Dose at the optic nerve can be reduced with CT-simulation in treatment planning for nasopharyngeal carcinoma / 中华放射肿瘤学杂志

ABSTRACT

Objective To compare and evaluate dose at optic structures (eyeballs, optic nerves and chiasma) between conventional and CT simulated treatment planning for nasopharyngeal carcinoma. Methods Thirty three patients with T1 to T4 nasopharyngeal carcinoma underwent simulated spiral CT with picker PQ 5000 and AcQ plan 4.1.1 software system. At first irradiation portals were delinated by the simulation films taken on the conventional X ray simulator using the digital reconstruction (DRR) of the treatment planning system (TPS) supplemented by the patient's CT (or MRI) and clinical findings. Then, the gross tumor volume (GTV),clinical target volume (CTV) and planned target volume (PTV) were delineated on every section of the CT scans. At the same time, the silhouette of the nearby important organs was drawn out ,paying special attention to the backward passage of the retrobulbar optic nerves up to the optic chiasma. The final portals were designed by the 3 dimensional relation between the tumor and its surrounding organs. Then dose distribution was separately calculated by two independent TPSs,with 100% at the target center. Finally,the maximum dose (D max ),average dose (D ave ) and median dose (D med ) of the tumor target area, eye ball,optic nerve and also the length of optic nerve included within the portal of these two technics were compared.Results The dose distribution in nasopharyngeal GTV and CTV were satisfactory both in CT simulated and conventional planning. However, the D max ,D ave and D med all showed significant differences. Those by CT simulated planning were obviously lower than those by conventional planning. CT simulated planning was able to spare more optic nerve by leaving it out of the irradiation portal,thereby,avoiding unwanted radiation.Conclusion CT simulated treatment planning is able to reduce the superfluous radiation to the optic nerve in external irradiation of nasophargngeal carcinoma.