Pattern of geometric changes of parotid gland in conventional and intensity-modulated radiotherapy in nasopharyngeal cancer patients

Introduction: This study compared the pattern of radiation induced parotid changes between conventional (ConRT) and intensity modulated radiotherapy (IMRT) in nasopharyngeal carcinoma patients. Methods: 56 adult NPC patients treated with IMRT (n=28) and conventional radiotherapy (n=28) were recruited. CT scans were acquired before radiotherapy, at 10th, 20thand 30thfractions, and 3 months after treatment. Parotid gland was delineated in the corresponding CT slices and its mean dose was calculated. The volumetric and geometric changes of the parotid gland at various time intervals were compared against the pre-treatment structure set. The pattern of changes was compared between the two techniques. Results: The mean parotid dose of IMRT (37.5±9.5 Gy) was significantly lower than ConRT (49.1±7.4 Gy). The parotid gland volume, DICE similarity coefficient and lateral dimension of patient head gradually decreased during the radiotherapy course and partially recovered in 3 months post-treatment. The differences between two groups were not significant until at 3 month after treatment, where IMRT showed significantly better volume recovery. Conclusion: Similar parotid gland size and location changes were observed during the treatment course in both ConRT and IMRT. However IMRT demonstrated better parotid volume recovery after treatment.