Simplifying intensity-modulated radiotherapy plans with fewer beam angles for the treatment of oropharyngeal carcinoma

This study is to investigate the feasibility of using fewer beam angles for the treatment of oropharygneal cancer (OPC) with inverse planned intensity-modulated radiation therapy (IP-IMRT) using treatment delivery efficiency, tumor coverage and critical structure avoidance as outcome measures. The treatment plans for five OPC patients previously treated with a Forward-Planned IMRT (FP-IMRT) technique were randomly selected as benchmarks for this study. The treatment goal for these patients were to deliver 70Gy to ≥ 95% of the planning gross tumor volume (PTV-70) and 59.4 Gy to ≥ 95% of the planning clinical tumor volume (PTV-59.4) simultaneously. The five cases were re-planned using IP-IMRT with five different beam angle arrangements, with IP-IMRT plans using seven or more beam angles referred to as complex IP-IMRT plans, and IP-IMRT plans using five beam angles referred to as the simple IMRT plans. The five different IP-IMRT plans were compared to each other, as well as to the FP-IMRT plans, by analyzing the dose coverage of the target volumes, plan homogeneity, the dose-volume histograms of critical structures, and the delivery parameters. When comparing these plans, we found that there was no difference among the complex IP-IMRT, simple IP-IMRT, or FP-IMRT plans for the tumor target coverage (PTV-70: p=0.56; PTV-59.4: p=0.20.). The plan homogeneity, measured by the mean percentage isodose, did not significantly differ among the IP-IMRT and FP-IMRT plans (p=0.08.) All IP-IMRT plans either met or exceeded FP-IMRT in terms of doses to the parotids, spinal cord and brainstem. Simple IP-IMRT plans significantly reduced the mean treatment time when compared to the complex IP-IMRT plans (maximum probability for 4 pairwise comparisons: p=0.0003). In Conclusion, this study demonstrated that simple IP-IMRT accomplishes similar target coverage and reduction of dose to critical structures as complex IP-IMRT when compared to FP-IMRT plans. Simple IP-IMRT significantly improved treatment delivery efficiency. However, this increased efficiency does not necessarily correlate with a decreased total MU.