The Dosimetric Outcome of a Rotational Planning Target Volume in Patients With Oropharyngeal Cancers

Abstract

An isotropic expanded Planning Target Volume (PTV) neglects patient's off-axis rotation. This study designs a rotational PTV that is used instead of the standard 3-mm Clinical Target Volume (CTV) expanded PTV in oropharyngeal cancers with the goal to reduce pharyngeal constrictor muscle (PCM) mean dose. 10 patients were retrospectively evaluated. For off-axis rotation, the image was rotated around the longitudinal axis (cervical spinal canal) ± 5 degrees. These new CTVs were combined to form the rotational PTV. The standard and rotational treatment plans were designed with the goal to keep the superior and middle PCM-CTV70 mean dose to less than 50 Gy. There were a 355 cGy reduction in the superior PCM mean dose (form 5332 to 4977 cGy) and a 506 cGy reduction in middle PCM mean dose (from 4185 to 3679 cGy). 60% of patients may have at least a 20% reduction in dysphagia probability based on a Normal Tissue Complication Probability (NTCP) formula. The superior and middle PCM mean dose were reduced to less than 50 Gy in 40 and 20% of cases. There was an association between superior PCM mean dose and overlap volume of PTV70 and superior PCM in both standard (r = 0.92, p = 0.001) and rotational (r = 0.84, p = 0.002) plans. This association was present for middle PCM and PTV70 (r = 0.52, p = 0.02 and r = 0.62, p = 0.006). Rotational PTV can lower the mean dose to superior and middle PCMs, ultimately leading to lower dysphagia rates.