Purpose: To estimate the radiotherapeutic dose equivalence of chemoradiotherapy in head and neck cancer.
Methods: The biologic equivalent dose (BED) of radiotherapy in nine trials of standard and five trials of modified fractionated radiotherapy with or without chemotherapy was calculated using the linear-quadratic formulation. Data from Radiation Therapy Oncology Group (RTOG) study 90-03 were used to calculate the relationship (S) between increase in locoregional control (LRC) and increase in BED with modified vs. standard fractionated radiotherapy. The increase in LRC with chemoradiotherapy vs. radiotherapy alone, the BED of the radiotherapy-alone arms, and the "S" value were used to calculate the BED contribution from chemotherapy and the total BED of chemoradiotherapy from each study.
Results: From RTOG 90-03, a 1% increase in BED yields a 1.1% increase in LRC. The mean BED of standard fractioned radiotherapy was 60.2 Gy(10) and 66 Gy(10) for modified fractionation. The mean BED of standard fractionated chemoradiotherapy was 71 Gy(10) (10.8 Gy(10) contributed by chemotherapy). The mean BED of modified fractionated chemoradiotherapy was 76 Gy(10) (10.4 Gy(10) contributed by chemotherapy).
Conclusions: Chemotherapy increases BED by approximately 10 Gy(10) in standard and modified fractionated radiotherapy, equivalent to a dose escalation of 12 Gy in 2 Gy daily or 1.2 Gy twice daily. Such an escalation could not be safely achieved by increasing radiation dose alone.