A Preliminary Study On Effect Of Split-Parotid Delineation Approaches On Dose Of Target And Parotid Gland In IMRT And Parotid Function In Patients With Nasopharyngeal Carcinoma

Part one Effect of Split-Parotid Delineation Approaches on Target and Parotid Gland of IMRT in Patients with Nasopharyngeal CarcinomaPurpose:Delineating whole parotid gland of patients with nasopharyngeal carcinoma（NPC）undergoing intensity modulated radiotherapy（IMRT）is not the optimal way to protect parotid function,therefore,we want to explore the dosimetric effect of three split-parotid delineation approaches on the target volume and parotid gland in IMRT for NPC.Methods:Seven cases of nasopharyngeal carcinoma were used to design4 IMRT plans,respectively,with the same prescription dose for tumor target and OARs（organ at risk）except of the parotid gland.Plan⁰:Whole parotid gland was outlined.The dose volume constraint of L-Parotid was D₅₀<30Gy and R-Parotid was D₅₀<30Gy.Plan¹:Parotid gland was divided into superficial lobe（Parotid-S）and deep lobe（Parotid-D）along the posterior border of the mandible.The dose volume constraint of whole parotid gland is the same as plan⁰,meanwhile the extra dose volume constraint for Parotid-S and Parotid-D was provided,which was D₅₀<25Gy.Plan²:Parotid gland was divided into target-overlapping lobe（Parotid-O）and non-target-overlapping lobe Parotid-NO)according to the contour line of PTV2.The overall parotid dose volume constraint is the same as plan⁰,at the same time the extra dose volume constraint for Parotid-NO and Parotid-O was given,which was D₅₀<25Gy.Plan³:Parotid gland was divided into front sub-segment（Parotid-F）and back sub-segment（Parotid-B）along a line through anterior margin of the retromandibular vein,The overall parotid dose volume constraint is the same as plan⁰,in the meantime the extra dose volume constraint for Parotid-F and Parotid-B was given,which was D₅₀<25Gy.The four plans were evaluated in terms of tumor volume and OARS,especially for parotid gland.Results:All 4 plans were in compliance with the target and normal tissue dose requests.There were no statistical differences among 4 plans for D_meanean and D₅₀.The mean dose(D_mean)of both sides parotid for plan¹,2,3,2,3 were below plan⁰.The exposure dose of both sides parotid for plan² was D_mean<31Gy,D₅₀<28Gy,which all were below plan⁰.Compared to Plan⁰,Plan²reduce most D_mean and D₅₀ of the left and right parotid glands.Conclusions:Under the premise of meeting the dosage requirements of target and OARs,split-parotid delineation by approach 2 is likely to minimize D_mean and D₅₀ of parotid gland significantly.Whether the delineation approach can reduce the radiation-induced parotid gland function damage is worth further research.Part two A Preliminary Study of non Target Overlapping Parotid Lobe-Sparing Delineation Approach for Nasopharyngeal Carcinoma Patients with Parapharyngeal Space Invasion and/or Lymph Node Metastasis in RegionⅡof Neck on the Parotid Gland FunctionPurpose: Exploring the effect of non target overlapping parotid lobe-sparing delineation approach on the function of parotid gland during and after radiotherapy in patients with nasopharyngeal carcinoma.Methods: In the test group,20 nasopharyngeal carcinoma patients with parapharyngeal space invasion and/or lymph node metastasis in regionⅡof neck were selected.Parotid gland was divided into target-overlapping lobe（Parotid-O）and non-target-overlapping lobe（Parotid-NO）according to the contour line of P-CTV2.Considering the Parotid-NO as organ at risk（OARS）,and the dose volume constraint was D50<30Gy.In the same enrollment condition,21 patients were selected for control group.In the control group,the whole parotid gland was outlined as OARS.The dose volume constraint of parotid was D50 <30Gy.Acute xerostomia scores according to the criteria of RTOG were recorded before radiotherapy,during radiotherapy（15 fractions）,at the end of radiotherapy,1 and 3 months after radiotherapy.Results: Firstly,it was statistical difference between the two groups that the subjective xerostomia was followed up at 1 month after radiotherapy.Secondly,on the whole,the percentage of grade 2 subjective xerostomia of the test group was more than the control group at each time point.The percentage of grade 3 subjective xerostomia of the test group was less than the control group and showed a gradual downward trend.These changes were more obvious at the end of and 1 month after radiotherapy.Conclusions: Only the protecting of the parotid gland outside the target area can significantly reduce the occurrence of grade 3 subjective xerostomia and relieve xerostomia symptom within 3 months after radiotherapy.This parotid gland delineation approach is worth further promoting for these NPC patients.