According to the eighth edition of the AJCC Cancer Staging Manual (AJCC8), a depth of invasion (DOI) >10mm is classified as pT3, representing a locally advanced tumour requiring postoperative radiotherapy (PORT). When node-negative, however, evidence regarding whether PORT improves loco-regional control or survival is unclear. To clarify this, two cohorts of patients were studied: (1) patients classified as pT3N0 by the seventh edition of the AJCC manual (AJCC7), with DOI >10mm and a tumour diameter >4cm (17 patients who received PORT), and (2) patients classified as pT1N0 and pT2N0 by AJCC7, with DOI >10mm and a tumour diameter <4cm (55 patients who did not receive PORT). Loco-regional control and survival were analysed. PORT was found not to impact overall survival or disease-free survival. It was also found not to impact local, regional, or distant recurrence. Although the two subsets of patients considered here (DOI >10mm with tumour diameter below or above 4cm) were previously distinct, they are both considered pT3 in AJCC8. Data from this study indicate that the routine administration of PORT to patients with a DOI >10mm may not be warranted in the absence of other risk features such as nodal disease or close margins.