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Evaluation of the Clinical Impact of Dark Current On the Delivery of Gated Treatments with An MR-Guided Radiotherapy System

A Shepard*, K Mittauer , J Bayouth , P Yadav , University of Wisconsin-Madison, Madison, WI


(Monday, 7/15/2019) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 5

Purpose: With the increasing usage of image guidance and gating in radiotherapy, it is necessary to consider the potential effects that dark current produced during beam-hold may have on patient treatments. This work aimed to quantify the dark current and its clinical impact on gated deliveries on an MR-guided radiotherapy system.

Methods: Dark current measurements were acquired on a monthly basis as a longitudinal assessment of the ViewRay MRIdian linac stability. Ion chamber measurements were used to determine the relative dark current contribution over the period of 60 seconds for an open field measurement with the beam in the “on� state, followed by a respective measurement with the beam maintained in the “hold� state. The dark current was characterized as the relative dose rate in the “hold� state compared to the nominal dose rate measured in the “on� state. The clinical impact was considered by retrospectively analyzing the total beam-hold time and the expected dose due to dark current for 51 plans and 320 total fractions.

Results: The observed dose rate due to dark current as analyzed monthly (N=8) was 64.4±7.87 µGy/s (0.120±0.024% of the measured nominal dose rate). Utilizing the average monthly dark current dose rate, the average fractional dark current contribution for 320 fractions and 7 clinical sites was 0.018±0.016 Gy. Correspondingly, the dark current contribution to the organs at risk relative to the target prescription dose was 0.26±0.23%. The maximum contribution relative to the prescription dose was 1.46% for an individual fraction and 0.76% for a patient. Minimal dependence was observed with varying treatment site.

Conclusion: The estimated dark current contribution had minimal impact on the delivery of the prescription dose, however, it is still recommended to verify its stability quarterly or semi-annually.

Funding Support, Disclosures, and Conflict of Interest: Dr Mittauer reports personal fees and consulting fees from ViewRay Inc. during conduct of this study. Dr Bayouth reports membership on scientific advisory board of ViewRay Inc. during conduct of this study.


Gating, Quality Assurance, Image-guided Therapy


IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance

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