2.1.

Experimental Setup

A batch of $n=10$ scintillator dosimeters were selected for this study. The scintillator discs ($15\mathrm{mm}Ø\times 1\mathrm{mm}$ thick) were composed of EJ-212 plastic and custom-manufactured (Elijen Technologies, Sweetwater, Texas). The rear-faces of these discs were painted with EJ-510 reflective paint (Elijen Technologies). All 10 of the dosimeters were tested to obtain baseline measurements of light output, thickness, and influence on surface dose. A 500-193-30 AOS Digital Caliper (Mitutoyo, Aurora, Illinois) was used to measure dosimeter thickness. Varian 2100 CD and Trilogy linacs (Varian Medical Systems, Palo Alto, California) were used for irradiation during phantom-based experiments and patient imaging sessions, respectively.

Five of the scintillators were kept unmodified as controls and the remaining five were modified with a protective coating and adhesive backing, according to the redesign aims—the modified scintillator group also underwent cleaning. These scintillators ($n=5$) were coated with oil-based glossy clear-coat (Type 280702, Rust-Oleum, Vernon Hills, Illinois). Scintillators were cleaned using Super Sani-Cloth Germicidal Disposable Wipes (PDI, Orangeburg, New York). As per current clinical and manufacturer-recommend standards, the entire scintillator was thoroughly wiped and wet with the disposable wipes. The chemical cleaning agent was left on the dosimeter for 2 min. Scintillator-sized ($0.18\mathrm{mm}\text{thickness}\times 15\mathrm{mm}Ø$) adhesive layers of 2477P double-coated thermoplastic elastomer silicone acrylate medical tape with premium liner (3M Maplewood, Minnesota) were placed on the rear faces of the modified scintillators to attach them to phantom and patient surfaces. A schematic of the redesigned scintillator is shown in the zoom-in panel of Fig. 1. The effect of protective coating and adhesive backing application on surface dose, dosimeter thickness, and light output were evaluated. A redesigned scintillator is shown side-by-side to a scintillator wrapped in cellophane in Fig. 1.

Fig. 1

Both “control” and “redesign” scintillator dosimeters mounted to a flat-faced phantom. “Control” scintillators are wrapped in cellophane and attached using medical tape. Zoomed-in view of the “redesigned” scintillator shows thicknesses of polyvinyl toluene (PVT) plastic, reflective paint, protective coating, and adhesive backing.

Following previously published methods, scintillators were placed directly on top of a calibrated PTW 23342 thin window ionization chamber (IC) (PTW, Freiburg, Germany) coupled to a Max 4000 electrometer (Standard Imaging, Middleton, Wisconsin) to evaluate the impact of the dosimeter thickness on surface dose.¹⁰ Samples were irradiated at 100 cm source-to-surface distance (SSD) using a $10\mathrm{cm}\times 10\mathrm{cm}$ 6-MeV electron field at 1000 monitor units (MU)/min. Dose rate was chosen to minimize experiment time, scintillator output is independent of dose rate.¹⁰ For reference, the impact of a standard optically stimulated luminescence dosimeter (OSLD) nanoDOT (Landauer, Glenwood, Illinois) on surface dose was also tested. Percent change in surface dose caused by placement of a dosimeter on-top of the IC was calculated.

The methodology for conducting scintillator imaging-based surface dosimetry (including imaging setup specifications, image processing, dose conversion algorithm, etc.) has been previously described in detail.^1,2,10 In general, a custom fitting algorithm is used to fit an ellipse-convolved Gaussian function to each scintillator within specified region-of-interest (ROI) per acquisition frame. The term, “light output,” is defined as the sum of the maximum amplitudes of the fitted profiles across all frames. To collect light output measurements, scintillators were attached to a flat-faced acrylic resin phantom and irradiated with a 6-MeV high dose total skin electron beam (888 MU/min). To mimic the patient imaging setup, the phantom was positioned using the geometry described below. Light output is converted to dose using an externally obtained calibration factor which accounts for Cherenkov light production in the disc, different calibration factors are necessary for “red”- and “blue”-sensitive cameras.^1,10

2.2.

Patient Imaging: Imaging System Modification

Previous work demonstrated the feasibility of obtaining skin surface dose information for patients undergoing total skin electron therapy (TSET) by imaging light output of plastic scintillator targets using a linac-synchronized, time-gated, intensified camera system.^1,2 In order to improve the sensitivity of this camera system to scintillator emission, the intensifier with red-light sensitive photocathode in the standard C-Dose camera (DoseOptics LLC, Lebanon New Hampshire) was replaced with one more sensitive to blue light. Otherwise, the chassis and design of the C-dose camera was not changed. For spectral sensitivity assessment, a calibrated tunable light source (Dynasil TLS-6, Newton, Massachusetts) was utilized. Measured spectral sensitivity was renormalized to quantum efficiency units using intensifier calibration records provided by the manufacturer (Photonis Technologies, Merignac, France). Comparative scintillator imaging was conducted on a patient being treated with TSET for a diagnosis of Mycosis fungoides. The patient was treated at a 3-m SSD and the camera was located 4 m away directly adjacent to the gantry head. Both red- and blue-sensitive camera systems were used to conduct surface dosimetry throughout the course of this patient’s treatment following previously described methods. In order to facilitate a fair comparison to previous data, scintillators were attached to the patient using a cellophane wrapping without addition of a protective coating. TSET scintillators were attached to the upper arm, lower arm, chest, mid-section, thigh, shin, and foot.¹

3.1.

Dosimeter Redesign

3.1.1.

Thickness and surface dose measurements

Prior to application of the protective coating, average scintillator thickness (including reflective paint) for both control and redesign groups was $1.06\pm 0.03\mathrm{mm}$. Addition of the protective coating was found to add an additional $0.11\pm 0.02\mathrm{mm}$ in thickness to the dosimeters. Compared to baseline IC surface dose measurements (shown as a green dot with corresponding standard deviation, STD, as vertical bar, Fig. 2), premodification control and redesign groups both showed an average $4.9\pm 0.1\%$ increase (blue and red horizontal dotted lines, Fig. 2). For reference, a standard OSLD was found to increase surface dose by an identical $4.9\pm 0.1\%$ (cyan star, Fig. 2). Following application of an adhesive backing, the postmodification redesigned scintillators increased the surface dose on average by $5.2\pm 0.1\%$ and $0.3\pm 0.1\%$, relative to IC baseline and OSLD, respectively. Thus, it was concluded that the impact on surface dose following addition of a protective coating and adhesive backing is comparable to that of an OSLD.

Fig. 2

Impact of scintillator ($n=10$) thickness on surface dose. Reading obtained with bare IC, no dosimeters placed on-top shown as green dot, vertical green bar represents STD in IC measurements. 5% increase in surface dose from this baseline measurement shown as horizontal dotted green line. Pre- and postmodification refers to redesign process: application of clear coat and adhesive backing. “Redesigned” group of scintillators underwent the redesign process, while the “control” group did not. It should be noted that the $n=5$ “redesigned” scintillators in the pre- and postmodification sections are the same dosimeters. Cyan star shows impact of a nanoDot OSLD on surface dose. Horizontal dotted lines in each section represent mean of corresponding sample groups.

3.1.2.

Light output testing and dosimeter cleaning

All scintillators were attached to a flat-faced phantom, left panel of Fig. 1. Control group scintillators were not removed in between steps of the modification and cleaning process; however, redesigned dosimeters were removed and reattached in between stages. Modified scintillators were measured before and after application of the protective coating and cleaning procedures, these dosimeters were attached using an adhesive backing during all stages. For the control group, light output was found to change on average $<0.02\%$ in between imaging sessions. An average $2.6\pm 0.3\%$ decrease in light output was measured following application of the protective clear coat when compared to baseline. A $<0.02\%$ decrease in light output for modified scintillators was noted following the cleaning procedure, Fig. 3. As such, it was determined that light output from redesigned scintillators was not affected by cleaning with Sani-Cloth wipes.

Fig. 3

Scintillator light output tracked over various stages of the redesigning process. Varying shades of red and blue represent individual scintillators. Mean of each sample group is represented as a color-coded dotted line. “Coating” and “cleaning” represent light output measurement obtained following application of protective coating and cleaning with Sani-Cloth, respectively. Control group was not removed from the flat-faced phantom, and this group did not undergo coating or cleaning.

3.2.

Imaging System Modification

3.2.1.

Improved sensitivity to scintillation

The spectra of light emitted from the scintillator dosimeter ranges from $\sim 400$ to 515 nm, where the wavelength of maximal emission was found to be 422 nm.¹⁰ A photocathode with overlap between maximum quantum detection efficiency and scintillator emission wavelength was selected. The quantum efficiency of the sensors was measured and is shown in Fig. 4—absolute response and coarse spectral profiles were measured by the photocathode manufacturer (Photonis, Merignac, France), while the fine relative spectra was measured by our research team. Increasing the sensitivity to scintillation emission can potentially allow for the use of smaller and thinner scintillating volumes during scintillation dosimetry. This would be especially important during treatment scenarios where minimizing field perturbation is more critical, such as when the field size is smaller and dose gradient is larger. Cherenkov emission produced during treatment is useful for monitoring and verifying radiation field geometry.¹³ Despite that the “blue”-sensitive camera suppresses Cherenkov light detection, one can still visualize Cherenkov emission signal by window and leveling in real-time using the camera acquisition CDose software.

Fig. 4

Emission spectra of the scintillator dosimeter¹⁵ (black solid line, right axis) overlaid with quantum detection efficiency spectra of the red- and blue-sensitive photocathodes (red and blue dashed lines, data correspond to left axis).

3.2.2.

Patient imaging

Scintillator dosimetry was conducted over the course of six TSET treatment positions (anterior-posterior, posterior-anterior, left and right-anterior-oblique, left and right-posterior oblique) for both “red”- and “blue”-sensitive cameras. Sample images for the anterior-posterior position are provided in Fig. 5. Previous reports provide details regarding the intensity distribution across the scintillators, fitting profiles, and associated errors.^1,10 Scintillators were attached to seven dosimetry sites across the body, in two cases during imaging with the “blue”-sensitive camera, scintillators were removed from the foot due to patient compliance issues. Thus, data from dosimetry sites (80 total) were considered for analysis. For reference, each scintillator was paired with an OSLD. Comparing dose measured by OSLD to scintillator, per dosimetry site, it was found that a linear relationship existed for both “red” and “blue” imaging systems, $R^2=0.96$ and 0.95, respectively. These data points are plotted for this patient (PT6), as well as others evaluated over the course of the last 1.5 years in a human pilot study.¹ In total, data for $n=6$ patients and 242 dosimetry sites are provided in Fig. 6. Data showed that irrespective of whether the red- or blue-sensitive camera was used, accurate surface dosimetry was achieved. The percent difference in dose measured by OSLD and scintillator was found to be $<5\%$ and $<3\%$ for 241/242 and 221/242 sites, see Fig. 6.

Fig. 5

Sample images of a patient undergoing TSET in the posterior-anterior position. (a) Color photograph, (b) background image, (c) cumulative image captured by “red”-camera, and (d) cumulative image captured simultaneously during the same imaging session as shown in (c) by “blue” camera. Intensity values of Cherenkov and scintillation intensity maps shown in (c) and (d) are identically scaled. The SNR of the lower back scintillator (scintillation signal) and $50\times 50\text{-}\text{pixel}$ square ROI in the center of back (Cherenkov light) increased and decrease by $25\times$ and $7\times$ when comparing (c) to (d), respectively.

Fig. 6

(a) Percent difference ($\pm 3\%=\text{green}$ and $\pm 5\%=\mathrm{red}$) between surface dose measured by scintillator and OSLD, per dosimetry site. (b) Relationship between surface dose measured by scintillator versus OSLD. Linear trendline and 95% confidence interval are displayed in black and green, respectively. Data points obtained by red- and blue-sensitive cameras are shown with different shades of red or blue color, respectively. $R^2$ and root-mean-square error for each patient data set are also shown.