What is the preferred dose and fractionation regimen for cutaneous squamous cell carcinoma of the thumb?  

No level-1 evidence likely exists for this relatively rare scenario, so common oncologic sense applies here: If appendage-conserving wide local excision (WLE) can be offered, do it. Post-surgical adjuvant RT is indicated if surgical pathology shows high-risk features for loco-regional recurrence...

Depending upon the tumor size and local anatomy, hypofractionated (40 Gy in 8 Fxs) electronic brachytherapy could be considered.