Beam and tissue factors affecting Cherenkov image intensity for quantitative entrance and exit dosimetry on human tissue

Abstract

This study's goal was to determine how Cherenkov radiation emission observed in radiotherapy is affected by predictable factors expected in patient imaging. Factors such as tissue optical properties, radiation beam properties, thickness of tissues, entrance/exit geometry, curved surface effects, curvature and imaging angles were investigated through Monte Carlo simulations. The largest physical cause of variation of the correlation ratio between of Cherenkov emission and dose was the entrance/exit geometry (˜50%). The largest human tissue effect was from different optical properties (˜45%). Beyond these, clinical beam energy varies the correlation ratio significantly (˜20% for X-ray beams), followed by curved surfaces (˜15% for X-ray beams and ˜8% for electron beams), and finally, the effect of field size (˜5% for X-ray beams). Other investigated factors which caused variations less than 5% were tissue thicknesses and source to surface distance. The effect of non-Lambertian emission was negligible for imaging angles smaller than 60 degrees. The spectrum of Cherenkov emission tends to blue-shift along the curved surface. A simple normalization approach based on the reflectance image was experimentally validated by imaging a range of tissue phantoms, as a first order correction for different tissue optical properties.

Keywords: Cherenkov imaging; Monte Carlo; optical calibration; radiation dose; radiation therapy.

Figure 1

The simulated dose as scored in voxelized slab and cylindrical phantoms are shown in (a, c) and sampling regions and sensitivities (probability normalized by sum of the quantity in all the voxels) of surface-escaped Cherenkov radiation are shown in (b, d). These examples show the sampling sensitivity of surface escaped Cherenkov radiation in the cases of entrance/exit surfaces of the slab and the side surface along the arc of the cylinder. The color map used was MATLAB Jet, with red highest and blue lowest. In (a, b), the beam was from coming down from the top, and in (c,d) the beam was from the side.

Figure 2

The correlation ratio factors for different skin color types are shown – with a variation between entrance and exit surface for different skin colors shown in (a, c) with corresponding sampling depths shown in (b, d) for x-ray and electron beams respectively.

Figure 3

The correlation factors are shown for x-ray and electron beams with different energies -correlation factors for x-ray beams generated from phase space files for different energies are shown in (a) with corresponding sampling depths shown in (b). Correlation factors for mono-energetic radiation beams are shown in (c, e) with corresponding sampling depths shown in (d, f) for x-ray and electron beam respectively. The energy spectrum with probability normalized by the sum for different phase space files is shown in (g) and with the number of Cherenkov photons generated in unit volume per Gy of radiation dose shown in (h) for x-ray and electron beams of different mono energies.

Figure 4

Correlation factors for different field sizes is shown in (a) for different field sizes with corresponding sampling depths shown in (b) for x-ray beams. The energy spectrum of the 6 MV phase space files with probability normalized by the sum for different field sizes are shown in (c) with beam hardening for smaller field sizes.

Figure 5

Correlation factors for a curved surface, along the arc of cylinders with different diameters are shown in (a) and (c) with corresponding sampling depths shown in (b) and (d) for x-ray and electron beams respectively.

Figure 6

Angular distributions, imaging angle and spectrum of emission are examined, with angular distribution of Cherenkov emission (normalized by the sum) at different positions along the arc of the cylinder with 10 cm diameter are shown in (a) with corresponding ratios to Lambertian distribution shown in (b) for x-ray beams. The spectrum of Cherenkov emission around the arc of a 10cm cylinder is shown in (c). Similar results exist for electron beams (not shown).

Figure 7

Correlation of reflectance and Cherenkov in an array of tissue phantoms -- The array were made with 1% Intralipid and gelatin with a range of blood concentrations for each column in (a), and a 200 micron layer was added with different concentration of melanin was added for each row in (b). The Cherenkov emission was imaged (c) while irradiated with a 25×25 cm² 6 MV X-ray beam. The Cherenkov image normalized by the reflectance image in the gray channel was shown in (d), to evaluate the potential for simple normalization based correction of the escaped signal.