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To the Editor:—
The article on supervoltage radiotherapy in The Journal, Sept. 12, page 133, deserves some comment. The unit mentioned by the author is described in the radiologic literature as a 4-mvp unit and not, as published, a 6-mev unit. Any radiation physicist know the marked difference between the two terms. The effective voltage is reportedly 1.6 mev. The isodose curves are somewhat misleading, since lack of adequate depth dose is not the limiting factor in radiotherapeutic cure of deepseated lesions; the limiting factors are lack of radiosensitivity, undue size of the tumor, and others. The data quoted from Fletcher and co-workers omit the important fact that they are not five-year survival rates. They are merely preliminary one-year, two-year, and three-year "controls" (Am. J. Roentgenol.81:375 [March] 1959). As long as Fletcher was being quoted the author might have added these figures from the same article (table 3):
Sisson MA. RADIOTHERAPY OF MALIGNANT DISEASE. JAMA. 1960;172(3):267. doi:10.1001/jama.1960.03020030060020