Radiation Biology

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SU_40_2406 - Assessment of the alpha/beta Ratio of the Optic Pathways

Sunday, October 21
1:15 PM - 2:45 PM
Location: Innovation Hub, Exhibit Hall 3

Assessment of the alpha/beta Ratio of the Optic Pathways
H. Speckter1, I. Miches1, J. Bido1, G. Hernandez1, D. Rivera1, L. Suazo1, S. Valenzuela1, J. M. Garcia2, and P. Stoeter1; 1CEDIMAT, Santo Domingo, Dominican Republic, 2Centro Gamma Knife Dominicano, Santo Domingo, Dominican Republic

Purpose/Objective(s): Hypofractionation has been recently considered an alternative to improve stereotactic radiosurgery of lesions in close proximity of the optic pathways. To estimate the intrinsic benefit from fractionation versus single dose radiosurgery for peri-optic lesions, the value of the alpha/beta ratio of the optic pathways has to be taken into account. Most studies expect and use an alpha/beta ratio of 2 Gy, or even 3 Gy, for the visual system. So far, to our knowledge, only 3 alpha/beta ratios have been published: In 1992 Goldsmith and Larson published an alpha/beta ratio of 3.06 Gy, which means that there would be no benefit of hypofractionation for most benign lesions. In 1994 Jiang estimated a ratio of 1.6 Gy based on relatively few data from a single center study. In 2010 Vernimmen derived a negative value of -0.6 Gy, and remarked that a negative value is not permitted by the LQ model.

Materials/Methods: The alpha/beta ratio of the optic pathways was estimated from a meta analysis of 388 studies published since 2000. We included 27 studies with fraction numbers between 1 and 31, considering the following inclusion criteria: frequency of radiation induced optical neuropathy RION between > 0 %, and < 10 %, follow up of at least 12 months, no tumor progression, no prior radiation, detailed dosimetric analysis for the visual system. Clinical data provided from our center were added, including 54 hypofractionated stereotactic radiosurgery (SRS) regimens (4 treatments with 5 sessions, 45 treatments with 4 sessions, and 5 treatments with 3 sessions) and 106 single session treatments of different lesions close to the optic pathways.

Results: The FE plot method revealed an alpha/beta ratio of the optic system of 1.11 Gy, confidence interval [0.39 – 3.59]. There is still not enough data in order to distinguish between alpha/beta ratios of the optic chiasm, the nerves and the tracts.

Conclusion: A significant intrinsic benefit from hypofractionation can be expected not only for malignant tumors located in the sellar region but for benign lesions as well, because of the very low alpha/beta ratio of the optic system of 1.11 Gy.

Author Disclosure: H. Speckter: IGKRF; IGKRF. I. Miches: None. J. Bido: None. G. Hernandez: None. L. Suazo: None. S. Valenzuela: None. J.M. Garcia: None. P. Stoeter: None.

Herwin Speckter, MS

Biography:
Herwin Speckter serves as medical physicist at the Dominican Gamma Knife Center since its opening in 2011 and as medical physicist at the medical center CEDIMAT since its opening in 1999, in Santo Domingo, Dominican Republic.

Herwin Speckter studied physics at the Johannes Gutenberg University in Mainz, Germany. He developed his thesis in high field magnetic resonance spectroscopy of brain and breast cancer at the Max Planck Institute for Polymer Research in Mainz, Germany.

Between 1992 and 1998, before moving to the Dominican Republic, he was faculty member of the departments of Nuclear Medicine and as well Neuroradiology at the University Hospital in Mainz.

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