An investigation of clinical treatment field delivery verification using cherenkov imaging: IMRT positioning shifts and field matching

Purpose: Cherenkov light emission has been shown to correlate with ionizing radiation dose delivery in solid tissue. An important clinical application of Cherenkov light is the real-time verification of radiation treatment delivery in vivo. To test the feasibility of treatment field verification, Cherenkov light images were acquired concurrent with radiation beam delivery to standard and anthropomorphic phantoms. Specifically, we tested two clinical treatment scenarios: (a) Observation of field overlaps or gaps in matched 3D fields and (b) Patient positioning shifts during intensity modulated radiation therapy (IMRT) field delivery. Further development of this technique would allow real-time detection of treatment delivery errors on the order of millimeters so that patient safety and treatment quality can be improved.

Methods: Cherenkov light emission was captured using a PI-MAX4 intensified charge coupled device (ICCD) system (Princeton Instruments). All radiation delivery was performed using a Varian Trilogy linear accelerator (linac) operated at 6 MV or 18 MV for photon and 6 MeV or 16 MeV for electron studies. Field matching studies were conducted with photon and electron beams at gantry angles of 0°, 15°, and 45°. For each modality and gantry angle, a total of three data sets were acquired. Overlap and gap distances of 0, 2, 5, and 10 mm were tested and delivered to solid phantom material of 30 × 30 × 5 cm³ . Phantom materials used were white plastic water and brown solid water. Tests were additionally performed on an anthropomorphic phantom with an irregular surface. Positioning shift studies were performed using IMRT fields delivered to a thoracic anthropomorphic phantom. For thoracic phantom measurements, the camera was placed laterally to observe the entire right side of the phantom. Fields were delivered with known translational patient positioning shifts in four directions. Changes in the Cherenkov fluence were evaluated through the generation of difference maps from unshifted Cherenkov images. All images were evaluated using ImageJ, Python, and MATLAB software packages.

Results: For matched fields, Cherenkov images were able to quantitate matched field separations with discrepancies between 2 and 4 mm, depending on gantry angle and beam energy or modality. For all photon and electron beams delivered at a gantry angle of 0°, image analysis indicated average discrepancies of less than 2 mm for all field gaps and overlaps, with 83% of matched fields exhibiting discrepancies less than 1 mm. Beams delivered obliquely to the phantom surface exhibited average discrepancies as high as 4 mm for electron beams delivered at large oblique angles. Finally, for IMRT field delivery, vertical and lateral patient positioning shifts of 2 mm were detected in some cases, indicating the potential detectability threshold of using this technique alone.

Conclusions: Our study indicates that Cherenkov imaging can be used to support and bolster current treatment delivery verification techniques, improving our ability to recognize and rectify millimeter-scale delivery and positioning errors.