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Correlation of Intrafraction KV Frames with VMAT Beam MU for Hypo-Lung Cancer Treatment

J Liang*, J Zhou , I Grills , Q Liu , D Yan , William Beaumont Hospital, Royal Oak, MI


(Tuesday, 7/31/2018) 10:00 AM - 10:30 AM

Room: Exhibit Hall | Forum 6

Purpose: To investigate relationship between the number of intrafraction KV frames and VMAT beam MU for hypo-lung cancer treatment under one global intrafraction parameter setting.

Methods: The acquisition interval is the most critical parameter that affects the number of KV frames acquired during beam ON time. We optimized it to be 0.1 degree per image in imaging preset. The actual acquisition interval varies and depends on beam delivery and KV panel’s fixed acquisition frequency. We tried to find a way to estimate number of KV frames based on beam MU so that we could determine using 3D or 4D reconstruction in prior. Totally 17 hypo-lung treatment plans were used for this intrafraction imaging study. KV beam span was preset to 200 degrees. MV beam gantry span ranges from 150 to 200 degrees, and mean beam MU is 1300±355 (842-2084).

Results: Number of KV frames were 781±162 (567-1116). The mean acquisition interval was 0.18 and 0.35 degree/image respectively for beams with the largest and smallest MU. Strong linear correlation (frames = 193.34 + 0.44*MU, R2=0.96) existed. KV frames were not enough to create acceptable quality of 4DCBCT when (1) smaller MV beam span range (<150 degree), and (2) smaller beam MU in which gantry moves at the fast speed with MV on. Further reducing this parameter has marginal effect because of the limitation of KV acquisition frequency. In these cases, 3DCBCT might be used instead for treatment position evaluation and treatment dose reconstruction.

Conclusion: Number of KV frames could be estimated based on treatment beam MU. This number could help therapist to decide whether to use 3D or 4D CBCT to evaluate target position. A unified acquisition interval in imaging preset is adequate and patient-specific preset is not necessary if a careful pre-treatment check is implemented.

Funding Support, Disclosures, and Conflict of Interest: Research sponsored by Elekta Company


Cone-beam CT, Image-guided Therapy


IM/TH- RT X-ray Imaging: combined KV/MV imaging

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