| ?Objective: To discuss the relationship between parotid/submandibular glands/thyroid glands morphologic changes and radiation dose in nasopharyngeal carcinoma (NPC).Material and Methods: A longitudinal study was conducted on 65 histologically-proven, non-metastatic NPC Patients treated by radical chemo-radiotherapy between February 2008 and December 2009. All patients received CT examination prior to radiotherapy. Subsequent follow up CT scans were performed at 6 months and 12 months after treatment (post-RT). Patients with recurrent or residual diseases who required a second course of radiotherapy were excluded from this study. All patients had no history of head and neck surgery or other salivary glands disease. Images generated from the scanner were transferred to the treatment planning system (TPS) for assessment of the gland volume and the radiation dose.Results: There was an overall decreasing trend in the parotid/submandibular glands/thyroid glands volume post-RT.1.The mean volume of parotid glands was reduced from 39.40±11.08 cm~3 before radiotherapy to 30.00±8.12 cm~3 at 6 months (p<0.01) and 26.55±8.30 cm~3 at 12 months (p<0.01) post-RT, an average reduction of 22.9% at 6 months and 32.1% at 12 months. The relative percentage volume reduction from 6 months to 12 months post radiotherapy was 9.2% and the difference was not statistically significant (p = 0.231). Comparing the percentage volume change of conventional radiotheraphy with those of IMRT,it is showed that only the difference at 6 months post-RT was significant (21.1%:27.2%,p<0.01),but not at 12 months (32.8%:30.6%,p=0.625). The mean parotid glands dose demonstrated a clear correlation with the percentage volume change at 6 months post-RT (p=0.028,r=0.239), but not at 12 months post-RT (p=0.171,r=-0.120). However, the percentage volume change did not show such correlation with the maximum dose received by the thyroid in the 6 and 12 months intervals (p=0.457 and 0.284,r=0.094 and -0.135, respectively). When applying linear regression on the mean doses with the percentage volume change in the group of intensity modulated radiation therapy (IMRT), the best fit curves at 6 months and 12 months post- RT were y = -0.0086x + 35.651 and y = -0.1215x + 32.173 respectively, where x was the percentage volume change and y was the mean dose of the parotid in Gy.2. The mean volume of submandibular glands was reduced from 16.65±4.58 cm~3 before radiotherapy to 10.58±3.10 cm~3 at 6 months (P<0.01) and 9.56±2.94 cm~3 at 12 months (P<0.01) post-RT, an average reduction of 35.3% at 6 months and 42.0% at 12 months. The relative percentage volume reduction from 6 months to 12 months post radiotherapy was 6.7% and the difference was not statistically significant (p = 0.268). Comparing the percentage volume change of conventional radiotheraphy with those of IMRT, there were no statistic differences both at 6 months and 12 months post-RT (p=0.426 and 0.845,respectively). Again at 6 and 12 months after treatment, the difference between percentage volume change and the mean dose was not statistically significant (p = 0.797 and p=0.636, r=0.032 and r=0.060, respectively), as well as in the maximum dose (p=0.128 and p=0.120,r=0.191 and r=0.195, respectively).3. The mean volume of thyroid glands was reduced from 16.50±4.88 cm~3 before radiotherapy to 13.80±4.21 cm~3 at 6 months (P=0.013) and 13.11±4.52 cm~3 at 12 months(P=0.001) post-RT, an average reduction of 15.5% at 6 months and 20.6% at 12 months. The relative percentage volume reduction from 6 months to 12 months post radiotherapy was 5.1% and the difference was not statistically significant (p = 0.738). Comparing the percentage volume change of conventional radiotheraphy with those of IMRT, it is showed that the difference was significant (P<0.01) both at 6 months and at 12 months post-RT.The mean thyroid glands dose demonstrated a clear correlation with the percentage volume change at 6 months and 12 months post-RT (P=0.006 and p=0.002,r=-0.336 and r=-0.380 respectively). However, the percentage volume change did not show such correlation with the maximum dose in the 6 and 12 months intervals (p=0.085 and 0.456,r=-0.215 and -0.094 respectively). When applying linear regression on the mean doses with the percentage volume change, the best fit curves at 6 months and 12 months post- RT were y = -0.245x + 35.221 and y = -0.2821x + 33.199 respectively, where x was the percentage volume change and y was the mean dose of the thyroid gland in Gy.Conclusions: The parotid/submandibular glands and thyroid glands shrunk post-RT, but the submandibular glands shrunk more than the other two glands. The results demonstrated that,parotid glands volume reductions at 6 months post-RT and thyroid glands volume reductions at 6 months and 12 months post-RT correlated significantly with the mean dose (P=0.028,0.006 and 0.002 respectively), but submandibular glands showed negative(p>0.05). This study suggests a reference threshold mean parotid dose of around 35.7 Gy at 6 months and 32.2 Gy at 12 months post-RT. For thyroid, a reference threshold mean thyroid dose of 35.2 Gy at 6 months post-RT, and 33.2 Gy at 12 months post-RT. |
