Fractionation sensitivity and the oxygen effect

Dose escalation to combat hypoxia in prostate cancer: a radiobiological study on clinical data

Earlier studies have demonstrated that hypoxic regions exist in human prostate cancer and the degree of hypoxia correlates with the treatment outcome of radiotherapy. Using the concept of the clinical oxygen enhancement ratio (COER), the linear-quadratic (LQ) model was extended to account for the effect of tumour hypoxia. The clinical data collected at the Fox Chase Cancer Center for prostate cancer were analysed based on the LQ model as well as the tumour control probability (TCP) model. The LQ and TCP parameters (alpha = 0.15 Gy (-1), alpha/beta = 3.1 Gy and the number of clonogens K = 10(6) approximately 10(7) cells) determined in earlier studies were used to derive the COER for prostate cancer: COER = 1.4 with a standard confidence interval (CI) of (1.2, 1.8). The result is consistent with the in vitro OER measurements of human tumour cell lines under chronic hypoxia conditions. This implies that a higher dose is needed to overcome tumour hypoxia. For prostate tumours, the prescription dose required to overcome tumour hypoxia is 165 Gy (CI: 153 approximately 186 Gy) for permanent 125I implants and 88 Gy (CI: 74 approximately 118 Gy) in 2 Gy fractions for external-beam radiotherapy. The impact of LQ parameters on the calculations of COER and dose escalation was discussed. This study provides a preliminary estimate of the dose escalation needed to overcome tumour hypoxia based on clinical data. More clinical data with better statistics and longer follow-up time are required to further tune the radiobiological modelling of hypoxia for prostate cancer.

The clinical radiobiology of brachytherapy

The unique geometrical features of brachytherapy, together with the wide variety of temporal patterns of dose delivery, result in important interactions between physics and radiobiology. These interactions exert a major influence on the way in which brachytherapy treatments should be evaluated, both in absolute and comparative terms. This article reviews the main physical and radiobiological aspects of brachytherapy and considers examples of their influence on specific types of treatment. The issues relating to the optimization of high dose rate brachytherapy are presented, together with the implications of multiphasic repair kinetics for low dose-rate and pulsed high dose rate brachytherapy. The opportunities for application of radiobiological principles to improve various brachytherapy techniques, together with the integration of brachytherapy with teletherapy, are also outlined. Equations for the numerical evaluation of brachytherapy treatments are presented in the Appendices.

Oxygen tension distributions are sufficient to explain the local response of human breast tumors treated with radiation alone

PURPOSE

Several factors are known to influence the probability of tumor control after radiation. These include tumor oxygen tension distribution, glutathione content, intrinsic radiation sensitivity, rate of repopulation, tumor size, physician skill, etc. The relative impact of oxygen on human tumor response is unknown. The purpose of this analysis is to determine to what extent the observed shape of the radiation response curve for human tumors can be predicted by the tumor oxygenation status.

METHODS AND MATERIALS

The radiation dose response curve for patients treated with radiation alone for breast cancer was calculated based on pooled data. Tumor control rates as a function of radiation dose were fitted to a probit curve. Twenty-two women with breast cancer in Mainz (Germany) and at Stanford University had pO2 measurements made of their tumors. An average of 87 +/- 58 (range 21 to 300) measurements were made from each patient. Hypoxia was assumed to be a purely dose modifying factor with a maximum oxygen enhancement ratio of 2.5. Assuming patients are treated with daily radiation doses of 2 Gy, the breast cancer alpha/beta ratio is 10 Gy, tumors have a mean of 10(8) stem cells, and using the linear quadratic formula for modelling surviving fraction, it was possible to estimate tumor control probability.

RESULTS

Tumor oxygenation was an extremely important modifier of the shape of the dose response curve and alone was sufficient to account for the slope of the observed dose response curve for human breast carcinoma. Tumor size distribution had a smaller effect on the shape and the slope of the dose response curve. Two models of radiation induced reoxygenation were tested, one that allowed full reoxygenation to the baseline state between the daily radiation fractions and another with no reoxygenation between fractions. The clinical data fell between these two models in accordance with the expected incomplete reoxygenation between treatments.

CONCLUSION

The results support the conclusion that in human breast carcinoma, oxygen tension distribution is a critical modifier of radiation treatment response.