SU-FF-T-535: Effects of Peripheral Dose Fall-Off On Biologically Equivalent Dose to Normal Brain for Intracranial Stereotactic Radiosurgery and Radiotherapy

Abstract

Purpose: The peripheral dose volume such as 10‐Gy or 12‐Gy volume has been reported to correlate with incidence of radionecrosis for stereotactic radiosurgery(SRS) or radiotherapy (SRT). In this study, we investigated whether variations in peripheral dose fall‐off of individual treatments would significantly affect normal tissue sparing for SRS or SRT. Method and Materials: A power relationship for measuring general dose fall‐off near the target was developed for both isocentric and non‐isocentric intracranial radiosurgery modalities. For the study, we sampled the ranges of variations in the peripheral dose distributions from patients treated with Gamma Knife Perfexion, Cyberknife, or linac‐based delivery system. Equivalent uniform biological effective dose (EUBED) for the normal braintissue was formulated to study the effect of dose‐fall variations on treatments of different targets of varying a/b values. Functional relationship of normal brain EUBED with increasing number of fractions was derived for treating either fast growing tumors with a/b of 10–20) or abnormal tissues such as AVM with possible a/b of 2–5. Results: The average g‐index from the derived power formula was found to be −1.49±0.25 with a range of −1.19 to −1.79. Based on such range, EUBED of the normal brain was calculated and found to decrease with increasing number of fractions for targets with a/b of 10–20. This decrease was most pronounced for fractions fewer than 10. However, the EUBED was found to slightly increase with increasing number of fractions for targets with a/b of 2–5. Conclusion: In delivering SRS or SRT, normal tissue EUBED was found to favor hypofractionated treatments for fast growing tumors with a/b of 10–20, but single fraction treatments for abnormal tissues with a/b of 2–5. The result was found to be insensitive to variations in dose fall‐off from individual cases regardless being treated with Gamma Knife, Cyberknife or Linac‐based modality.