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Sensitivity Evaluation of Organ Based Registration for Pancreatic IGRT

W Yang

W Yang*, R Tuli , H Sandler , B Fraass , Cedars-Sinai Medical Center, Los Angeles, CA


SU-E-J-132 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Soft-tissue-based CBCT registration is not always straightforward. Typical image guidance is based on use of generic registration criteria that may not be adequately sensitive to PTV and critical organ positioning errors. This work presents a novel organ-based registration criterion to help quantify registration results and guidance.

Ten pancreatic cancer patients treated using IGRT were included. Five individual CBCTs for each patient were manually registered to the planning CT by an expert. Instead of calculating the cross-correlation (CC) for the entire imaging volume, the CC values of the PTV and critical normal tissue (limited to a 1-cm ring around the PTV), were calculated as references. The CBCT images were then intentionally shifted in three axes by 1, 3, 5, 8, and 10 mm. New CC values were computed between the shifted CBCT and planning CT. The organ based CC was called sensitive to a given shift size when the p-value between new and reference CC values for the 5 CBCTs was <0.05.

The average reference CC was 0.23 (0.06-0.41). Despite the low correlation values, the CC-PTV is sensitive to average registration errors ≥ 5, 3 and 5 mm in the sup/inf, ant/post and right/left directions, respectively. Critical organ cross-correlation is sensitive to registration errors ≥ 7, 4 and 6 mm in these directions. 3/10 patients showed particularly poor sensitivity (≥ 8 mm) due to low reference CC, heavy contrast presence in the planning CT or small PTV volume.

CBCT of upper abdomen tends to have very low similarity to the planning CT, affecting registration accuracy and reliability. This organ-based registration method focuses on the PTV and the most critical tissues, and shows improved sensitivity to positioning errors that may not be easily resolved by visual inspection.

This work supported in part by grant 1R03CA173273-01

Funding Support, Disclosures, and Conflict of Interest: This work supported in part by grant 1R03CA173273-01

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