TUMOR VOLUME AND LOCAL-CONTROL PROBABILITY - CLINICAL-DATA AND RADIOBIOLOGICAL INTERPRETATIONS

Purpose: It is well established for certain human tumor histologies th at increasing tumor volume leads to a decreasing probability of tumor control, The simplest explanation for these findings is that the numbe r of tumor clonogens that must be sterilized to control a tumor increa ses with tumor volume, In this investigation we consider whether clini cal evidence favors a further hypothesis, namely, that clonogen number increases in direct proportion to tumor volume.Methods and Materials: Previously published data on the volume-cure relationship for breast tumors, neck nodes, malignant melanoma, and squamous cell carcinomas o f the oropharynx and the uterine cervix were analyzed. Results: We fou nd in all these data sets evidence that the effect of tumor volume on tumor control probability was less than what would be expected under t he assumption of proportionality between number of clonogens and volum e, We describe good reasons to believe that this is the result of pati ent-to-patient variability in radiocurability, and possibly other fact ors as well. Conclusions: Clinical data do provide evidence for a high ly significant reduction of tumor control probability with increasing tumor volume, However, because of heterogeneity in patient and tumor c haracteristics, the volume effect is less pronounced than would be exp ected from a simple proportionality between number of clonogens and vo lume, In principle this simple proportionality does hold in individual patients, so that standard approaches for treatment plan optimization in individuals may still be valid.