Calculation of isoeffective doses and the α /β value by comparing results following radiosurgery and radiotherapy for arteriovenous malformations of the brain

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Object

The authors sought to assess the relationship between obliteration rate and different dose parameters following fractionated radiotherapy for arteriovenous malformations (AVMs). A comparison of the results of radiosurgery and radiotherapy for AVMs was made to calculate the best fit α/β value, which would then be used as a model for predicting the treatment outcome, independent of the number of fractions applied.

Methods

Data from 1453 patients were analyzed: 1154 treated with radiosurgery and 300 with fractionated radiotherapy. The relationships between dose and obliteration rate after 3 years were calculated, and the best fit curve to the empirical results was defined. The higher the dose per fraction, biologically effective dose, and the lower the total dose, the higher the obliteration rate. The isoeffective doses when comparing radiotherapy and radiosurgery independent of the α/β value could not be defined. The dose per fraction had the best predictive value, independent of the number of fractions.

Conclusions

Dose per fraction seems to be the decisive parameter for the treatment response following both radiotherapy and radiosurgery. A larger number of fractions did not increase the obliteration rate. The data indicate that higher doses per fraction should be used when irradiating AVMs.

Abbreviations used in this paper: AVM = arteriovenous malformation; BED = biologically efficient dose; CI = confidence interval; CT = computed tomography; MR = magnetic resonance.

Article Information

Address reprint requests to: Bengt Karlsson, M.D., Ph.D., Department of Neurosurgery, West Virginia University, Morgantown, West Virginia 26506. email: nykuttram@yahoo.se.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Graph showing the relationship between dose per fraction and obliteration rate. Gray lines denote single and black lines multiple fraction treatments. The thin lines denote logarithmic curve fits and the thick lines represent clinical data. Both curves fall within the bars representing the 95% CIs, suggesting no significant difference in dose response between radiosurgery and radiotherapy. The dose–response rates as well as the curve fits are similar following radiosurgery and radiotherapy. RS = radiosurgery; RT = radiotherapy.

  • View in gallery

    Graph showing the relationship between total dose and BED and obliteration rate for the AVMs treated with radiotherapy. Total dose equals dose per fraction multiplied by number of fractions, and the BED is calculated for α/β=–50. The bars represent the 95% CIs. There is a negative correlation between both total dose and BED on the one hand and obliteration rate on the other.

  • View in gallery

    Graph showing the relationship between radiosurgery (black curves) and radiotherapy (gray curves), expressed as the BED and obliteration rate. Two different values for α/β are used: 2 Gy (thick line) and 4 Gy (thin line). The biological response for similar BEDs varies significantly depending on whether radiosurgery or radiotherapy is used.

  • View in gallery

    Graph showing a comparison of the probability of AVM obliteration for the same BED values when treating with radiosurgery and radiotherapy for different α/β values as well as for different doses per fraction. Ideally, the curve should be equal to x = y. The dose per fraction has the closest fit to x = y. The lowest of the α/β values has the best fit to x = y for the BED values.

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