Improving organ at risk sparing in oropharyngeal treatment planning by increasing target dose heterogeneity: A feasibility study.

Target dose homogeneity has historically been a priority in radiotherapy treatment planning. However, in an era of more advanced modulated techniques, there is now greater flexibility in shaping dose distributions suggesting that allowing controlled target dose heterogeneity may consequently improve organ at risk (OAR) sparing. This study sought to determine the feasibility of allowing an increase in target dose heterogeneity in oropharyngeal VMAT plans, and to examine the dosimetric impact this has on target coverage and OARs such as the parotid glands, spinal cord, brainstem and mandible. Nineteen oropharyngeal patients' plans were created with homogeneous dose distributions specified in the London Cancer Head and Neck Radiotherapy Protocol. The upper dose constraint (UDC) objective of the primary planning target volumes (PTV) for each plan were increased in increments of 10% until a maximum of 150% of the prescribed dose was reached. These plans were dosimetrically compared to plans with a uniform dose distribution in terms of OAR sparing and target coverage. Minimal coverage was not compromised, with the largest median changes being a 0.81% decrease [98.6 to 97.8%] to the PTV_70Gy D98% and a 2.86% decrease [99.81 to 96.96%] to the PTV_54Gy D98% at a UDC of 150% of the prescription dose. An OAR sparing effect was observed for the parotid glands, spinal cord and oral cavity sub PTV. Mandible and brainstem Dmax values increased as the PTV UDC increased. Changes in brainstem dose were not statistically significant. All other differences were statistically significant for UDC's above 130%. Target coverage was not compromised as a result of increased target dose heterogeneity. The OAR sparing effect was promising for most organs, however further research with a larger dataset is necessary surrounding the effect on organs that overlap with the PTV.