Evaluating Bacterial Response in Sarcoma Management Using Fluorescence Imaging

Optical Tracking System

For this study, PRODIGI was combined with a commercial optical tracking system (OTS, Polaris, NDI Medical, Waterloo, Ontario, Canada) to track the movement of the device in space relative to a patient over time. This addition is in the form of an infrared-light camera, which tracks four IR reflective spheres (NDI Medical, Waterloo, Ontario, Canada) that are fixed to the external housing of PRODIGI device.

The OTS has been described previously. Briefly, it consists of optical tracking technology, which provides 6 degrees of freedom (x, y, z, pitch. yaw, roll), attached to the camera along with software developed in house to register and visualize the tracked camera pose relative to a previously acquired radiological volumetric image data. The in-house software platform GTxEyes performs tracking and navigation of the imaging camera, camera calibration (including any image distortion), registration of the camera coordinates with respect to the CT images, and co-visualization (e.g. visual overlay) of the camera and CT images. Radiation dose planning information can also be spatially co-registered and overlaid with the hybrid optical-CT images/videos using methods described previously by our group.

After loading a CT image of the patient, the OTS is registered to the CT coordinate space by identification of known fiducials using a conventional pointer tool. With registration complete, the coordinates of the PRODIGI camera can be tracked in real time relative to the CT image. Viewing options include orthogonal views through the CT image, corrected PRODIGI image and a virtual image based on the CT surface rendering viewed from the perspective of a virtual camera at the PRODIGI coordinates. Radiation dose can be displayed on the real and virtual camera views as either isodose lines or colorwash.

Patient Population

Patients will be recruited from the Princess Margaret Cancer Centre, University Health Network (Ontario, Canada) sarcoma clinic to be treated with pre-operative external beam RT followed by surgical resection of lower limb soft-tissue sarcoma. Informed consent was obtained according to institutional Research Ethics Board (REB) requirements and Good Clinical Practice (GCP) (ClinicalTrials.gov NCT02270086). Patients with pre-existing skin issues, who received prior radiotherapy or required chemotherapy were not eligible to the study.

Imaging Procedures

At the radiotherapy planning stage, a CT scan of the patient sarcoma site is acquired for standard treatment planning purposes. Prior to the CT scan, small radio-opaque fiduciary markers (Suremark TM skin marking labels) were placed on the patient's six radiation treatment setup points, making the <1 mm diameter points easy to identify in the CT images. After the CT scan, these markers were replaced with ink tattoos and used during radiation treatment to align the patient with radiation therapy machine reference frame. In addition to the CT fiducial markers placed at the treatment setup points, a flexible radio-opaque wire was overlaid on the planned surgical incision. This enabled localization of the entire surgical scar during the radiation treatment plan procedure. A radiotherapy stereotactic mask was also made at that stage. Following CT simulation scan acquisition, an appropriate radiotherapy plan was designed as per institutional clinical standard guidelines.

Imaging with PRODIGI and the OTS was performed throughout the sarcoma management, i.e. during radiotherapy and in the operating room. Imaging was performed at three time points during RT: fractions 0, 12 and 25, i.e. at the beginning, middle and end of the treatment. Four out of six treatment setup points marked with radio-opaque fiduciary markers on the CT scans were used to perform the optical to CT co-registration. An optically-tracked pointer tool using four IR reflective spheres (identical to the ones fixed to the PRODIGI system) was used to register the patient in space with respect to the IR camera. For this, the pointer was placed sequentially on each tattoo mark, identified and spatially registered by the tracking IR camera and visualized in real-time on the CT scans using the custom-built software GTxEyes. The locations of the tattoo points in the optical tracking coordinate system were then registered to the corresponding points in the CT image using the fiduciary CT markers. Once registration was completed, the planned surgical scar was drawn on the patient's skin with a marker by superimposing the optically-tracked pointer on the scar visible on the CT scans. An imaging session consisted of both WL and corresponding AF imaging of the planned skin surgical incision and surrounding tissue. Room lights were turned off during AF imaging to avoid background signal and artifacts. The four reflective spheres on the PRODIGI device were pointed towards the IR camera to ensure proper tracking in 3D during the entire session.

To perform PRODIGI imaging in the operating room, a sterilization method approved by UHN's control and processing department using a sterile drape was found to be the most effective way to ensure proper sterile conditions without damaging the instrument or affecting its performance. An elongated sterile drape (Cardinal Health Canada, 29-59029) was used to cover the entire PRODIGI imaging system, i.e. the camera and electrical power cord. Six strong neodymium magnets (Super Magnets, 8 mm diameter) were embedded into the emission filter slider and six corresponding magnets were autoclaved prior to each use and placed on the outside of the drape to hold it in place to avoid image quality degradation. Steri-strips (3M, R1547) were applied on the outside of the draped device to tighten the drape around the IR reflective spheres to insure adequate tracking during imaging. Imaging was performed at five time points: before and after sterilization of the surgical site (OR1, OR2), once the flap was raised (OR3), after tumor excision (OR4) and after closure (OR5).

The combined WL and AF images were part of a superset of data recorded using the tracking system. The superset also included preoperative patient CT and the patient's RT dose volume. A skin surface model was generated from the patient's CT, where each surface point held a quantitative tuple that contained the surface normal vector, the RT dose, the WL scalar, the AF scalar, and the camera pose corresponding to the AF scalar. Overlay of WL and AF images on the skin surface model was visualized using the software ParaView.