The relationship between the parotid glands function and the dose-volume effect in nasopharyngeal carcinoma patients with intensity-modulated radiation therapy

Abstract

To study the preservation of parotid glands function and relationship between parotid glands function and dose-volume histogram (DVH) in nasopharyngeal carcinoma (NPC) patients treated by intensity modulated radiation therapy (IMRT). From August 2002 to December 2004, the excretion index (EI) and uptake index (UI) of parotids in 48 NPC patients underwent radical IMRT was examined by ECT at the beginning, the end of and the 3 months after radiotherapy. The relationship between parotid function (EI and UI) and DVH were analyzed. The mean doses to the contralateral parotid and ipsilateral parotid were 22.8 +/- 4.5 Gy and 31.9 +/- 4.1 Gy, respectively. The symptom of xerostomia was mild at the end of radiotherapy. ECT showed EI of contralateral parotid were 0.35 +/- 0.25, 0.31 +/- 0.24 and 0.33 +/- 0.22 at the beginning, the end of and 3 months after radiotherapy (RT), respectively. UI were 7.12 +/- 3.56, 5.81 +/- 2.25 and 5.72 +/- 2.81 at the same intervals. This shows no statistical difference. The EI and UI of ipsilateral parotid at the completion of radiotherapy declined significantly (0.21 +/- 0.16 and 4.87 +/- 2.45, respectively) compared with those of pre-treatment (0.36 +/- 0.27 and 8.02 +/- 3.89, respectively) (P < 0.05). DVH showed: at the end of RT, the EI was significant difference between mean dose < 26 Gy and > or = 26 Gy group (P = 0.009) and decreased significantly in the group of V25 (the percentages of parotid volume irradiated with < 25 Gy) > or = 50% compared with the group of V25 < 50% (P < 0.01). The UIs were no significant difference in two groups (P > 0.05). 26 Gy is a threshold dose for the preservation of parotid glands function. There is also a threshold volume irradiated for the preservation of the parotid glands function. Based on the precondition of assuring significant dose to the target volume (PTV), we should reduce the irradiated volume and dose to parotid glands as possible as we can so as to preserve its function.