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Effect of Radiation Therapy Planning Scan Registration On the Dose in Lung Cancer Patient CT Scans

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A Cunliffe

A Cunliffe1*, C Contee1 , B White1 , J Justusson1 , S Armato1 , R Malik2 , H Al-Hallaq2 , (1) Department of Radiology, The University of Chicago, Chicago, IL, (2) Department of Radiation & Cellular Oncology, The University of Chicago, Chicago, IL

Presentations

TU-F-BRF-3 Tuesday 4:30PM - 6:00PM Room: Ballroom F

Purpose: To characterize the effect of deformable registration of serial computed tomography (CT) scans on the radiation dose calculated from a treatment planning scan.

Methods: Eighteen patients who received curative doses (≥60Gy, 2Gy/fraction) of photon radiation therapy for lung cancer treatment were retrospectively identified. For each patient, a diagnostic-quality pre-therapy (4-75 days) CT scan and a treatment planning scan with an associated dose map calculated in Pinnacle were collected. To establish baseline correspondence between scan pairs, a researcher manually identified anatomically corresponding landmark point pairs between the two scans. Pre-therapy scans were co-registered with planning scans (and associated dose maps) using the Plastimatch demons and Fraunhofer MEVIS deformable registration algorithms. Landmark points in each pre-therapy scan were automatically mapped to the planning scan using the displacement vector field output from both registration algorithms. The absolute difference in planned dose (|ΔD|) between manually and automatically mapped landmark points was calculated. Using regression modeling, |ΔD| was modeled as a function of the distance between manually and automatically matched points (registration error, E), the dose standard deviation (SD_dose) in the eight-pixel neighborhood, and the registration algorithm used.

Results: 52-92 landmark point pairs (median: 82) were identified in each patient's scans. Average |ΔD| across patients was 3.66Gy (range: 1.2-7.2Gy). |ΔD| was significantly reduced by 0.53Gy using Plastimatch demons compared with Fraunhofer MEVIS. |ΔD| increased significantly as a function of E (0.39Gy/mm) and SD_dose (2.23Gy/Gy).

Conclusion: An average error of <4Gy in radiation dose was introduced when points were mapped between CT scan pairs using deformable registration. Dose differences following registration were significantly increased when the Fraunhofer MEVIS registration algorithm was used, spatial registration errors were larger, and dose gradient was higher (i.e., higher SD_dose). To our knowledge, this is the first study to directly compute dose errors following deformable registration of lung CT scans.

Funding Support, Disclosures, and Conflict of Interest: Supported, in part, by NIH Grant Nos. S10 RR021039, P30 CA14599, and T32 EB002103-23, the Virginia and D. K. Ludwig Fund for Cancer Research, Imaging Research Institute, Biological Sciences Division, The University of Chicago, and a 2013 AAPM Minority Undergraduate Summer Experience Fellowship.


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