Anatomical changes and dosimetric analysis of the neck region based on FBCT for nasopharyngeal carcinoma patients during radiotherapy.

BACKGROUND:The study aimed to investigate anatomical changes in the neck region and their impact on dose distribution in patients with nasopharyngeal carcinoma (NPC) undergoing intensity modulated radiation therapy (IMRT), as well as to determine the optimal time for replanning during treatment. METHODS:Twenty NPC patients received IMRT with weekly pretreatment in-room kV fan beam computed tomography (FBCT) scans. Metastasized lymph nodes in the neck region and organs at risk (OARs) were recontoured based on the FBCT scans. The original treatment plan (PLAN0) was copied to each FBCT scan to create new plans of PLAN 1-6, correspondingly. The dose-volume histograms (DVH) of the new plans and the original plan were compared. One-way repeated measure ANOVA was employed to define threshold(s) at any timepoint. The presence of a threshold(s) would indicate significant anatomical change(s) such that replanning should be suggested. RESULTS:Progressive volume reductions in the neck region, gross target volume for metastatic lymph nodes (GTVnd), submandibular glands, and parotids were observed over time. Compared to PLAN0, Dmean of GTVnd-L significantly increased in PLAN5, while the D95% of PGTVnd-L showed a significant decrease from PLAN3 to PLAN6. Similarly, the Dmean of GTVnd-R significantly increased from PLAN4 to PLAN6, whereas the D95% of PGTVnd-R exhibited a significant decrease from PLAN3 to PLAN6. Furthermore, a gradual increase in the dose delivered to the bilateral parotid glands, bilateral submandibular glands, brainstem, and spinal cord from PLAN0 to PLAN6. CONCLUSION:Significant anatomic and dosimetric changes were observed in the target volumes and OARs. Based on the identified thresholds, replanning at approximately 20 fractions is crucial to ensure adequate target volumes dose and avoid overdosing to the OARs. This approach is clinically feasible and strongly recommended, particularly for centers without access to an adaptive planning system.