Interlaced x-ray microplanar beams: A radiosurgery approach with clinical potential

TLDR

X‑ray microbeam arrays can spare normal CNS tissue but are limited to synchrotron sources and other practical constraints. The study aimed to evaluate CNS tolerance to thicker microbeams as a step toward clinical feasibility. An interlaced geometry of two orthogonal microbeam arrays was used to create a contiguous high‑dose region while preserving surrounding tissue. Rats tolerated 400‑Gy microbeams and 170‑Gy arrays without neurological deficits, and MRI showed focal damage only at 120‑ and 150‑Gy doses with a 30‑µm dose falloff, indicating that interlaced microbeams could treat tumors with minimal collateral damage.

Abstract

Studies have shown that x-rays delivered as arrays of parallel microplanar beams (microbeams), 25- to 90-microm thick and spaced 100-300 microm on-center, respectively, spare normal tissues including the central nervous system (CNS) and preferentially damage tumors. However, such thin microbeams can only be produced by synchrotron sources and have other practical limitations to clinical implementation. To approach this problem, we first studied CNS tolerance to much thicker beams. Three of four rats whose spinal cords were exposed transaxially to four 400-Gy, 0.68-mm microbeams, spaced 4 mm, and all four rats irradiated to their brains with large, 170-Gy arrays of such beams spaced 1.36 mm, all observed for 7 months, showed no paralysis or behavioral changes. We then used an interlacing geometry in which two such arrays at a 90-degree angle produced the equivalent of a contiguous beam in the target volume only. By using this approach, we produced 90-, 120-, and 150-Gy 3.4 x 3.4 x 3.4 mm(3) exposures in the rat brain. MRIs performed 6 months later revealed focal damage within the target volume at the 120- and 150-Gy doses but no apparent damage elsewhere at 120 Gy. Monte Carlo calculations indicated a 30-microm dose falloff (80-20%) at the edge of the target, which is much less than the 2- to 5-mm value for conventional radiotherapy and radiosurgery. These findings strongly suggest potential application of interlaced microbeams to treat tumors or to ablate nontumorous abnormalities with minimal damage to surrounding normal tissue.