INFLUENCE OF RADIOTHERAPY TREATMENT TIME ON CONTROL OF LARYNGEAL-CANCER - COMPARISONS BETWEEN CENTERS IN MANCHESTER, UK AND TORONTO, CANADA

A comparison has been made of the influence of treatment time on tumou r control rates for 496 (T-2 and T-3) larynx cancer cases in Mancheste r, UK and 1001 (T-1-T-4) cases in Toronto, Canada. Both series of pati ents were treated in fairly short overall times, commonly 3 weeks in M anchester and 4-5 weeks in Toronto. All the tumour control data were a nalysed using the same method to obtain values of fitted dose, fractio nation and time parameters. The analysis showed the following. (a) Dif ferences between the total combined (T-2 + T-3) data sets from the two centres, fitted using direct analysis and the LQ model incorporating a parameter for overall treatment time, were not significant (p = 0.17 ) and close similarity in control rates was observed using treatment r egimens common to both series. (b) The Manchester series over 9-41 day s and the Toronto series over 14-84 days are both consistent in showin g for (T-2 + T-3) tumours the presence of a mean time factor of 0.6-0. 8 Gy/day required to abrogate the decrease in tumour control concomita nt with an increase in overall treatment time from the minimum to the maximum employed in each series. (c) When a parameter was included in the model to test for the possible presence of a lag period before the time factor became operative, the lag was not significant for the Tor onto data, in contrast to a significant lag for the Manchester data al one (T-2 + T-3 data). Hence, even with the large combined number of 10 73 (T-2 + T-3) patients from the two centres it was not possible to re solve separate time factors for different periods of the overall treat ment times. Also, there was no significant difference overall between the responses of glottic versus supraglottic tumours. (d) The data for all stages (T-1 - T-4) in Toronto and T-3 (but not T-2) in Manchester are compatible with considerable heterogeneity among the population o f tumours which results in a fairly flat dose-response curve (mean D-a about 30 Gy, albeit with very wide confidence intervals). Hence the t ime factor could be interpreted as a reflection of a low 'effective' r ate of increase in tumour clonogen number during treatment for the pop ulation of patients as a whole, with a higher and unknown rate for ind ividual patients.