Total skin electron therapy (TSET): A reimplementation using radiochromic films and IAEA TRS-398 code of practice

Non-contact scintillator imaging dosimetry for total body irradiation in radiotherapy

Objective.The goal of this work was to assess the potential use of non-contact scintillator imaging dosimetry for tracking delivery in total body irradiation (TBI).Approach. Studies were conducted to measure the time-gated light signals caused by radiation exposure to scintillators that were placed on tissue. The purpose was to assess efficacy in conditions common for TBI, such as the large source to surface distance (SSD) commonly used, the reduced dose rate, the inclusion of a plexiglass spoiler, angle of incidence and effects of peripheral patient support structures. Dose validation work was performed on phantoms that mimicked human tissue optical properties and body geometry. For this work, 1.5 cm diameter scintillating disks were developed and affixed to phantoms under various conditions. A time-gated camera synchronized to the linac pulses was used for imaging. Scintillation intensity was quantified in post processing and the values verified with simultaneous thermolumiescent dosimeter (TLD) measurements. Mean scintillation values in each region were compared to TLD measurements to produce dose response curves, and scatter effects from the spoiler and patient bed were quantified.Main results.The dose determined by scintillators placed in TBI conditions agreed with TLD dose determinations to within 2.7%, and did so repeatedly within 1.0% standard deviation variance. A linear fit between scintillator signal and TLD dose was achieved with anR2= 0.996 across several body sites. Scatter from the patient bed resulted in a maximum increase of 19% in dose.Significance.This work suggests that non-contact scintillator imaging dosimetry could be used to verify dose in real time to patients undergoing TBI at the prescribed long SSD and low dose rate. It also has shown that patient transport stretchers can significantly influence surface dose by increasing scatter.

In vivo Dosimetry for Dose Verification of Total Skin Electron Beam Therapy Using Gafchromic® EBT3 Film Dosimetry

Background and Purpose

Total skin electron beam therapy (TSEBT) is an important skin-directed radiotherapeutic procedure done in the treatment of cutaneous T-cell lymphomas, namely, mycosis fungoides (MF). This procedure is usually done at larger source-to-surface distances with the patient standing on a rotatory platform. As the patient has to stand in different positions without any rigid immobilization devices, there are chances that the total skin may not get uniformly irradiated which could lead to nonuniform dose distributions. Therefore, all the necessary arrangements should be made to evaluate the dose for different regions of the skin using suitable in vivo dosimeters at the radiotherapy centers offering these treatments. This study aimed to evaluate the consistency between the delivered and planned doses in vivo during TSEBT using Gafchromic EBT3 film dosimetry.

Materials and Methods

The surface dose for the six MF patients treated for TSEBT at our hospital from 2018 to 2022 was measured and evaluated. 2 cm × 2 cm Gafchromic® EBT3 films were used to measure skin dose at reference body positions of clinical interest. All the patients were treated with the modified Stanford technique. The irradiated film strips were analyzed for the dose using the IMRT OmniPro software. The doses at respective positions were expressed as mean dose ± standard deviation and the deviation was calculated as the percentage of the prescribed dose.

Results

One hundred and fifty-four Gafchromic® EBT3 film strips irradiated on six TSEBT patients showed a maximum dose variation of 2.00 ± 0.14 Gy, in the central body regions. The dose variation in the peripheral areas such as hands and ears was larger. A variation of 2 ± 0.32 Gy was observed on the hands and ears. The uniformity of the dose delivered to maximum body parts was within -7% and +16% for the peripheral areas like hands. The American Association of Physicists in Medicine recommends a dose uniformity of 8% and 4% in the vertical and horizontal patient plane for direct incident beam; however, for oblique incidences like in the modified Stanford technique, the dose variation is about 15%.

Conclusion

In vivo dosimetry using Gafchromic EBT3 film dosimetry for TSEBT yields objective data to find the under or overdose regions. That can be useful to provide quality treatment, especially when treatments tend to be as complex as TSEBT.

Dosimetry, Optimization and FMEA of Total Skin Electron Irradiation (TSEI)

PURPOSE

Total Skin Electron Irradiation (TSEI) is a method for treating malignant cutaneous T-cell lymphomas. This work aims to implement and optimize the total skin technique established at Strahlenklinik Erlangen, Germany on two new linear accelerators and to quantify the risks using failure mode and effects (FMEA) analysis.

MATERIAL AND METHODS

TSEI is performed at a VersaHD accelerator (Elekta, Stockholm) with 6MeV in the "high dose rate mode" HDRE and a nominal field size of 40×40cm². To reach the entire skin surface, the patients perform 6 different body positions at a distance of 330cm behind an acrylic scatter plate, with two overlapping irradiation fields being radiated at 2 gantry angles per position. The irradiation technique was commissioned according to the recommendation of AAPM report 23. With the help of a reference profile at 270°, 2 gantry angles were calculated, which in total resulted in an optimal dose distribution. This was metrologically verified with ion-chamber measurements in the patient's longitudinal axis. The influence of the shape of the acrylic scatter plate and the distance between the acrylic scatter plate and patient was determined by measurements. The dose homogeneity was verified using an anthropomorphic disc phantom equipped with GafChromic films. The workflows and failure modes of the total skin technique were described in a process map and subsequently quantified with a FMEA analysis.

RESULTS

An optimal dose distribution is achieved at a distance of SSD=330cm, using the gantry angles 289° and 251°. The previously used segmented acrylic scatter plate was replaced by a flat plate (200×120×0.5cm³), which is placed at a distance of 50cm in front of the patient. The densitometric evaluation of the GafChromic films in the anthropomorphic disc phantom revealed an expected dose distribution of 3Gy at a depth of up to 1.5cm below the skin surface, with a homogeneity of ±10% over the phantom's longitudinal axis. By FMEA a maximum risk priority number of 30 was determined.

CONCLUSION

Based on the calculations and measurements performed on the new accelerators as well as the risk analysis, we concluded that total skin therapy can be implemented clinically.

[Application of Real-time Variable Shape Tungsten Rubber for Nail Radiation Protection in the Total Skin Electron Beam (TSEB) Therapy]

PURPOSE

This study investigated whether real-time variable shape tungsten rubber (STR) could be applied for nail radiation protection in total skin electron beam (TSEB) therapy.

METHODS

Simulated finger phantoms were made from syringes filled with physiological saline of volumes 5, 10, 20, and 30 ml (inner diameters of 14.1, 17.0, 21.7, and 25.3 mm, respectively). Gafchromic film was applied to the phantom, and lead (thickness 1-3 mm) or STR (thickness 1-4 mm) with an area of 4´1.5 cm was used to cover the film. A 6 MeV electron beam with an 8 mm acrylic board was then used to irradiate the phantom. The source-surface distance (SSD) was 444 cm, the field size was 36´36 cm at SSD of 100 cm without an electron applicator, and the monitor unit was 2000 MU. The shielding rates were obtained from the dose profiles.

RESULTS

The mean values of the shielding rate values for all phantoms were 50.1, 97.6, and 98.7% for 1, 2, and 3 mm of lead, respectively, and -13.6, 53.9, 91.2, and 99.4% for 1, 2, 3, and 4 mm of STR, respectively.

CONCLUSION

STR with a thickness of 4 mm had the same shielding properties as lead with a thickness of 3 mm, which was an approximately 100% shielding rate. STR could therefore be used in TSEB therapy instead of lead.

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

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.

Moving gantry method for electron beam dose profile measurement at extended source-to-surface distances

A novel method has been put forward for very large electron beam profile measurement. With this method, absorbed dose profiles can be measured at any depth in a solid phantom for total skin electron therapy. Electron beam dose profiles were collected with two different methods. Profile measurements were performed at 0.2 and 1.2 cm depths with a parallel plate and a thimble chamber, respectively. 108 cm×108 cm and 45 cm×45 cm projected size electron beams were scanned by vertically moving phantom and detector at 300 cm source‐to‐surface distance with 90° and 270° gantry angles. The profiles collected this way were used as reference. Afterwards, the phantom was fixed on the central axis and the gantry was rotated with certain angular steps. After applying correction for the different source‐to‐detector distances and incidence of angle, the profiles measured in the two different setups were compared. Correction formalism has been developed. The agreement between the cross profiles taken at the depth of maximum dose with the ‘classical’ scanning and with the new moving gantry method was better than 0.5 % in the measuring range from zero to 71.9 cm. Inverse square and attenuation corrections had to be applied. The profiles measured with the parallel plate chamber agree better than 1%, except for the penumbra region, where the maximum difference is 1.5%. With the moving gantry method, very large electron field profiles can be measured at any depth in a solid phantom with high accuracy and reproducibility and with much less time per step. No special instrumentation is needed. The method can be used for commissioning of very large electron beams for computer‐assisted treatment planning, for designing beam modifiers to improve dose uniformity, and for verification of computed dose profiles.

PACS numbers: 87.53.Bn, 87.53.Jw, 87.56.jf

Dosimetric comparison of 4 MeV and 6 MeV electron beams for total skin irradiation

BACKGROUND

In this study, dosimetric aspects of TSEI consisting of a 4 MeV beam with no spoiler were investigated in comparison to a nominal 6 MeV beam with spoiler, and the potential for clinical applications was evaluated.

METHODS

The TSEI technique is based on the Stanford technique, which utilizes a beam configuration of six-dual fields. MOSFETs were used to measure the optimal gantry angle, profile uniformity, and absolute dose at the calibration point. The depth dose curve of the central axis was measured in the treatment plane using EBT2 film. Photon contamination was measured as the dose at 5 cm depth in a solid water phantom relative to the maximum dose using a parallel plate ion chamber. A MOSFET dosimeter placed on the surface of a humanoid phantom, and EBT2 films inserted into a humanoid phantom were used to verify the TSEI commissioning.

RESULTS

Dosimetric aspects of the 4 MeV TSEI beam, such as profile uniformity (±10%) and relative photon contamination (<0.001%), were comparable to those of a 6 MeV TSEI beam. The relative depth dose of the 4 MeV electrons was 81.4% at the surface and 100% at 0.4 cm. For the 6 MeV electrons, the relative depth dose was 93.4% at the surface and 100% from 0.2 cm to 0.4 cm. The calculated B-factor of the 4 MeV TSEI beam was 1.55, and 1.53 for the 6 MeV TSEI. 80% of the prescribed dose was obtained at 0.22 cm depth for the 4 MeV TSEI beam and 0.53 cm for the 6 MeV TSEI beam in the humanoid phantom measurement.

CONCLUSIONS

The suggested 4 MeV beam for TSEI could be applied to shallow depth skin diseases and to electron boost as second treatment course.

Review of the results of the in vivo dosimetry during total skin electron beam therapy

This work reviews results of in vivo dosimetry (IVD) for total skin electron beam (TSEB) therapy, focusing on new methods, data emerged within 2012. All quoted data are based on a careful review of the literature reporting IVD results for patients treated by means of TSEB therapy. Many of the reviewed papers refer mainly to now old studies and/or old guidelines and recommendations (by IAEA, AAPM and EORTC), because (due to intrinsic rareness of TSEB-treated pathologies) only a limited number of works and reports with a large set of numerical data and proper statistical analysis is up-to-day available in scientific literature. Nonetheless, a general summary of the results obtained by the now numerous IVD techniques available is reported; innovative devices and methods, together with areas of possible further and possibly multicenter investigations for TSEB therapies are highlighted.

Dosimetric perturbation from cloth and paper gowns for total skin electron irradiation

Traditionally, total skin electron patients remove all clothing for treatment. It is generally assumed that this is best for the treatment of superficial skin lesions out of concern clothing may significantly perturb dose. We investigate the dosimetric effect of patient gowns and determine the necessity of treating patients naked. Using GAFCHROMIC EBT2 film, dose to a cylindrical phantom was measured with cloth, paper, and tri-layer cloth gowns, compared to no covering. A 6 MeV electron beam with spoiler accessory was used at ~ 4 meters source-to-skin distance. The gantry was angled at 248° and 292°. The phantom was rotated at -60°, 0°, and 60° relative to the beam's central axis, simulating the Stanford technique. This was also repeated for films sandwiched between the phantom's discs. Using a Markus chamber, the effect of air gaps of 0 to 5 cm in cloth and paper gowns was measured. The water equivalent attenuation of the gowns was determined through transmission studies. Compared to no covering, films placed on the phantom surface revealed an average increase of 0.8% in dose for cloth, 1.8% for tri-layered cloth, and 0.7% for paper. Films sandwiched within the phantom showed only slight shift of the percent depth-dose curves. Markus chamber readings revealed 1.4% for tri-layered cloth, and < 0.2% for single layer cloth or paper. Air gaps appeared to have a minimal effect. Transmission measurements found that one layer of cloth is equal to 0.2mm of solid water. Cloth and paper gowns appear to slightly increase the dose to the skin, but will not introduce any significant dose perturbation (<1%). Gowns having folds and extra layers will have a small additional perturbation (<2%). To minimize perturbation, one should smooth out any folds or remove any pockets that form extra layers on the gown.