Refining Elective Contralateral Neck Radiation Volumes for Oropharynx Cancer.

PURPOSE/OBJECTIVE(S) Judicious reduction in elective nodal volumes for patients with oropharyngeal squamous cell carcinoma (OPSCC) has the potential to reduce acute and long term toxicity while maintaining high rates of regional control. We postulated that the contralateral N0 neck region at highest risk for containing occult microscopic lymph node (LN) metastases in patients with OPSCC could be best defined by the location of the initial positive ipsilateral LN. We sought to identify and target this N0 elective neck region and assess impact on dose to organs at risk compared to using a consensus-defined contralateral elective nodal volume. MATERIALS/METHODS We performed a retrospective review to identify patients with OPSCC and N2c disease as defined by a single radiographically positive LN. LN centroids were used to define the anatomic boundaries of the novel contralateral elective clinical target volume (ceCTV). Factors associated with LN centroid locations were assessed by the Fisher exact test. We used an external testing cohort to evaluate the robustness of ceCTV. The dosimetric impact of ceCTV compared to consensus elective nodal clinical target volumes (conCTV) was assessed with the t-test. RESULTS We identified 25 patients that fit the selection criteria of which 84% had a single contralateral LN in level II and 16% in level III. We did not identify factors associated with LN centroid location. In the external data set, 97% (33/34) of the LN centroids were located within the novel elective contralateral target volume. Mean radiation dose to the contralateral parotid gland was 15.1 Gy (range 13.0-17.0 Gy) versus 21.0 Gy (range 18.0-24.0 Gy), P < 0.001 using the novel versus consensus elective contralateral nodal clinical target volumes, respectively. CONCLUSION We present a systematic approach for considering the contralateral nodal regions at highest risk of harboring subclinical disease in patients with oropharyngeal squamous cell carcinoma. These data support clinical study to evaluate this novel contralateral elective clinical target volume in patients with oropharyngeal squamous cell carcinoma who warrant elective contralateral neck radiotherapy.