Hypofractionated Brain Radiosurgery With Hundreds of Isocenters.

Abstract

PURPOSE/OBJECTIVE(S)\nTo report our initial clinical implementation of a hypofractionated brain radiosurgery (hSRS) approach via packing hundreds of isocenters for treatment of a large target with a substantial GTV expansion in order to reduce dose hot spots in eloquent brain.\n\n\nMATERIALS/METHODS\nA high-density isocenter packing (HDIP) technique similar to onion peeling was developed for hSRS, where a large target volume of 14.1 cc was sequentially shrunk or peeled by a margin of 2 to 4-mm depending on its size. The isocenters were densely packed inside each layer or each peel of the target, especially within the GTV-to-PTV margin. Once packed, the isocenter location, shapes and weights of the beams were inversely optimized and manually adjusted to achieve an optimal dose distribution. This technique was clinically implemented on a frameless SRS unit for the purpose of expediting the workflow in treating brain lesions with GTV-to-PTV expansion of at least 5 mm.\n\n\nRESULTS\nHDIP enabled the target dose to be prescribed to the 80-90% isodose line (normalized to the dose maximum), thus reducing dose hot spots greater than 120% of prescription dose inside the GTV expansion volume comprised mostly of normal brain tissue. Compared to the conventional approach of using only 1/3 to 1/4 as many isocenters, HDIP achieved identical dose coverage (> 98%), dose conformality (<\u202f=\u202f2.8) and Paddick gradient indices (<\u202f=\u202f3.5) for the PTV. An important caveat in implementing HDIP was that when globally increasing the total number of isocenters, the dose contributions from some isocenters decreased rapidly, rendering their beam-on time to fall below the machine limit of 0.01 min. To overcome such a problem, manual blocking of beamlets associated with the affected isocenters was implemented to effectively lower the dose rate, propelling the treatment time at these isocenters > 0.01 min to enable clinical implementation. This strategy negligibly impacted the total treatment time of 20-30 min of a typical HDIP treatment with a dose rate of 2.0 Gy/min of our LGKI unit.\n\n\nCONCLUSION\nHDIP is technically feasible and successfully implemented for the first time at our institution, enabling us to expedite the hSRS workflow to care for patients with targets in eloquent brain requiring substantial GTV-to-CTV/PTV margin expansion.\n\n\nAUTHOR DISCLOSURE\nN. Cho: Employee; Thermo Fisher Scientific. D. Raleigh: None. B.P. Ziemer: None. T. Nano: None. P.V. Theodosopoulos: None. P.K. Sneed: None. L. Boreta: None. S.E. Braunstein: Advisory Board; Radiation Oncology Questions, LLC.L. Ma: Patent/License Fees/Copyright; University of California Regents.