An examination of some dose-time relationships in therapeutic radiology

A re-analysis of available dose-response and time-dose data in Hodgkin's disease

A re-analysis of all available dose-response data on Hodgkin's disease, compiled recently by Vijayakumar and Myrianthopoulos (Vijayakumar, S. and Myrianthopoulos, L.C. An updated dose-response analysis in Hodgkin's disease. Radiother. Oncol. 24: 1-13, 1992), fails to demonstrate any dose-response relationship at doses higher than 32.5 Gy. Thus, in contrast with these authors, we find no evidence that local control will be improved by radiation doses of more than 32.5 Gy. A review of the available data on the time-dose relationship in Hodgkin's disease indicates that overall treatment time, at least up until 7 weeks, is not of major importance. Further, there is some indication that the sensitivity to changes in dose per fraction is low. This allows the fraction size to be selected from considerations of the level of late treatment related morbidity.

Predicting normal tissue injury in radiation therapy

We tested three radiobiologic models, the nominal standard dose (NSD), the biologic index of reaction (BIR), and the linear quadratic (LQ) models to determine which best predicts normal tissue injury in radiation therapy. Clinical data for radiation myelopathy, rib fracture and pericardial effusion were used for all three models to predict injury. We assumed that on the average injuries occurred at higher equivalent doses of radiation than were received by patients who were not injured. We used a t-test to determine whether there were in fact significant differences in the mean values of the equivalent doses among the injured and non-injured. The means were calculated for the four sets of injury by the three models. For the LQ model it was necessary to choose a value for the parameter alpha/beta; the results were not sensitive to the choice over the range of 1/2 to 12 Gy. None of these models showed a significant difference between injured and non-injured patients for all four sets of data. The BIR model showed significant differences in three sets, the LQ model was significant in two and marginally significant in one set, and the NSD was significant in two sets. This analysis illustrates therefore, that the linear quadratic model can be adopted for analysis of clinical data with results that are no worse and possibly better than the NSD model.

The relationship of dose to prognosis in squamous-cell carcinoma of the aerodigestive tract

How much does radiation dose determine prognosis of patients requiring radiotherapy for squamous-cell carcinoma of the aerodigestive tract? Using a statistical test of the correlation of dose versus outcome, several results of radiation treatment reported in the literature were examined. The radiation dose received was found to be a poor predictor of which patients would be cured.

Radiotherapy of liposarcomas

A review of the literature of time-dose relationships of liposarcoma is given. The radiotherapeutic data on 15 measurable liposarcoma sites, together with 45 sites collected from the literature, are subjected to a Strandqvist plot analysis using the modified Ellis' formula TSD = D.T-alpha. The recovery coefficient alpha is thought to be rather high, of the order alpha = 0.5. This suggests that the use of a few large fractions in therapy may be beneficial.

Analysis of dose, dose-rate and treatment time in the production of injuries by radium treatment for cancer of the uterine cervix

A retrospective study has been made of 15 patients who had developed severe complications to the bladder, rectum and ureters after radiation therapy for carcinoma of the cervix. Comprehensive dose distribution in the pelvis were calculated with computer assistance to visualize how the normal organs had been irradiated. The risk of normal tissue injury was analysed graphically with respect to the maximum radium dose, dose-rate and irradiation time. The two groups of data representing the injured and non-injured organs separate much better in a dose-rate versus dose plot than in a conventional Strandqvist type dose-time plot. This implies that in radium therapy, the dose-rate rather than the treatment time, is the important parameter in modifying the risk of normal tissue injury. Our results show a good separation of injured from non-injured cases only when the maximum values of dose and dose-rate were used. This suggests that clinically significant injury may develop from relatively small regions of high dose and dose-rate. Since the dose-rate varies from point to point in the pelvis (unlike treatment time), comprehensive determinations of dose-rate distributions are required in order to locate the sites of potential injuries.