Clinical radiobiology of glottic T1 squamous cell carcinoma

Radiation therapy is the treatment of choice for early glottic squamous cell cancer in many institutions over the world. Despite a relatively homogenous clinical model of T1 glottic tumors for the fractionation studies, the relationships between dose-time parameters remain unclear. To analyze the influence of fractionation parameters and hemoglobin level on tumor cure, this study has been performed.

This is a retrospective review of 235 patients with T1N0M0 glottic cancer treated by radiation therapy alone given in a conventional schedule with 5 fractions each week. The individual total dose, dose per fraction, and overall treatment time (OTT) ranged from 51-70 Gy, 1.5-3.0 Gy, and 24-79 days, respectively. The median follow-up was 48 months. Patient data--total dose, dose per fraction, OTT, and hemoglobin level (Hb) measured before the radiation treatment--were fitted by the mixed LQ/log-logistic model.

The 5-year local relapse-free survival rate was 84%. All parameters included in the mixed LQ/log-logistic model improved the fit significantly. The dose-response curve for 235 patients with T1 glottic cancer was well defined and steep, and showed significant decrease in tumor control probability (TCP) when total doses were below 61 Gy. The 10-day prolongation of OTT, from 45 to 55 days, decreased the TCP by 13%. The dose of 0.35 Gy/day, compensated repopulation during the 1 day of prolongation, which indicates a potential doubling time (Tpot) for glottic T1 tumor clonogens of 5.5 days. The drop of Hb level of 1 g/dl (from 13.8 g/dl to 12.8 g/dl) gave a 6% decrease of TCP, provided that OTT was 45 days.

The significant correlation between the total dose, overall treatment time, hemoglobin concentration, and tumor control probability has been found for T1 glottic cancer.