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A Comparison of Two Immobilization Systems for Prostate Radiotherapy

F Mostafaei*, S Dougherty, R Hamilton, University of Arizona, Department of Radiation Oncology, Tucson, AZ


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: systems and their corresponding set-up errors influence the clinical target volume to planning target volume (CTV-PTV) margins and thus may result in undesirable treatment outcomes. This study compared the reproducibility of patient positioning with a Civco CombiFix (CCF) system and a Medical Intelligence Vacloc (MIV) system for localized prostate cancer.

Methods: consecutive patients (5 fractions per patient) with localized prostate cancer and implanted marker seeds in the prostate were selected. Three patients were immobilized with each system. Patients were treated on a Novalis linear accelerator using the ExacTrac6D Robotics (ETR) system for positioning. Once a patient was positioned at the isocenter, planar x-ray images were acquired pre-treatment with the ETR and another set were acquired following treatment delivery. The planar x-ray images were registered with DRRs from the treatment planning CT by matching the implanted marker seeds either manually or automatically. The residual differences between planning DRR and x-ray images in the vertical, longitudinal, and lateral directions were recorded for the pre-treatment and post-treatment images. To find the intrafraction displacements of the prostate, the post-treatment values were subtracted from the pre-treatment values.

Results: average displacements in lateral, longitudinal and vertical directions, respectively, for patients immobilized with CCF were 0.9 ± 0.2 mm, 1.7 ± 0.4 mm, and 1.3 ± 0.3 mm, and for patients immobilized with MIV were 0.5 ± 0.2 mm, 0.6 ± 0.2 mm, and 0.8 ± 0.2 mm. The total three-dimensional displacements averaged over all patients and treatment fractions, were 2.8 ± 0.4 mm for the CCF system and 1.3 ± 0.3mm for the MIV system.

Conclusion: use of MIV resulted in smaller intra-fractional displacements compared to the CCF. This may require adjustment of PTV margins to accommodate this movement when using CCF. However, the substantial cost difference of CCF versus MIV must also be considered.


Immobilization, Prostate Therapy


TH- External Beam- Photons: Motion management - intrafraction

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