Purpose: To estimate the alpha/beta ratio for rectal cancer according to the outcome of three fractionation schedules of preoperative radiotherapy.
Methods and materials: Between 1996 and 2002, 168 patients with locally advanced rectal cancer were treated as follows: 53 patients received 25 Gy in 5 Gy per fraction, 45 received 30 Gy in 3.0 Gy per fraction, and 70 were treated with accelerated hyperfractionation (42 Gy, 1.5 Gy per fraction, given twice daily). No patients received concurrent chemotherapy. The clinical characteristics of the groups were comparable. Surgery was performed shortly after radiotherapy. Crude data on locoregional tumor control were fitted directly using a linear-quadratic model, and the actuarial data were analyzed using Cox model.
Results: A linear-quadratic model provided an alpha estimate of 0.339 (SE 0.115) and beta estimate of 0.067 (SE 0.027), which resulted in an alpha/beta ratio of 5.06 Gy (95% confidence interval -0.1 to 10.3). In all three schemes the overall radiation treatment time was short, which limits the rationales for incorporating time effect into the model. If, however, time was incorporated the alpha/beta ratio was 11.1 Gy and the dose increment required to compensate for repopulation was 0.15 Gy/day. The actuarial analysis provided similar alpha/beta estimates.
Conclusion: Although because of the retrospective character of the study, nonrandomized selection of fractionation schedule, and uncontrolled quality of surgery the present results can be regarded as hypothesis generating only, the control rates obtained in the pelvis are consistent with a moderately low alpha/beta ratio for rectal cancer.