Alpha/beta ratio for arteriovenous malformations estimated from obliteration rates after fractionated and single-dose irradiation.

Background And Purpose: Results from single-dose radiosurgery have failed to yield reasonable alpha/beta ratios for obliteration rates of arteriovenous malformations (AVMs) in the framework of the linear-quadratic approach. We used outcome data from single-dose and fractionated radiotherapy for AVM to approach this problem.

Patients And Methods: AVM obliteration rates observed in an updated historical series of fractionated radiotherapy and from six recent series of single-dose stereotactic radiosurgery were analyzed. Reciprocal total doses (1/D) and fraction sizes (d) of isoeffective fractionation schemes were entered into the rearranged form of the linear-quadratic equation: 1/D = (alpha/E) + (beta/E)d, and alpha/beta-ratios were calculated from the parameters of the regression line.

Results: Fractionated radiotherapy with 20 Gy/4 Gy fractions, 50 Gy/2 Gy fractions and single-dose radiosurgery of approximately 13 Gy were isoeffective with crude obliteration rates of 13%. The analysis yielded an alpha/beta-ratio of 3.5 Gy. For small-sized AVMs (<3 cm), alpha/beta-ratios of 4.6-6.4 Gy were obtained.

Conclusion: These results support the view that radiosurgery for AVM can be understood as a typical late tissue effect with a high fractionation sensitivity. Fractionated radiotherapy is ineffective for AVMs and should be evaluated carefully in other benign targets.