What dose and fractionation do you use for definitive radiation for SCC in situ of glottic larynx?

Per NCCN, treatment options include endoscopic resection (preferred) or definitive RT. The recommended dose is 60.75 Gy in 27 fractions for in-situ vs. 63Gy in 28fx for T1. Is there available data to suggest that in situ disease can be treated to a lower dose than T1 disease?