The function of multi-leaf collimators (MLC) is to modulate and shape the intensity of a radiotherapy beam by either blocking or unblocking beamlets. A variation on this functionality is tested in this work wherein the MLC is split into layers, with each layer attenuating the beam by a different amount. In this design, full blocking of a beamlet occurs only if all layers are blocked. This work suggests that such a device, a multi-layer MLC (MLMLC), can deliver dose distributions like a single layer MLC can deliver while requiring less time and monitor units (MU) Methods: Optimal fluences were made for prostate plans using the Eclipse v13.6. An algorithm was developed to create step-and-shoot MLMLC patterns to match these optimal fluences when using up to six layers of MLC. Twelve MLMLC plans were made in total. These patterns were imported back into Eclipse as equivalent tungsten compensators and doses were calculated.Dosevolume histogram (DVH) values,total monitor units (MU),and total time to deliver were compared between arc-style MLMLC plans and nine-field step and shoot IMRT plans created completely in Eclipse using a single layer MLC. Results: When using three or more layers, specified DVH values between the two sets agreed to within 5% while requiring roughly half as much time to deliver and about 20% fewer MU. Conclusions: Demonstrated that having multi-layer MLC can deliver dose distributions like a single layer MLC with less time and monitor units.

This study investigated a single institution's experience with volumetric modulated arc therapy (VMAT) directed stereotactic ablative body radiotherapy (SABR) for vertebral metastases. From 2010 to 2014, 95 lesions of spinal metastases in 73 patients were treated with SABR using VMAT. Clinical local control, pain level, and use of steroid medication were employed to evaluate treatment responses. The majority (79%) of patients were treated with a radiation dose of 20 Gy in a single fraction. However, when normal tissue constraints could not be achieved, the dose was reduced to 18 Gy (11%) or 16 Gy (8%) in 1 fraction. At the median follow up of 12.7 months (mean 18.0, range 1-56 months), clinical local control was 97% (92 out of 95). There was a mean 81% (median 100%, range 28-100%) decrease in subjective pain score. Seventy-seven percent of patients had a decrease in narcotic pain medication use. Pain was completely resolved at the treatment site for 69% (66/95) of patients. Prior to the SABR treatment, 33% (31/95) of patients had epidural extension of tumor. Among patients with epidural involvement, 45% (14/31) exhibited neurologic impairment prior to treatment. Twenty-three percent (7/31) experienced spinal cord compression. Prior to treatment, 34 patients experienced some form of neurologic impairment. Of these patients, 24% (8/34) experienced improved motor functioning; the remaining 76% (26/34) of patients' neurological dysfunction were stable. Our results indicate the SABR regimen using VMAT technique is clinically effective in achieving clinical local control and palliation. This is the first publication reporting clinical outcomes of VMAT directed SABR.

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