Initially accelerated fractionation of the R1H tumor of the rat increases local tumor control and might reduce impact of overall treatment time.
OBJECTIVE
To study the effect of treatment time prolongation following initial dose acceleration on the response of subcutaneously growing R1H tumor.


METHODS
Continuous standard fractionation (30 fractions/40 days) was compared to initially accelerated treatment (30 fractions/21 days) followed by five to two fractions per week yielding total treatment times from 40 to 72 days. Local tumor control was assessed as endpoint.


RESULTS
Radiation dose to control 50% of the tumors (TCD50%) decreased statistically significant from 83.5 Gy (95% confidence interval [CI]: 78.6 .. 88.4) for standard fractionation to 74.1 Gy (95% CI: 72.7 .. 75.5) determined for all accelerated treatment arms (p = 0.003). Prolongation of treatment time after initial acceleration from 40 to 72 days led to a small but statistically not significant increase in TCD50% from 72.0 Gy (95% CI: 71.0 .. 72.9) to 76.2 Gy (95% CI: 69.9 .. 82.4) corresponding to a repopulated dose of 0.9 Gy per week. This time factor is considerably smaller than for conventional radiation treatment as determined in previous experiments.


CONCLUSIONS
The results indicate that initially accelerated irradiation not only improves local tumor control but also minimizes the negative effect of treatment time prolongation. This might be due to changes in tumor cell repopulation kinetics.
Raabe
Annette
A
Laboratory for Radiobiology and Experimental Radiation Oncology, Department of Radiotherapy and Radiation Oncology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany. araabe@uke.uni-hamburg.de
Petersen
Cordula
C
Krüll
Andreas
A
D000818:Animals; D019583:Dose Fractionation, Radiation; D004305:Dose-Response Relationship, Drug; D051381:Rats; D012207:Rhabdomyoma / Q000473:pathology* / Q000532:radiotherapy*; D013997:Time Factors; D016896:Treatment Outcome