Prognostic Significance of Extent of Tumor Regression at Completion of Radiation Therapy

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Abstract
If among tumors of a given volume and anatomical site there were subpopulations which differed in their local radiocontrollability or TCD502 (radiation dose that Yields a 50 per cent local control rate) and if these groups had distinguishing clinical characteristics, the radiotherapist could administer reduced radiation doses to the low TCD50 cases. This would lessen the likelihood of complication in that group of cases for a specified local control rate. For the higher TCD50 groups, either the treatment could be carried to maximum tolerable radiation doses by complex technics or the patient could be referred for surgery. In our center, decision as to the final radiation dose to be used in the treatment of individual squamous-cell carcinomas of the head and neck region has been based on the tacit assumption that each tumor at a specified site and a given volume could be classified as of a low, standard, or high local radiocontrollability group by observing the extent of regression of tumor volume at the completion of a basic or minimum course of therapy. The extent of tumor regression at that point has been considered a direct indicator of the local radiocontrollability. That is, if all tumor had disappeared at or before completion of this minimum therapy, no additional radiation would be thought necessary to achieve an acceptable local control rate. For example, in a stage T3N0 squamous-cell carcinoma of the lateral pharyngeal wall, a minimum course of treatment of 6,000 rads in six weeks would be planned. If all clinical evidence of disease had disappeared by the fourth week of therapy, the lesion would be considered as “radiosensitive” and therapy stopped at the 6,000 rad point. In contrast, if induration and nodularity persisted at the end of the basic treatment, the lesion would have been classed as “radioresistant” and therapy continued for another 1,250 to 1,500 rads through reduced fields. An evaluation of this use of tumor regression will be presented. It is based on: (a) a laboratory experiment to determine the relationship between tumor regression and probability of local control of the spontaneous mammary carcinoma in the C3H mouse treated by single radiation doses and (b) retrospective analysis of extent of tumor regression at completion of external beam irradiation and incidence of local control of tumor in locally advanced oropharyngeal and oral cavity squamous carcinomas. Animal Tumor Study Materials and Methods Animal Tumor System: Spontaneous Mammary Carcinoma: Adult female mice of the inbred C3H strain (mammary tumor agent positive) carrying a single spontaneous mammary carcinoma were supplied by the Roscoe B. Jackson Memorial Laboratories, Bar Harbor, Maine, and the Texas Inbred Mouse Company, Houston, Texas. The animals were maintained at a constant temperature and humidity. Standard Purina pellets and water were supplied ad libitum; the animals were housed 5 per box. Pregnant mice were excluded from this study.

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