Cherenkov imaging method for rapid optimization of clinical treatment geometry in total skin electron beam therapy

Abstract

Purpose: A method was developed utilizing Cherenkov imaging for rapid and thorough determination of the two gantry angles that produce the most uniform treatment plane during dual-field total skin electron beam therapy (TSET).

Methods: Cherenkov imaging was implemented to gather 2D measurements of relative surface dose from 6 MeV electron beams on a white polyethylene sheet. An intensified charge-coupled device camera time-gated to the Linac was used for Cherenkov emission imaging at sixty-two different gantry angles (1° increments, from 239.5° to 300.5°). Following a modified Stanford TSET technique, which uses two fields per patient position for full body coverage, composite images were created as the sum of two beam images on the sheet; each angle pair was evaluated for minimum variation across the patient region of interest. Cherenkov versus dose correlation was verified with ionization chamber measurements. The process was repeated at source to surface distance (SSD) = 441, 370.5, and 300 cm to determine optimal angle spread for varying room geometries. In addition, three patients receiving TSET using a modified Stanford six-dual field technique with 6 MeV electron beams at SSD = 441 cm were imaged during treatment.

Results: As in previous studies, Cherenkov intensity was shown to directly correlate with dose for homogenous flat phantoms (R(2) = 0.93), making Cherenkov imaging an appropriate candidate to assess and optimize TSET setup geometry. This method provided dense 2D images allowing 1891 possible treatment geometries to be comprehensively analyzed from one data set of 62 single images. Gantry angles historically used for TSET at their institution were 255.5° and 284.5° at SSD = 441 cm; however, the angles optimized for maximum homogeneity were found to be 252.5° and 287.5° (+6° increase in angle spread). Ionization chamber measurements confirmed improvement in dose homogeneity across the treatment field from a range of 24.4% at the initial angles, to only 9.8% with the angles optimized. A linear relationship between angle spread and SSD was observed, ranging from 35° at 441 cm, to 39° at 300 cm, with no significant variation in percent-depth dose at midline (R(2) = 0.998). For patient studies, factors influencing in vivo correlation between Cherenkov intensity and measured surface dose are still being investigated.

Conclusions: Cherenkov intensity correlates to relative dose measured at depth of maximum dose in a uniform, flat phantom. Imaging of phantoms can thus be used to analyze and optimize TSET treatment geometry more extensively and rapidly than thermoluminescent dosimeters or ionization chambers. This work suggests that there could be an expanded role for Cherenkov imaging as a tool to efficiently improve treatment protocols and as a potential verification tool for routine monitoring of unique patient treatments.

FIG. 1.

Experimental setup for rapid TSET optimization using Cherenkov imaging.

FIG. 2.

(a) Mean Cherenkov intensity (relative surface dose) vs coefficients of variation for a subset of angle pairs. The historical angle pair used at the institution (255.5° and 284.5°) is labeled by a red square, and the minimum coefficient of variation at a symmetrical angle pair occurred at 252.5° and 287.5°, denoted by the blue star. (b) Calculated coefficients of variation at symmetric angle spreads. The composite Cherenkov images for the original and optimal angles spreads are shown in (c) and (d), respectively, on the same normalization scale; the self-normalized Cherenkov images are shown in (e) and (f). The image in (e) is overlaid with an image of a TSET patient for size scale. The white box in (e) shows the analyzed ROI.

FIG. 3.

(a) White light image of the phantom with embedded ionization chamber placed at position 5. (b) Self-normalized, superimposed, composite Cherenkov image of the phantom irradiated with the two fields historically used for treating patients at our institution (255.5° and 284.5°); approximate locations of a patient’s wrist, back, and heel are shown for reference. (c) Cherenkov mage of the phantom irradiated with treatment angles producing minimum coefficient of variation (gantry at 252.5° and 288.5°). (d) Mean Cherenkov intensity calculated from the ROIs outlined in black shown in [(b), red squares] and [(c), blue circles] versus the ionization chamber measurements, at each of the nine positions shown, each normalized to the respective value at position 5 (prescription point). Black square outlines were superimposed on (b) and (c) to illustrate ROI size and placement.

FIG. 4.

(a) Ionization chamber charge measurements for increasing depth of Solid Water between the original treatment angles and the selected optimized treatment angles, demonstrating the decrease in relative dose delivered for the new angles. (b) Calculated percent depth dose between the original treatment angles and the optimized treatment angles.

FIG. 5.

Linear dependence of optimal angle spread on the SSD, using symmetric angle pairs. Images were acquired using 1° increments.

FIG. 6.

Composite Cherenkov images for three patients in the six Stanford technique treatment positions. All images are represented on the same color scale. Patients #1 and #2 were treated with angle spread of 29° at SSD = 441 cm and were wearing cloth shorts. Patient #3 was treated with angle spread of 35° at SSD = 441 cm, sitting on a bicycle seat for patient stabilization (no cloth shorts).

FIG. 7.

(a) Placement of ten TLDs on patient #1 (posterior position only). TLDs were also placed on patient #3 in similar approximate positions. (b) Plot comparing average Cherenkov intensity of ROIs directly on top of the TLD with the measured dose from the TLDs. Each data set is normalized to its respective highest measurement (at the hips). Measurements on opposing wrists and opposing heels are each averaged together for display clarity. Data from patients #1 and #3 are shown for comparison, although images (a) and (c) correspond to patient #1 only. (c) The two sets of ten 25 × 25 pixel regions of interest (ROIs) were analyzed for average Cherenkov intensity on patient #1. The white regions are directly on top of the tape affixing the TLDs, and the black regions are offset from the taped regions. There was no statistically significant difference between the mean Cherenkov intensity in the black and white labeled regions; the white labeled regions were used in (b).