Calculating Optic Nerve Planning Organ at Risk Volume Margins for Stereotactic Radiosurgery Using Optic Nerve Motion determined using MRI.

OBJECTIVES: The combination of sharp dose gradients in stereotactic radiosurgery (SRS) and minute optic nerve motion may significantly increase dose to the optic nerves when treating perioptic lesions. The aim of this study was to calculate optic nerve planning organ at risk volume (PRV) margins for CyberKnife SRS treatment planning. METHODS: MRI scans were taken of 10 healthy volunteers looking left, right, up, down, and straight ahead to measure optic nerve motion. The measured optic nerve motion and the uncertainties in the technical accuracy of CyberKnife were used to calculate optic nerve PRV margins. RESULTS: Two optic nerve PRV margins were calculated: a non-isotropic margin of mL/R,PRV=3mm, mSup/Inf,PRV=2mm and mAnt/Post,PRV=1mm which considers the full range of motion measured in a worst case scenario; and an isotropic margin of mPRV=1mm which considers a scenario where patients are asked to look neutrally during imaging and treatment. Applying these PRVs to 8 historical sphenoid wing meningioma CyberKnife plans showed tolerance levels may be exceeded due to optic nerve motion. CONCLUSIONS: Optic nerve PRV margins may be needed in CyberKnife planning to reduce risk to the optic nerves. The use of a mPRV=1mm PRV to account for organ motion, along with instructing patients to hold their gaze neutrally during imaging and treatment, may be a suitable organ sparing strategy. ADVANCES IN KNOWLEDGE: Measured optic nerve motion and the technical accuracy of the CyberKnife system have been used to calculate optic nerve PRV margins.

Determination of output factor for CyberKnife using scintillation dosimetry and deep learning.

Objective. Small-field dosimetry is an ongoing challenge in radiotherapy quality assurance (QA) especially for radiosurgery systems such as CyberKnifeTM. The objective of this work is to demonstrate the use of a plastic scintillator imaged with a commercial camera to measure the output factor of a CyberKnife system. The output factor describes the dose on the central axis as a function of collimator size, and is a fundamental part of CyberKnife QA and integral to the data used in the treatment planning system.Approach. A self-contained device consisting of a solid plastic scintillator and a camera was build in a portable Pelicase. Photographs were analysed using classical methods and with convolutional neural networks (CNN) to predict beam parameters which were then compared to measurements.Main results. Initial results using classical image processing to determine standard QA parameters such as percentage depth dose (PDD) were unsuccessful, with 34% of points failing to meet the Gamma criterion (which measures the distance between corresponding points and the relative difference in dose) of 2 mm/2%. However, when images were processed using a CNN trained on simulated data and a green scintillator sheet, 92% of PDD curves agreed with measurements with a microdiamond detector to within 2 mm/2% and 78% to 1%/1 mm. The mean difference between the output factors measured using this system and a microdiamond detector was 1.1%. Confidence in the results was enhanced by using the algorithm to predict the known collimator sizes from the photographs which it was able to do with an accuracy of less than 1 mm.Significance. With refinement, a full output factor curve could be measured in less than an hour, offering a new approach for rapid, convenient small-field dosimetry.

First UK patient cohort treated with stereotactic ablative radiotherapy for primary kidney cancer.

Aims: Stereotactic ablative radiotherapy (SABR) for primary renal cell carcinoma (RCC) is a promising non-invasive ablative treatment option. A prospective interventional clinical trial published showed that treatment was feasible and well tolerated. We present the first single-institution UK cohort of patients with primary RCC receiving protocol-based SABR with prospective follow-up. We also present a protocol that could be used to facilitate more widespread use of the treatment. Materials and methods: Nineteen biopsy-proven primary RCC patients were treated with either 42 Gy in three fractions on alternate days or 26 Gy in a single fraction based on predefined eligibility criteria using either Linear Accelerator or CyberKnife platform. Prospective toxicity data using CTCAE V4.0 and outcome data such as estimated glomerular filtration rate (eGFR) and tumour response using CT thorax, abdomen and pelvis (CT-TAP) were collected at 6 weeks, 3, 6, 12, 18 and 24 months post treatment. Results: The 19 patients had a median age of 76 years (interquartile range [IQR] 64-82 years) and 47.4% were males, and they had a median tumour size of 4.5 cm (IQR 3.8-5.2 cm). Single and fractionated treatment was well tolerated and there were no significant acute side effects. The mean drop from baseline in eGFR at 6 months was 5.4 ml/min and that at 12 months was 8.7 ml/min. The overall local control rate at both 6 and 12 months was 94.4%. Overall survival at 6 and 12 months was 94.7% and 78.3%, respectively. After a median follow-up of 17 months, three patients experienced a Grade 3 toxicity, which was resolved with conservative management. Conclusion: SABR for primary RCC is a safe and feasible treatment for medically unfit patients, which can be delivered in most UK cancer centres using standard Linear Accelerator as well as CyberKnife platforms.

The Expanding Role of Radiosurgery for Brain Metastases.

Stereotactic radiosurgery (SRS) has become increasingly important in the management of brain metastases due to improving systemic disease control and rising incidence. Initial trials demonstrated SRS with whole-brain radiotherapy (WBRT) improved local control rates compared with WBRT alone. Concerns with WBRT associated neurocognitive toxicity have contributed to a greater use of SRS alone, including for patients with multiple metastases and following surgical resection. Molecular information, targeted agents, and immunotherapy have also altered the landscape for the management of brain metastases. This review summarises current and emerging data on the role of SRS in the management of brain metastases.

Real-time Cherenkov emission portal imaging during CyberKnife® radiotherapy.

The feasibility of real-time portal imaging during radiation therapy, through the Cherenkov emission (CE) effect is investigated via a medical linear accelerator (CyberKnife(®)) irradiating a partially-filled water tank with a 60 mm circular beam. A graticule of lead/plywood and a number of tissue equivalent materials were alternatively placed at the beam entrance face while the induced CE at the exit face was imaged using a gated electron-multiplying-intensified-charged-coupled device (emICCD) for both stationary and dynamic scenarios. This was replicated on an Elekta Synergy(®) linear accelerator with portal images acquired using the iViewGT(™) system. Profiles across the acquired portal images were analysed to reveal the potential resolution and contrast limits of this novel CE based portal imaging technique and compared against the current standard. The CE resolution study revealed that using the lead/plywood graticule, separations down to 3.4 ± 0.5 mm can be resolved. A 28 mm thick tissue-equivalent rod with electron density of 1.69 relative to water demonstrated a CE contrast of 15% through air and 14% through water sections, as compared to a corresponding contrast of 19% and 12% using the iViewGT(™) system. For dynamic scenarios, video rate imaging with 30 frames per second was achieved. It is demonstrated that CE-based portal imaging is feasible to identify both stationary and dynamic objects within a CyberKnife(®) radiotherapy treatment field.