Radiotherapy in Squamous Cell Carcinoma of the Lip: A Systematic Review of Reported Practices, Clinical Outcomes, and Gaps in Consensus.

Background/Objectives: Squamous cell carcinoma (SCC) of the lip occupies an ambiguous position between cutaneous and oral-cavity oncology and lacks dedicated radiotherapy guidelines, resulting in considerable practice variability. We systematically reviewed the reported radiotherapy practices, clinical outcomes and gaps in consensus at this site. Methods: Following PRISMA 2020 and prospectively registered in PROSPERO (CRD420261349994), we searched PubMed/MEDLINE, Scopus, the Cochrane Library (Trials) and ClinicalTrials.gov (20 March 2026). Studies of at least ten patients with lip SCC treated with external beam radiotherapy or brachytherapy were eligible. Risk of bias was assessed with ROBINS-I; the synthesis was narrative, following the Synthesis Without Meta-analysis (SWiM) reporting framework. Results: Nine studies (approximately 3904 patients; 1964-2021; six countries) met the criteria. Interstitial brachytherapy was the dominant modality (seven studies), with high local control (approximately 93-97% in modern actuarial series), favourable toxicity and good functional/aesthetic outcomes; external beam radiotherapy served mainly as adjuvant treatment. Doses, fractionation, nodal management and outcome reporting were markedly heterogeneous, and eight of nine studies were at serious risk of bias. Conclusions: Radiotherapy-in the identified literature, in effect, largely historical interstitial brachytherapy-appears to be an effective, organ-preserving option in early-stage lip SCC; no conclusions can be drawn for modern external beam techniques such as IMRT. The limited, brachytherapy-dominated evidence warrants cautious interpretation and justifies protocol standardisation, a core outcome set with validated quality-of-life instruments, and multi-institutional prospective registries.