Med Phys. 2012 Jun;39(6):3189-201. doi: 10.1118/1.4712224.

What do we know about the α/β for prostate cancer?

S M Oliveira1,2, N J Teixeira1,3 and L Fernandes3,4

1Faculdade de Ciências Médicas, Universidade Nova de Lisboa, Campo Mártires da Pátria, 130, 1169-056 Lisbon, Portugal

2MedicalConsult, SA, Campo Grande, 56-8ºA, 1700-093 Lisbon, Portugal

3Escola Superior de Tecnologia da Saúde de Lisboa, Instituto Politécnico de Lisboa, Av. D. João II, lote 4.69.01, 1900-096 Lisbon, Portugal

4Instituto Gulbenkian de Ciência, Rua da Quinta Grande, 6, 2780-156 Oeiras, Portugal

 

Abstract

Since last decade, the debate on the parameter which reflects prostate cancer sensitivity to fractionation in a radiotherapy treatment, the a/b, has become extensive. Unlike most tumors, the low labeling indices and large potential doubling time that characterize the prostate tumor led some authors to consider that it may behave as a late responding tissue. So far, the existing studies with regard to this subject point to a low value of a/b, around 2.7 Gy, which may be considered as a therapeutic gain in relation to surrounding normal tissues by using fewer and larger fractions. The aim of this article is to review several estimates that have been made in the last few years regarding the prostate cancer a/b both from clinical and experimental data, as well as the set of factors that have potentially influenced these evaluations.

© 2012 American Association of Physicists in Medicine.

PMID: 22755703

 

Supplement

The α/β ratio is a parameter of the Linear Quadratic model of cell survival after irradiation which quantifies the tissue sensitivity to a given fractionation scheme in Radiotherapy. Although α/β values are typically higher for tumors (10 Gy) than for late responding normal tissues (3 Gy), this parameter value for the slowly growing prostate cancer has been highly debated in the Radiobiology and Radiotherapy communities.

Since 1999 there is a clinical evidence that prostate cancer should have a low α/β ratio. On the other hand, if this value is proven to be lower than that for late complication, it would be expected an improvement in the therapeutic ratio using hypofractionated regimens than conventional schedules in the treatment of prostate cancer with radiation.

Although many studies reported estimates on the α/β ratio for prostate cancer based ether in the clinical outcome of patients or experimental in vitro data and exploiting different factors that influence the calculations, a reliable value to use in the clinical practice was still controversial.

In this study we performed a comprehensive review of all published data reporting on the α/β ratio parameter for prostate cancer since 1999. By using the reported estimates with a meaningful 95% confidence interval, a consensus value of 2.7 Gy was obtained by taking the mean of this evaluations. Considering that this values is lower than that for late rectal complications (5.4 Gy), a therapeutic gain may be achieve with the use of fewer and larger fractions in the Radiotherapy protocols for treatment of prostate cancer.

Summary of reported a/b values and the corresponding 95% CI. The dash-dot line represents the arithmetic mean of the a/bvalues (2.73 Gy).

Nuno Teixeira fig1

 

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