Technical challenges of sparing infrahyoid swallowing organs at risk in oropharynx squamous cell cancer treated with IMRT.

This study reports clinical performance in the sparing of infrahyoid swallowing organs at risk (SWOARs) in oropharynx cancer intensity-modulated radiation therapy (IMRT) plans. Rates of meeting dose-volume planning goals are reported and compared with geometry-based estimates of what is achievable. This study also develops 3 measures of target-SWOAR geometry and tests their usefulness in providing geometry-based dose-volume planning goals. A total of 50 oropharynx cancer IMRT plans were reviewed. Success rates in meeting institutional dose-volume goals were determined for the glottic larynx (G), postcricoid pharynx (P), and esophagus (E). The following 3 measures of target-SWOAR geometry were investigated as methods of identifying geometry-based planning goals: presence of gross disease in neck levels 3 to 4, target-SWOAR overlap, and a 3-dimensional (3D) measure of target-SWOAR geometry. Locally advanced disease was predominant in this patient population with target volumes overlapping SWOARs in 68% to 98% of cases. Clinical rates of success in meeting dose-volume goals varied by SWOAR (16% to 82%) but compared well with estimated potentially achievable rates in most cases (14% average difference between clinical and potential). Cases grouped by the presence of levels 3 to 4 neck nodes or target-SWOAR overlap did not have significantly different SWOAR doses. Cases grouped using a 3D measure of target-SWOAR geometry differed significantly, providing useful geometry-based planning goals (e.g., mean Glottis dose <45Gy was achieved 19%, 44%, or 81% of the time in each of 3 groups). This study describes the technical challenge of sparing SWOARs and investigates several potential methods for grouping cases to assist with treatment plan evaluation. Quantifying the 3-D relationship between the targets and SWOARs is a promising way of approaching this complex problem. Data presented in this paper may be useful to evaluate treatment plans using objective geometry-based goals.