Assessment of imaging Cherenkov and scintillation signals in head and neck radiotherapy

Abstract

The goal of this study was to test the utility of time-gated optical imaging of head and neck (HN) radiotherapy treatments to measure surface dosimetry in real-time and inform possible interfraction replanning decisions. The benefit of both Cherenkov and scintillator imaging in HN treatments is direct daily feedback on dose, with no change to the clinical workflow. Emission from treatment materials was characterized by measuring radioluminescence spectra during irradiation and comparing emission intensities relative to Cherenkov emission produced in phantoms and scintillation from small plastic targets. HN treatment plans were delivered to a phantom with bolus and mask present to measure impact on signal quality. Interfraction superficial tumor reduction was simulated on a HN phantom, and cumulative Cherenkov images were analyzed in the region of interest (ROI). HN human patient treatment was imaged through the mask and compared with the dose distribution calculated by the treatment planning system. The relative intensity of radioluminescence from the mask was found to be within 30% of the Cherenkov emission intensity from tissue-colored clay. A strong linear relationship between normalized cumulative Cherenkov intensity and tumor size was established ([Formula: see text]). The presence of a mask above a scintillator ROI was found to decrease mean pixel intensity by >40% and increase distribution spread. Cherenkov imaging through mask material is shown to have potential for surface field verification and tracking of superficial anatomy changes between treatment fractions. Imaging of scintillating targets provides a direct imaging of surface dose on the patient and through transparent bolus material. The first imaging of a patient receiving HN radiotherapy was achieved with a signal map which qualitatively matches the surface dose plan.

Figure 1:

(A) Thermoplastic mask fitting over body phantom wrapped in molding clay; detailed view of differing mass sizes in the neck region. (B) Thermoplastic mask fitting over phantom, showing scintillators underneath the mask. (C) Photograph of patient imaging setup.

Figure 2:

Relative, background-corrected, normalized radioluminescence spectra for purple, white, and orange thermoplastic mask samples, as well as transparent bolus, tissue-colored clay, and wavelength-dependent relative camera sensitivity.

Figure 3:

Relative intensity and SNR values for scintillator, tissue-colored clay, transparent bolus, and mask samples. Luminescence images and photographs of orange (B), purple (C), and white (D) thermoplastic mask samples. Colorbar is presented in photons cm⁻² sr⁻¹ s⁻¹.

Figure 4:

Cumulative images of Cherenkov intensity from VMAT treatment of the neck of an anthropomorphic phantom with an artificial tumor of decreasing size in order from (A) to (D). (E) Plot of Cherenkov emission intensity in the ROI compared to tumor volume. Coefficients of determination for both fits are shown in the lower left corner. Colorbar and vertical axis are presented in photons cm⁻² sr⁻¹ s⁻¹.

Figure 5:

Cumulative images showing various combinations of treatment materials applied to a phantom during irradiation, with and without scintillators. The same head and neck VMAT plan was administered in all eight scenarios. The top row (A-D) shows images from treatment with transparent bolus, while the bottom row (E-F) shows images from treatment without bolus. Colorbar is presented in photons cm⁻² sr⁻¹ s⁻¹.

Figure 6:

Coregistered intensity maps from patient treatment plan and treatment delivery. (A) Dose distribution overlaid with 3D simulation CT data from the patient treatment plan. (B) Cumulative Cherenkov intensity distribution captured through the immobilization mask acquired real-time during treatment. Colorbar is presented in photons cm⁻² sr⁻¹ s⁻¹.