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A Radiobiological Model Based Approach to Evaluate Brain Radiotherapy Fractionation Regimens

N Xiao

N Xiao^{1, 3}*, F Kong², S Ryu¹, I Chetty¹, J Burmeister³, M Joiner³, J Jin¹, (1) Henry Ford Health System, Detroit, MI, (2) University of Michigan, Ann Arbor, MI, (3) Wayne State University School of Medicine, Detroit, MI

SU-E-T-293 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate various brain radiotherapy fractionation regimens by calculating relative damaged volume (RDV) of normal brain while giving the same biologically effective dose (BED) to tumor.

Methods:The RDV was computed for 5 brain cases with the tumor sizes varying between 0.9-63.2 cm³ for 17 different fractionation regimens (1-8, 10, 12, 15, 20, 25, 30, 35, 40, and 50 fractions). A 20Gy single fraction regimen was used as the tumor control probability reference. The LQ model with correction for tumor regrowth effect (Tpot) was used to determine BED to tumor. The RDV was calculated from patients brain DVHs and a logistic local response function, which is characterized by DL50, the dose required to produce 50% local damage. Various Tpot (5 -30 days) and DL50 (20 - 50Gy) values were used to calculate RDV for each patient. Assuming a homogeneous brain structure, we used RDV as a surrogate for brain normal tissue complication probability (NTCP). A smaller RDV value represents lower NTCP and higher therapeutic ratio.

Results:The minimum RDV among 17 studied fractionation regimens varied with Tpot, DL50, and tumor size. Stereotactic radiosurgery (SRS) is the favorable treatment regimen for low DL50 (20Gy). For a small sized tumor (<5.8 cm³, in this study), SRS is preferred up to medium DL50 (35Gy) and low Tpot (5 days). The optimal number of fractions (a regimen with minimum RDV) increases up to 35 fractions with increasing DL50, Tpot, and tumor size. Greater than 10% RDV reductions were achieved in some situations compared to reference fractionation.

Conclusion:This study demonstrates that the RDV with the same tumor BED could be used to evaluate different fractionation regimens. An optimal fractionation regimen, such as SRS vs. hypofractionation, may be determined by the DVH, DL50, and Tpot of a patient to improve the therapeutic ratio.

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