Using efficient immobilization and dedicated beam collimation devices, stereotactic radiosurgery ensures highly conformal treatment of small tumours with limited microscopic extension. One contribution to normal tissue irradiation remains the radiological penumbra. This work aims at demonstrating that intermediate energy photons (IEP), above orthovoltage but below megavoltage, improve dose distribution for stereotactic radiosurgery for small irradiation field sizes due to a dramatic reduction of radiological penumbra. Two different simulation systems were used: (i) Monte Carlo simulation to investigate the dose distribution of monoenergetic IEP between 100 keV and 1 MeV in water phantom; (ii) the Pinnacle3 TPS including a virtual IEP unit to investigate the dosimetry benefit of treating with 11 non-coplanar beams a 2 cm tumour in the middle of a brain adjacent to a 1 mm critical structure. Radiological penumbrae below 300 microm are generated for field size below 2 x 2 cm2 using monoenergetic IEP beams between 200 and 400 keV. An 800 kV beam generated in a 0.5 mm tungsten target maximizes the photon intensity in this range. Pinnacle3 confirms the dramatic reduction in penumbra size. DVHs show for a constant dose distribution conformality, improved dose distribution homogeneity and better sparing of critical structures using a 800 kV beam compared to a 6 MV beam.

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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.

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