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Cone-Beam CT Frequency in Image Guided Radiation Therapy of Pancreatic Cancer Patients

J Zhou

J Zhou*, G Lasio , A Gopal , W D'Souza , K Prado , B Yi , Univ. of Maryland School Of Medicine, Baltimore, MD


MO-L-GePD-J(B)-4 (Monday, July 31, 2017) 1:15 PM - 1:45 PM Room: Joint Imaging-Therapy ePoster Lounge - B

Purpose: To provide a decision framework for determining if Cone Beam CT frequency needs increasing in image-guided radiotherapy of pancreas patients.

Methods: At our institution image guidance for pancreatic cancer patients includes bony anatomy alignment with orthogonal 2D kV images, followed by soft tissue alignment with kV Cone Beam CT daily for the first 3 fractions, and daily 2D kV plus weekly CBCT thereafter. We selected 35 patients who received at least 8 CBCTs in their treatment course (364 CBCT scans in total). The post-CBCT shift magnitude in the first 3 fractions was used to estimate the need to increase CBCT frequency above once per week. An additional CBCT was acquired if the action threshold was exceeded at least once among the first three treatments. We measured the fraction of the total CBCTs exceeding the action threshold in at least 60% of the scans (total match TM). We propose that the CBCT frequency should be increased if the action threshold is exceeded in 2 or more of the initial CBCTs.

Results: 82.8%, 88.6% and 91.4% of the patients belonged to the TM group, when the setup thresholds selected were 3, 4 and 5mm respectively. The specificities for determining no need for increased CBCT frequency from the first three fractions were 0.89, 0.91 and 0.94 with thresholds 3, 4 and 5 mm. The sample size was too small, at this stage, to derive meaningful sensitivities.

Conclusion: The categorization approach we proposed identified candidates for increased CBCT frequency if a 5 mm action threshold is used. Patients for which CBCT frequency should not be increased were identified with higher confidence for all three action thresholds. The specificity improves for all action thresholds if a tighter criterion of at least one of the initial CBCTs exceeding the action threshold.

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