Studies of tumour thickness in patients with all clinical tumour and node categories, treated with primary surgery for squamous cell carcinoma of the oral cavity, and with at least two-years of follow-up for those with untreated necks, were eligible for inclusion in the review. Tumour thickness had to be described in relation to the risk of cervical lymph-node involvement. The term positive lymph-node declaration was used to describe different methods of detection of cervical lymph-node involvement at immediate or delayed time points.
The included patients had various disease sub-sites, including oral cavity, tongue, buccal mucosa, floor of mouth, and lower lip. Most patients were classified as tumour (T)1, T2 or both cases. Tumour thickness was clearly defined in the majority of studies, and (where reported) it was measured using an ocular micrometer. All studies used formalin-fixed, paraffin-embedded tissue samples from primary surgery. Excluded studies were of patients with non-oral cavity cancers or histologies other than squamous cell carcinoma; those treated primarily with nonsurgical therapies; those with adjuvant or neoadjuvant treatment (other than surgery) for the untreated neck; those with tumour thickness measurement other than upon pathologic examination; or where absolute numbers were not reported.
Two independent reviewers selected studies for inclusion, and disagreements were resolved by reassessing the article.