd (L)), left parotid and right parotid decreased significantly (P < 0.05). Additionally, the centroids of the left and right parotid moved towards the centre, abdomen and head. Compared to Plan(1,) the D(95) and V(100) of PTVnd (R) and PTVnd (L) of Plan(1-2) was significantly lower, also significant differences were observed in the D(mean) and D(50) of the left and right parotid, and the D(1cc) of the spinal cord (P < 0.05). In comparison with Plan(2) and Plan(1-2), except for the D(mean) and D(95) of Planning Target Volume nodal extension (PTVnx) and the D(95) of PTVnd (R), significant dosimetric differences were observed in the target area and all organs at risk indicators (P < 0.05). In the re-designed plan, the exposure dose to organs at risk was reduced. CONCLUSIONS: Due to significant anatomical changes in the volume and position of both target volumes and organs at risk during the later fractions of radiotherapy for NPC, the dose coverage of lymph-node regions can no longer meet clinical requirements. Adaptive radiotherapy ensures adequate target coverage while simultaneously reducing the dose to surrounding critical structures.