Purpose: To correct several elementary radiobiologic errors in the otherwise admirable article by Kasibhatla, Kirkpatrick, and Brizel (2007) on estimating the equivalent radiation effect of the concomitant chemotherapy in head-and-neck chemoradiotherapy.
Methods and materials: (1) Their equation was wrong because it omitted the lag or onset time of repopulation in tumors, Tk. Instead of zero days this should be 18-35 days. (2) Instead of a doubling time of 5 days, at most 3 days should be used for head-and-neck tumors. (3) Their slope "S" (the gamma-50 slope) for head-and-neck tumors should be 1.7, not 1.1. The same percentages of increased locoregional control as quoted by Kasibhatla et al. are used.
Results: The average time-corrected biologically effective dose for the 16 schedules listed should be 72.4 instead of 63.1 Gy(10). The average gains in locoregional tumor control are the equivalent of 8.8 Gy(10), not 10.6 Gy(10) (p = 0.05).
Conclusions: The equivalent number of 2-Gy fractions of concomitant chemotherapy as used in the 16 listed schedules is 3.6 (95% confidence interval, 2.7-4.1), not 5 as claimed by Kasibhatla et al. The difference is statistically significant (p < 0.001).