Intermediate energy photons ( 1 MV ) to improve dose gradient, conformality, and homogeneity: Potential benefits for small field intracranial radiosurgery

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.