Superficial dosimetry imaging based on Čerenkov emission for external beam radiotherapy with megavoltage x-ray beam

Abstract

Purpose: Čerenkov radiation emission occurs in all tissue, when charged particles (either primary or secondary) travel at velocity above the threshold for the Čerenkov effect (about 220 KeV in tissue for electrons). This study presents the first examination of optical Čerenkov emission as a surrogate for the absorbed superficial dose for MV x-ray beams.

Methods: In this study, Monte Carlo simulations of flat and curved surfaces were studied to analyze the energy spectra of charged particles produced in different regions near the surfaces when irradiated by MV x-ray beams. Čerenkov emission intensity and radiation dose were directly simulated in voxelized flat and cylindrical phantoms. The sampling region of superficial dosimetry based on Čerenkov radiation was simulated in layered skin models. Angular distributions of optical emission from the surfaces were investigated. Tissue mimicking phantoms with flat and curved surfaces were imaged with a time domain gating system. The beam field sizes (50 × 50-200 × 200 mm(2)), incident angles (0°-70°) and imaging regions were all varied.

Results: The entrance or exit region of the tissue has nearly homogeneous energy spectra across the beam, such that their Čerenkov emission is proportional to dose. Directly simulated local intensity of Čerenkov and radiation dose in voxelized flat and cylindrical phantoms further validate that this signal is proportional to radiation dose with absolute average discrepancy within 2%, and the largest within 5% typically at the beam edges. The effective sampling depth could be tuned from near 0 up to 6 mm by spectral filtering. The angular profiles near the theoretical Lambertian emission distribution for a perfect diffusive medium, suggesting that angular correction of Čerenkov images may not be required even for curved surface. The acquisition speed and signal to noise ratio of the time domain gating system were investigated for different acquisition procedures, and the results show there is good potential for real-time superficial dose monitoring. Dose imaging under normal ambient room lighting was validated, using gated detection and a breast phantom.

Conclusions: This study indicates that Čerenkov emission imaging might provide a valuable way to superficial dosimetry imaging in real time for external beam radiotherapy with megavoltage x-ray beams.

Figure 1

(a) Position of detectors on flat phantom with respect to the beam field. (b) and (c) Energy spectra of charged particles for entrance and exit planes. (d) and (e) Images (self-normalized by the maximum pixel value) of superficial dose and local Čerenkov emission for entrance plane. (h) and (i) Images (self-normalized by the maximum pixel value) of superficial dose and local Čerenkov emission for exit plane. (f) and (j) CP and IP profiles of radiation dose and local Čerenkov emission for entrance and exit planes. (g) and (k) Discrepancy of CP and IP profiles of radiation dose and local Čerenkov emission for entrance and exit planes.

Figure 2

(a) Placement of detectors along the arc of the cylindrical phantom. (b) Energy spectra of charged particles along the arc. (c) and (d) Images (self-normalized by the maximum pixel value) of radiation dose and local Čerenkov emission of the central transection of the cylindrical phantom. (e) and (g) Profiles of superficial dose and local Čerenkov emission for entrance (0°–90°) and exit (90°–180°) plane (As indicated by dashed lines in Figs. 2(c) and 2(d), profiles were calculated based on a layer of 3 mm along the arc of the cylindrical phantom.). (f) and (h) Discrepancies between profiles of superficial dose and local Čerenkov emission for entrance (0°–90°) and exit (90°–180°) plane.

Figure 3

(a) The sampling depth distribution of Čerenkov photons and corresponding exponential fitting for the three types of skin [skin 1: lightly pigmented skin (∼1% melanin in epidermis), skin 2: moderately pigmented (∼12% melanin in epidermis), skin 3: darkly pigmented (∼30% melanin in epidermis)] for entrance plane. (b) The spectra of Čerenkov emission from the entrance surface. (c) Effective sampling depth for different wavelength ranges on the entrance surface.

Figure 4

(a) The sampling depth distribution of Čerenkov photons and corresponding exponential fitting for the three types of skin for exit plane. (b) The spectra of Čerenkov emission from the exit surface. (c) Effective sampling depth for different wavelength ranges on the exit surface.

Figure 5

(a) Simulation of the angular distribution of Čerenkov photons escaped the surfaces for flat phantom with pencil beams. (b) Simulation of the angular distribution of Čerenkov photons escaped the surfaces for the cylindrical phantom. (c) Angular distribution for different skin types. (d) Angular distributions for skin 1 with different incident angles from 0° to 70°. (e) Angular distributions for skin 1 along the arc of the cylindrical phantom. (f) Angular distributions for different tissue thickness from 0.1 to 100 mm on the entrance surface.

Figure 6

(a) Time domain gating system. (b) Acquisition speed of one frame of image with accumulation of different number of radiation bursts. (c) Signal to noise ratio for different acquisition procedures. (d) Čerenkov images (self-normalized by the maximum pixel value) of flat phantom with field sizes from 20 × 20 to 200 × 200 mm². (e) Čerenkov images (self-normalized by the maximum pixel value) of flat phantom with incident angles from 0° to 70°.

Figure 7

(a) A picture of the breast phantom placed on an anthropomorphic torso phantom (entrance, tangential, and exit imaging region indicated in the picture). (b) A picture shows the ambient light tested for time domain gating. (c)–(e) Čerenkov images (ambient light off) of the entrance, tangential, and exit region of the breast phantom under whole breast radiotherapy. (f) Čerenkov image of the exit region with ambient light on.