2018 - Intra-Op Imaging Method for Treatment Planning for HDR Brachytherapy

Purpose/Objective(s): Interstitial HDR brachytherapy involves precise, localized delivery to targets with high dose gradients, sparing adjacent organs at risk (OAR). The process of treatment planning can be time-intensive, with much of the workflow spent waiting on the patient to arrive at the department, followed by manually contouring of regions-of-interest (ROIs) and reconstructing the catheters in the planning system. A preliminary investigation was proposed to use the O-arm imager in the OR, which is currently used for verification fluoroscopy at the end of the surgery, to create a CT for treatment planning. The purpose of this study is to assess the quality and ease-of-use of using O-arm images for HDR interstitial plans and determine whether pre-treatment imaging is suitable for reconstructing the catheters and regions of interest used in planning to reduce the time burden on department staff.

Materials/Methods: An O-arm is a mobile, large FOV CBCT scanner typically used in orthopedic, trauma, neurosurgery, and veterinary surgeries, & able to accurately image bony and soft tissue anatomy quickly. For the proposed workflow, while the patient is in the operating room, & after the implant is completed, the imaging technologist acquires a volume image using the O-arm imager as a verification of needle position. Once the imaging procedure is completed, the technologist uploads the images into the hospital image repository (PACS) for the radiation oncology department to query and retrieve. The treatment planning process then proceeds, using the O-arm images as a preliminary plan. Once the patient is cleared for transport and is imaged on the department CT simulator, the image sets are subsequently registered using the fiducial markers as a guide, the preliminary plan is adjusted, and the workflow continues.

Results: 3D displacement was 0.76 cm ± 1.6 cm (n = 511) with the O-arm vs the approved CT-sim plan. Angular deflection increased, with a difference of 1.22° ± 0.74° (n = 42). Where the O-ring was used for a preliminary plan, the time required of physics for planning was on average 3:16 ± 0:33 (h:mm) vs 1:54 ± 0:31 for the standard workflow. For plans with metal needles, it was easier to reconstruct the needle vs plastic catheters (0.597cm ± 0.33cm vs 0.92cm ± 2.2 cm) but still unacceptable.

Conclusion: The time it takes to create a treatment plan for interstitial HDR continues to be one of the most important factors for quality treatments. Adding the O-arm to the interstitial HDR workflow did not improve efficiency. The proposed workflow increased physics burden, as plans were having to be redone from scratch and adding an extra 30-40 minutes of physics time that could be allocated elsewhere. The proposed workflow is promising, and rather than abandoning it altogether, the institution will revisit the workflow once a new, updated O-arm has been installed. Future work will include refining this workflow and working with the vendor to create imaging protocols that improve catheter visualization and soft tissue contrast.