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Analysis of Varian RPM Reproducibility During Deep Inspiration Breath Hold with AlignRT Monitoring

E Wright*, K Bota, S Karnas, S Gaede, London Regional Cancer Program, London, ON, CA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

To assess the reproducibility of external marker-based tracking during deep inspiration breath-hold (DIBH) breast cancer radiotherapy by monitoring with surface-guided radiation therapy.

Six breast cancer patients were treated with whole breast tangents under DIBH guided by the Varian Real-Time Position Management (RPM) system on a True Beam Linear Accelerator (Varian Medical Systems, Palo Alto, USA). AlignRT (Vision RT, London, UK) was used to simultaneously monitor the position of a user-defined region-of-interest (ROI) on the patient surface relative to a pre-defined reference surface. Patients performed DIBH for each tangent separately. For each treatment fraction, the difference in mean vertical position of the RPM block and the mean real-time delta values of the ROI in all 6 dimensions generated with AlignRT between each tangent during the beam-on time were determined.

Correlation coefficients between the difference in RPM block height between tangents and the difference in AlignRT real-time deltas between tangents were greater than 0.7 in 4/6 patients for the vertical parameter, and 1/6 patients for longitude and roll parameters. Other AlignRT parameters were not strongly correlated in any patient. When all 74 total fractions were analyzed together, RPM block height difference was significantly correlated (p<0.001) with the difference in AlignRT vertical (r=0.43). However, correlation was poor for other parameters, especially rotation, roll, and pitch (r=0.0001-0.03).

While we expected differences of the vertical position of the RPM block between each tangent to be less than the intended gating window width, differences in real-time delta values monitored by AlignRT between successive breath-holds suggest that patient motion other than breathing may drive the marker block into the gating window (ex. arching the back). These factors may affect the ability to pull the heart out of the treatment field as intended. Monitoring the motion in all 6 dimensions are recommended.


Gating, Patient Movement, Patient Positioning


TH- RT Interfraction Motion Management: external markers-based

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