Stereotactic radiosurgery (SRS) treatment is characterized by high doses per fraction and extremely steep dose gradients. This requires a great degree of accurate localization to the appropriate treatment position, and continuous immobilization during the treatment session. In the case of Trigeminal Neuralgia (TGN) treatment this is especially true as the very small target volume makes positional accuracy critical. In this study we carried out a quantitative analysis of patient motion during the full treatment fraction within a radiosurgery immobilization mask system. Patient cranial movement was assessed by using the image guidance stereo x-ray cameras on a CyberKnife (CK) M6 robotic radiosurgery system (Accuray, Sunnyvale, CA). A total of five patients received treatments for either right or left TGN. The duration of treatment varied from 24-64 minutes. Orthogonal images were taken every 15 seconds during the treatment to assess patient movement. Approximately 60 stereo images were taken per patient and a total of 560 images were analyzed in this study. The mean absolute movement in each of longitudinal, lateral or vertical directions was approximately 0.3 mm for the duration of the treatment; however, on occasion much greater movement was observed during a fraction. The maximum displacement was in the longitudinal direction and reached 2.4 mm compared to the initial setup. Images taken at the end of the treatment session showed that the patients typically return to a position closer to the original setup position than the maximum excursion that occurred. This data suggests that although this mask system appears stable during much of the treatment session; for some patients there may be momentary patient movements that take place. Frequent imaging and correction can help mitigate the effect of this movement. It is important to understand the limitations of non-invasive mask systems when used for very high precision treatment.

High plan quality for complex spinal radiosurgery was achieved among all systems and all participating centers in this planning challenge. This study concludes that simple IMAT techniques can generate significantly better plan quality compared to previous established CKRS benchmarks.

To develop a compact, proof-of-concept kilovoltage intensity modulated radiotherapy platform to deliver contrast-enhanced radiotherapy (CERT). Methods: The proof-of-concept requirements were threefold: (i) develop a compact means to generate a kilovoltage x-ray broadbeam capable of targeting the K-edge of heavy metal contrast, (ii) investigate the potential clinical application of such a beam in silico, and (iii) construct a limited working prototype of the platform to identify weaknesses in the approach. The x-ray broadbeam was created with a standard kilovoltage x-ray tube heavily filtered with 0.156 mm tungsten. Depth dose and treatment characteristics of the beam were investigated using GEANT4 Monte Carlo framework. The working prototype included a miniaturized multi-rod collimator (MRC) constructed with tungsten carbide rods, 3D-printed gear rack, and an in-house GUI/microprocessor assembly. Measurements of simulated treatments were made with EBT3 radiochromic film. Results: Monte Carlo simulations of pseudo-clinical treatments on a homogeneous spherical Lucite phantom produced dose distributions that indicated surface dose was less than target dose. When simulated-bone was added to the calculations, dose-to-bone was found to be the probable limiting factor in clinical treatment. Nevertheless, when iodine contrast enhancement was accounted for, dose was escalated in the tumor compared to standard megavoltage treatments even with dose-to-bone limitations. Experimentally, after commissioning the MRC and calibrating the radiochromic film, surface dose was compared to maximum dose measured on each film. Surface dose measurements were higher than those in a megavoltage beam, but below the skin toxicity threshold. Dose at depth was reasonable despite increased beam attenuation at these energies from photoelectric interactions in bone. IMRT and non-coplanar dose falloffs around the target were steeper than kV 3D conformal dose falloff. Conclusions: The compact kilovoltage treatment platform presented here proved clinically feasible in both simulation and measurement. It is reasonable to pursue this modality further as a potential CERT platform.

There are numerous commercial radiotherapy systems capable of delivering single fraction spine radiosurgery/SBRT. We aim to compare the capabilities of several of these systems to deliver this treatment when following standardized criteria from a national protocol. Four distinct target lesions representing various case presentations of spine metastases were contoured in both the thoracic and lumbar spine of an anthropomorphic SBRT phantom. Single fraction radiosurgery/SBRT plans were designed for each target with each of our treatment platforms. Plans were prescribed to 16 Gy in one fraction to cover 90% of the target volume using normal tissue and target constraints from RTOG 0631. We analyzed these plans with priority on the dose to 10% of the partial spinal cord and dose to 0.03 cc of the spinal cord. Each system was able to maintain 90% target coverage while meeting all the constraints of RTOG 0631. On average, CyberKnife was able to achieve the lowest spinal cord doses overall and also generated the sharpest dose falloff as indicated by the Paddick gradient index. Treatment times varied widely depending on the modality utilized. On average, treatment can be delivered faster with Flattening Filter Free RapidArc and Tomotherapy, compared to Vero and Cyberknife. While all systems analyzed were able to meet the dose constraints of RTOG 0631, unique characteristics of individual treatment modalities may guide modality selection. Specifically, certain modalities performed better than the others for specific target shapes and locations, and delivery time varied significantly among the different modalities. These findings could provide guidance in determining which of the available modalities would be preferable for the treatment of spine metastases based on individualized treatment goals.

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