BWH T staging system for CSCC (adapted from Ruiz et al108)
| T stage | Primary tumor (T) |
| T1 | 0 high-risk factors* |
| T2a | 1 high-risk factor* |
| T2b | 2–3 high-risk factors* |
| T3 | 4 high-risk factors or bone invasion* |
*BWH high-risk factors include tumor diameter ≥2 cm, poorly differentiated histology, perineural invasion of nerve(s) ≥0.1 mm in caliber, or tumor invasion beyond subcutaneous fat (excluding bone invasion, which upgrades tumor to BWH stage T3).
BWH, Brigham and Women’s Hospital; CSCC, cutaneous squamous cell carcinoma.