Purpose: To compare, in patients with brain tumors, surgical bed biologically effective dose (BED) from GammaTile (GT), a permanently implanted collagen tile embedded with Cs-131 seeds (GT Medical Technologies, Tempe, AZ) versus BED with commonly used stereotactic radiosurgery (SRS) protocols.
Methods: In four patients who underwent GT implants (60Gy prescribed at 5mm depth from the operative bed surface), absolute dose was calculated on post-operative thin-slice CT using a commercial treatment planning software (MIM Symphony, MIM Software Inc, Beachwood, OH) and AAPM TG-43U1 formalism. Voxel dose was converted to BEDGT using a validated linear/quadratic model (α/β=10) that accounted for half-life (9.7days), cellular repair half-life (1.5hours), and tumor doubling time (5.4days). Absolute dose and BED covering 90% (DGT90% and BEDGT90%) and 95% (DGT95% and BEDGT95%) of the target volumes (tumor bed [TB]+0-5mm) were calculated. Prescription doses to target volumes (TB+2-3mm) commonly used for single- and fractionated-SRS -- 12, 14, 15, 17, 18, or 20Gy/1 fraction; 27Gy/3 fractions; and 30Gy/5 fractions -- were converted to BED1fx, BED3fx, and BED5fx, respectively, using the external beam linear/quadratic model (α/β=10). BEDGT90% and BEDGT95% were compared to BED1fx, BED3fx, and BED5fx.
Results: BED for SRS was 26.4–60Gy (12-20/1fx), 51.3Gy (27 Gy/3fx), and 48Gy (30 Gy/5fx). For TB+margins of 0, 1, 2, 3, 4, and 5 mm, mean BEDGT90% was 145.5±56.9Gy, 116.3±40.2Gy, 92.9±29.9Gy, 78.5±25.1Gy, 65.6±19.9Gy, and 55.2±17.1Gy, respectively. Mean BEDGT95% for TB+0, 1, 2, 3, 4, and 5mm was 126.5±51.8Gy, 100.6±35.2Gy, 80.6±26.0Gy, 67.9±21.7Gy, 56.7±17.1Gy, and 47.8±15.0Gy, respectively.
Conclusion: This preliminary work indicates that GT delivers equivalent or higher BED than commonly used SRS protocols. Investigations with a larger sample size are underway, and may be informative for interpreting efficacy and safety results using this novel brachytherapy device, which is FDA-cleared to deliver radiation therapy in patients with newly-diagnosed malignant intracranial neoplasms and recurrent intracranial neoplasms.
Funding Support, Disclosures, and Conflict of Interest: Adam Turner: Physics Consultant for GT Medical Technologies, Inc Mehee Choi: Director of Medical Science for GT Medical Technologies, Inc David Brachman: Chief Technology Officer for GT Medical Technologies, Inc