[What is the impact of IMRT of nasopharyngeal carcinomas on glandular structures?].

PURPOSE: The aim of this work is to evaluate the anatomical changes of the glandular structures during the NPC IMRT and to study their dosimetric impacts. PATIENTS AND METHODS: Twenty patients receiving IMRT for NPC were included. For each patient, a second dosimetric CT was performed at a dose of 38Gy, which was fused with the initial planning dosimetric CT. We calculated the volume percent change, the positional and dosimetric variation between the 2 scanners for the glandular structures (parotid, submaxillary, thyroid and pituitary). RESULTS: We observed a decrease in the volume of right and left parotids (-27.9% and -27.54%). It was correlated with the initial dose planned at its level. For the sub maxillary glands, the decrease was -36.1% on the right and -27.28% on the left. The value of reduction of the thyroid gland was -18.01%. A medial supra-millimeter migration of 2 and 1.15mm was found for right and left parotid glands respectively, correlated with GTV N reduction volume. We found a significant increase in mean doses for the parotid glands. It was 1.8+/-2.3Gy for the right and 1.5+/-2.7Gy for the left. For the right sub maxillary gland, the increase was about 0.35+/-2Gy and 3.79+/-5.2Gy for the thyroid. CONCLUSION: The modifications observed for glandular structures during NPC IMRT can explain the different toxicities caused by radiation. It seems also that a careful adaptation of the treatment plan should be considered during therapy.