While treating patients with postoperative radiotherapy (RT) for squamous cell carcinoma of the oral tongue or floor of mouth, we hypothesized a low risk of tumor recurrence in the contralateral neck in the absence of contralateral neck RT when the primary tumor is >1 cm from the midline regardless of tumor thickness (TT) or depth of invasion (DOI)>4 mm.

The policy at our institution for many years has been to limit postoperative RT to the ipsilateral side when the primary tumor does not cross the midline, regardless of TT or DOI. We report the rate of isolated contralateral neck failure in this group.

We retrospectively reviewed all patients treated with postoperative RT at our institution between 1998 and 2014 for pathologic stage T1-T4 N1-N2b squamous cell carcinoma of the oral tongue or floor of mouth that did not cross the midline and thus were treated with RT limited to the primary site +/- ipsilateral neck.

Our study population included 32 patients: 75% with close (<5 mm) or positive margins and 38% with perineural invasion. No patients had a tumor that crossed the midline. TT was ≥4 mm in 75% of patients. DOI was >5 mm in 68% of patients. There were no isolated contralateral neck recurrences with a median follow-up of 5 years.

While delivering postoperative RT for oral tongue or floor of mouth cancer with pathologic neck stage N0-2b, the risk of not irradiating the contralateral neck is very low when the primary tumor does not cross the midline, regardless of other factors at the primary site, such as TT and DOI.