Abstract

PurposeTo evaluate the accuracy of monitoring intrafraction motion during stereotactic radiotherapy with the optical surface monitoring system. Prior studies showing a false increase in the magnitude of translational offsets at non‐coplanar couch positions prompted the vendor to implement software changes. This study evaluated two software improvements intended to address false offsets.MethodsThe vendor implemented two software improvements: a volumetric (ACO) rather than planar calibration and, approximately 6 months later, an improved calibration workflow (CIB) designed to better compensate for thermal drift. Offsets relative to the reference position, obtained at table angle 0 following image‐guided setup, were recorded before beam‐on at each table position and at the end of treatment the table returned to 0° for patients receiving SRT.ResultsPrior to ACO, between ACO and CIB, and after CIB, 223, 155, and 436 fractions were observed respectively. The median magnitude of translational offsets at the end of treatment was similar for all three intervals: 0.29, 0.33, and 0.27 mm. Prior to ACO, the offset magnitude for non‐zero table positions had a median of 0.79 mm and was found to increase with increasing distance from isocenter to the anterior patient surface. After ACO, the median magnitude was 0.74 mm, but the dependence on surface‐to‐isocenter distance was eliminated. After CIB, the median magnitude for non‐zero table positions was reduced to 0.57 mm.ConclusionOngoing improvements in software and calibration procedures have decreased reporting of false offsets at non‐zero table angles. However, the median magnitude for non‐zero table angles is larger than that observed at the end of treatment, indicating that accuracy remains better when the table is not rotated.

Highlights

  • Frameless radiosurgery has been used to improve patient comfort during treatment and has been shown to have the same level of positional accuracy as framed treatments.[1,2,3,4] Our institution has previously published on the use of flattening filter free (FFF)‐VMAT stereotactic radiotherapy (SRT) to significantly reduce treatment time 5 as well as utilizing a single isocenter for the treatment of multiple brain metastases.[6]

  • The optical surface monitoring system (OSMS) system presents the deviations from this reference surface, which are referred to as real time deltas (RTDs)

  • We present the aggregate data of intrafraction motion during frameless FFF‐VMAT SRT captured via OSMS before and after two updates provided by the vendor: (a) a vendor performed volumetric, rather than planar, calibration, called advanced camera optimization (ACO) and (b) a user performed cold camera state calibration detailed in a customer information bulletin (CIB)

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Summary

Introduction

Frameless radiosurgery has been used to improve patient comfort during treatment and has been shown to have the same level of positional accuracy as framed treatments.[1,2,3,4] Our institution has previously published on the use of FFF‐VMAT stereotactic radiotherapy (SRT) to significantly reduce treatment time 5 as well as utilizing a single isocenter for the treatment of multiple brain metastases.[6]. Optical surface guidance has been used for patient positioning and monitoring for a range of clinical sites,[9,10,11,12] including intracranial SRS.[4,13] Detailed descriptions of this technology have been previously published[14] and only a basic overview is presented here. OSMS uses three pods, each with two cameras and a projector, to monitor the patient during treatment. The projector is used to project a pseudo‐speckle pattern on the patient that is used for the three‐dimensional reconstruction of the patient’s position. The patient is monitored with a reference surface, called the region‐of‐interest (ROI), which is created from the patient’s CT scan or can be captured before treatment begins. Treatment thresholds can be set that will gate the beam if these thresholds are exceeded

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