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Evaluation of MR Images as the Planning and Reference Dataset for Daily CBCT-Based IGRT of the Prostate

A Doemer

A Doemer*, T Nurushev, M Siddiqui, M Elshaikh, B Movsas, I Chetty, Henry Ford Health System, Detroit, MI

TH-C-141-11 Thursday 10:30AM - 12:30PM Room: 141

Purpose: An important question rarely discussed in the CT vs. MRI simulation debate is whether MRI reference images provide an adequate image-set to use with daily localization such as cone-beam CT (CBCT). This study compares clinical couch shifts based on daily CBCT images to shifts measured from MR images as the reference dataset for prostate IMRT treatment.

Methods: Eight patients undergoing a pilot study had MR imaging along with CT simulation with the intent of evaluating a MR-simulation process. Patients had T1, T2 and bTFE (balanced Turbo Field Echo) sequences. The remainder of the treatment planning process continued using traditional procedures using CT. Therapists used only the CT scan as a reference for localization. Retrospectively, an observer measured shifts between daily CBCT images and MR reference images.

Results: The differences in shift positions for the cohort between therapists and the observer are -0.16cm ± 0.25cm (AP), 0.04cm ± 0.19cm (SI), and -0.01cm ± 0.14cm (LR). The mean group error for the therapists and the observer were less than 2 mm in all directions. Based on these shifts, the calculated margins for the therapists would be 0.87cm (AP), 0.65cm (SI), and 0.71cm (LR) and for the observer would be 1.1cm (AP), 0.66cm (SI), and 0.70cm (LR). For SI and LR directions both sets of margins are very close to one another. An outlier impacted the AP margin difference by 2.3mm and should be investigated further. This initial analysis suggests that each modality can be considered clinically sufficient for daily localization.

Conclusion: The results of this study suggest that MR reference image-sets can be used for daily image-guided localization of prostate cancers with at least the same accuracy as current methods. MR simulation provides substantial soft-tissue contrast and can improve tissue targeting in radiation oncology, as a result further investigation is warranted.

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