Segmental IMRT for oropharyngeal cancer in a clinical setting

Background and purpose : To develop a segmental intensity-modulated radiotherapy (IMRT) technique for the treatment of oropharyngeal cancer. Patients and methods : Eight patients previously treated for oropharyngeal cancer were replanned with segmental IMRT. The dose distribution was optimized using beam geometries consisting of 3, 5, 7 and 9 equiangular beams. The optimization procedure resulted in a theoretical fluence for each beam. In order to vary the number of segments, the optimized fluence was divided into four different equidistant levels. The final dose distribution was calculated using clinically deliverable segments obtained from optimized fluence. Results : For our segmental IMRT technique the dose homogeneity within the target volumes improved when the total number of segments increased and reached a saturation level at approximately 150 segments. Seven beams were sufficient to achieve the saturation level for dose homogeneity. The mean dose to the parotid glands depended on the beam geometry and tumor location and did not depend on the number of segments. On average the mean dose to the contralateral parotid gland was 35.7Gy (27.1–39.9Gy) for all seven beam plans. Conclusions : Seven beams are sufficient to achieve an acceptable dose homogeneity within the target volumes and significant parotid sparing. These results will be used to introduce IMRT in routine clinical practice.