11% J. Radiation Omd~Biol. Phys.. Vol. 10, pp. 1995-1996 Ptinted in the U.S.A. All ri@s rexwed.

0360.3016/84 $03.00 + .X0 Copyright C 1984 F’crgamon Press Ltd.

??Correspondence

LE’ITER TO THE EDITOR

To the Editor: Walker and Suite’ indicate two reasons for not considering a proportional haxards analysis as a method for employing censored survival data in dose-response assays. They are:

6. . . . first bccausc the hazards of local failure in different treatment groups do not stand in a fixed ratio to one another over time; second because the proportion of tumors controlled at the end of the fixed observation time, is, in fact, the measurement of interest, and proposed methods for calculating survival curves from pro- portional hazards coefficients presuppose a known failure time distribution.*’

The purpose of this letter is to point out that, while the authors are strictly correct in their first reason, the proportional hazards model developed by Cox’ can be modified in such a manner that it is applicable in situations where the ratio hazards are time dependent. By allowing for time by treatment interactions in the n relative hazard Cox’s model becomes non-proportional and can be used to advantage in the situation where hazards do not stand in a fixed ratio to one another over time. We have used this in the preclinical evaluation of a combination of cytotoxic agents6 and in the analysis of a clinical trial.’ Walker and Suit are too harsh in their second criticism of proportional hazards models. Several authors’.‘.’ have addressed the estimation of the underlying hazard function so that once an estimate of the model parameters has been obtained one can indeed estimate the survival curves. Again we have reported on this6

With the exception of the quotation given in this letter, we found the paper by Walter and Suit interesting and feel that it makes a nice contribution by showing how improved estimates of the response rate can be obtained. However, we were concerned that if their statement concerning proportional hazard models went unchallenged, some readers would not appreciate the versatility of such models.

DONALD M. STABLEIN

The Emmes Corporation Potomac, MD 20854

WALTER H. CARTER JR. Department of Biostatistics Medical College of Virginia Virginia Commonwealth University Richmond. VA 23298

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Breslow, N.: Analysis of survival data under the proportional haxards model. Int. Star. Rev. 43: 45-58, 1975. Carter, W.H., Jr., Wampler, G.L., Stablein, D.M.: Regression Analysis oJSurvival Dara in Cancer Chemotherapy. New York, Marcel Dekker, Inc., 1983. Cox, D.R.: Regression models and life tables. J. Roy. Stat. Sot. B 34: 187-220, 1972. Oakes, D.: Discussion oJcox. J. Roy. Stal. Sot. B 34: 187-220, 1972. Stablein, D.M., Carter, W.H., Jr., Novak, J.W.: Analysis ofsurvival data with nonproportional hazard functions. Con!. C/in. Trials 2: 149-159, 1981.

6. Stablein, D.M., Carter, W.H., Jr., Wampler, G.L.: Survival analysis of drug combinations using a hazards model with time-dependent covariates. Biometrics 36: 537-546, 1980.

7. Walker, A.M., Suit, H.D.: Assessment of local tumor control using censored tumor response data. Int. J. Radial. Oncol. Biol. Ph.vs. 9: 383-386, 1983.

RESPONSE

To fhe Editor: Stablein and Carter discussed in their letter our rejection of a “proportional haxard analysis as a method for employing censored survival data in dose response assays”. In the particular system which we addressed, biological interest has focused on the cumulative recurrence (or control) of tumors at the end of a fixed follow-up period. And so, we were led to the methods which we described. We fully share Stablein’s and Catter’s enthusiasm for the proportional hazards model in a wide variety of survival analyses.

ALEXANDER WALKER

HERMAN D. SUIT Department of Radiation Medicine Massachusetts General Hospital Boston, MA 02 1 I4

A TECHNIQUE TO AVOID UNNECESSARY PERINEAL REACI’ION

To the Editor: When using a four field pelvic technique for the treatment of rectal carcinoma following abdominoperineal resection, the fields include the perineal skin through which the operation has been performed. It is undesirable, however, to include the external genitalia.

With the patient lying prone in the treatment position the perineum can be displaced cephalad several centimeters by mechanical means, thus allowing the lower margins of the treatment fields to be placed so that they avoid the anterior perineal structures. A device to achieve this is shown in the accompanying photographs. Its application is self evident. The perspex plate, which can be varied to suit the individual patient, can be used as the lower margin of the treatment fields and it also serves the purpose of build-up at the same time. When so used it has been found that the perineal skin reacts briskly as required, but the external genitalia escape unnecessary radiation reaction. The use of oblique anterior and posterior fields to avoid the anterior part of the perineum also solves this problem, but only at the expense of the inclusion of intra-abdominal structures such as small intestine.

K. W. MEAD Director Institute of Oncology and Radiotherapy The Prince of Wales Hospital High Street Randwick, N.S.W., 203 1 Australia

1995

1996 Radiation Oncology ??Biology 0 Physics October 1984. Volume 10. Number IO

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