Intermediate energy photons to improve dose gradient, conformality, and homogeneity: Potential benefits for small field intracranial radiosurgery

Abstract

A previously conceived and demonstrated principle of reducing penumbra for small radiosurgical dose fields is here now applied to a multiple beam arrangement in a stereotactic head phantom. In this work it is found that the fourfold reduction in radiological penumbra of small, single 1 MV x-ray fields translates to a more conformal, homogeneous dose distribution in the more complex beam arrangements. The film dosimetry is conducted with a high resolution digital microscope to quantify the sharp dose gradients. Further, the Gafchromic EBT film measurements in phantom are compared to calculations using the Xknife RT3 (Radionics, Burlington, MA) treatment planning software (TPS) with modeled 1 MV beam data. An orthogonal pair of coplanar beams and an 18-beam coplanar arc irradiation both yielded agreement between the modeling within the TPS and the film work. Conventional 6 MV modality is compared alongside 1 MV throughout. The 90%-50% and 90%-10% dose gradients at the intersection of the orthogonal beam pair were 1.7 and 4.7 mm for 6 MV versus 0.5 and 1.3 mm for 1 MV for an identical setup. The 18-beam coplanar arc plan yielded 90%-80% and 90%-50% dose gradients of 0.84 and 2.2 mm for 6 MV versus gradients of 0.29 and 1.36 mm for 1 MV for the midaxial slice coplanar with all beamlet axes. Uncertainties in gradient measurements were +/- 0.15 mm. The 18-beam coplanar beam arrangement represented a worst case scenario for penumbra overlap deteriorating the dose distribution. In brief, 1 MV x-rays provided superior homogeneity, conformality, and dose fall-off to 6 MV for the irradiations examined.

Feasibility of using intermediate x‐ray energies for highly conformal extracranial radiotherapy

“…It is expected that 2 MV flattening filter free x rays will show greater strengths in these regards. Keller et al 37 recently modeled intermediate-energy x rays for small field intracranial conformal treatments. They showed that 1 MV x-rays provided markedly superior homogeneity, conformality, and dose falloff when compared against 6 MV.…”

Technical Note: Dose gradients and prescription isodose in orthovoltage stereotactic radiosurgery

“…That work was completed using a 6 MV cone-based Linac calculation with data fit to an analytical approximation. The increased interest in SRS delivered using orthovoltage energies [18][19][20] motivated the examination of this relationship in the orthovoltage range. This work expanded on previously published methods by using Monte Carlo methods to derive pencil kernels, and then using these updated kernels to examine a range of beam energies in the orthovoltage range.…”

“…Multiple studies in SBRT and SRS have shown promise for the use of orthovoltage energy photons, with both theoretical and physical orthovoltage systems showing improvements in dose gradient, conformality, and homogeneity of stereotactic plans compared to standard megavoltage energy photons. [16][17][18][19][20] Previous work in the megavoltage energy range has examined the trade-off between prescription isodose and dose gradient index in a 6 MV Linac SRS delivery. 21 This work proposes to examine such a trade-off in orthovoltage SRS.…”

Design of a modulated orthovoltage stereotactic radiosurgery system

“…In general, the focal spot sizes for orthovoltage units are larger than those for standard 6 MV linear accelerators; however, the geometric penumbra may be minimized through the use of collimating cones near the surface of the volume to be irradiated. Multiple studies in SBRT and SRS have shown promise for the use of orthovoltage energy photons, with both theoretical and physical orthovoltage systems showing improvements in dose gradient, conformality, and homogeneity of stereotactic plans compared to standard megavoltage energy photons . In addition, orthovoltage energies have been noted as the ideal energy range for small animal irradiations .…”

“…Multiple studies in SBRT and SRS have shown promise for the use of orthovoltage energy photons, with both theoretical and physical orthovoltage systems showing improvements in dose gradient, conformality, and homogeneity of stereotactic plans compared to standard megavoltage energy photons. [8][9][10][11][12] In addition, orthovoltage energies have been noted as the ideal energy range for small animal irradiations. 13 Keeping the aforementioned improvements in mind, this work optimized orthovoltage energy fluence in SRS cones.…”