Evaluation of eye shields made of tungsten and aluminum in high-energy electron beams

Evaluation of the clinical outcomes and patient satisfaction related to the use of internal eye shields for electron external beam radiation therapy

INTRODUCTION

Cancers around the eye are often treated using orthovoltage machines or by plastic surgery, neither of which are widely available in regional Australia. External beam radiation therapy (EBRT) using electrons and an internal eye shield is an alternative, relatively underreported technique which can provide similar cosmetic and functional outcomes. This report aimed to describe the process for the use of internal eye shields at GenesisCare Fraser Coast Radiation Oncology (GCFCRO) and the associated clinical outcomes and patient perceptions of the delivery and results of this procedure.

METHODS

This project was conducted in two phases. Phase I was an audit of the departmental technique and short-term clinical outcomes of 17 patients who received EBRT for skin cancer near the eyes at GCFCRO in partnership with Wide Bay Hospital and Health Service (WBHHS). Phase II was a survey of nine of those patients to elicit the patient perspective of the delivery and long-term outcomes of the treatment.

RESULTS

Phase I revealed the departmental procedures for simulation, planning and treatment at GCFCRO are consistent with other departments published protocols. Phase II results detailed positive patient perspectives regarding cosmetic outcomes and receipt of EBRT for skin cancer near their eyes.

CONCLUSION

EBRT with an internal eye shield is an acceptable alternative modality to surgery for squamous cell carcinomas (SCC) and basal cell carcinomas (BCC) around the eye in the definitive and adjuvant setting. This is particularly important in regional locations to facilitate patients receiving high-quality care and outcomes locally.

Review of recent advances in managing periocular skin malignancies

Management of cutaneous malignancies can be particularly challenging when they are located in the periocular region. The standard of care for localized disease is complete surgical excision, but this may not be possible without significant disruption to visual structures and facial appearance. Definitive radiation may be an option for some patients who cannot or do not wish to undergo surgery. Advances in systemic treatment options for locally advanced and metastatic skin cancers in the past 10 years have prompted investigation into neoadjuvant treatment of periocular cancers. The use of chemotherapy, immune checkpoint inhibitors, and targeted therapies have all been reported with varying degrees of success. For many patients, targeted therapies or immune checkpoint inhibitors should be considered depending on the cancer type, symptoms, and goals with the input of a multidisciplinary cancer care team. In this article, we systematically review the latest updates in surgical, radiotherapeutic, and medical management of periocular malignancies.

Personalized Radiation Attenuating Materials for Gastrointestinal Mucosal Protection

Cancer patients undergoing therapeutic radiation routinely develop injury of the adjacent gastrointestinal (GI) tract mucosa due to treatment. To reduce radiation dose to critical GI structures including the rectum and oral mucosa, 3D-printed GI radioprotective devices composed of high-Z materials are generated from patient CT scans. In a radiation proctitis rat model, a significant reduction in crypt injury is demonstrated with the device compared to without (p < 0.0087). Optimal device placement for radiation attenuation is further confirmed in a swine model. Dosimetric modeling in oral cavity cancer patients demonstrates a 30% radiation dose reduction to the normal buccal mucosa and a 15.2% dose reduction in the rectum for prostate cancer patients with the radioprotectant material in place compared to without. Finally, it is found that the rectal radioprotectant device is more cost-effective compared to a hydrogel rectal spacer. Taken together, these data suggest that personalized radioprotectant devices may be used to reduce GI tissue injury in cancer patients undergoing therapeutic radiation.

Artifact-free CT images for electron beam therapy using a patient-specific non metallic shield

Patient's CT images taken with metallic shields for radiotherapy suffer from artifacts. Furthermore, the treatment planning system (TPS) has a limitation on accurate dose calculations for high density materials. In this study, a Monte Carlo (MC)-based method was developed to accurately evaluate the dosimetric effect of the metallic shield. Two patients with a commercial tungsten shield of lens and two patients with a custom-made lead shield of lip were chosen to produce their non-metallic dummy shields using 3D scanner and printer. With these dummy shields, we generated artifact-free CT images. The maximum CT number allowed in TPS was assigned to metallic shields. MC simulations with real material information were carried out. In addition, clinically relevant dose-volumetric parameters were calculated for the comparison between MC and TPS. Relative dosimetry was performed using radiochromic films. The dose reductions below metallic structures were shown on MC dose distributions, but not evident on TPS dose distributions. The differences in dose-volumetric parameters of PTV between TPS and MC for eye shield cases were not clearly shown. However, the mean dose of lens from TPS and MC was different. The MC results were in superior agreement with measured data in relative dosimetry. The lens dose could be overestimated by TPS. The differences in dose-volumetric parameters of PTV between TPS and MC were generally larger in lip cases than in eye cases. The developed method is useful in predicting the realistic dose distributions around the organs blocked by the metallic shields.

Estimation of Backscatter from Internal Shielding in Electron Beam Radiotherapy Using Monte Carlo Simulations (EGSnrc) and Gafchromic Film Measurements

PURPOSE

The purpose of the study was to estimate the backscatter electron dose in internal shielding during electron beam therapy using Monte Carlo (MC) simulations and Gafchromic film measurements.

MATERIALS AND METHODS

About 6 and 9 MeV electron beams from a Varian 2100C linac were simulated using BEAMnrc MC code. Various clinical situations of internal shielding were simulated by modeling water phantoms with 2 mm lead sheets placed at different depths. Electron backscatter factors (EBF), a ratio of dose at tissue-shielding interface to the dose at the same point without the shielding, were estimated. The role of 2 mm aluminum in reduction of backscatter was investigated. The measurements were also performed using Gafchromic films and results were compared with MC simulations.

RESULTS

For particular beam energy, the EBF value initially increased with depth in the buildup region and then decreased rapidly. The highest value of EBF for both the energies is nearly same though at different depths. Decreased EBF was observed for 9 MeV beam in comparison to the 6 MeV beam for the same depth of shielding placement. Two millimeter aluminum reduced the backscatter by nearly 25% at maximum backscatter condition for both the energies, though the effectiveness slightly decreased at higher energy. The range of backscatter electrons was varying from 5 to 12 mm in the upstream direction from the interface. The Gafchromic film-measured EBF and MC-simulated EBF were matching well within the clinically acceptable limits except in close vicinity of tissue-lead interface.

CONCLUSIONS

This study provides an important clinical data to design internal shielding at the local clinical setup and confirms applicability of MC simulations in backscatter dose calculations at interfaces where physical measurements are difficult to perform.

Absorbing materials with applications in radiotherapy and radioprotection

The radiotherapy centres are using linear accelerators equipped with multi-leaf collimators (MLCs) for treatments of various types of cancer. For superficial cancers located at a maximum depth of 3 cm high-energy electrons are often used, but MLC cannot be used together with electron applicators. Due to the fact that the tumour shape is not square (as electron applicators), searching for different materials that can be used as absorbents or shields for the protection of adjacent organs is of paramount importance. This study presents an experimental study regarding the transmitted dose through some laboratory-made materials when subjected to electron beams of various energies (ranging from 6 to 15 MeV). The investigated samples were composite materials consisting of silicon rubber and micrometre aluminium particles with different thicknesses and various mass fraction of aluminium. The measurements were performed at a source surface distance of 100 cm in the acrylic phantom. The experimental results show that the transmitted dose through tested samples is ranging between ∼1.8 and 90%, depending on the electron beam energy, sample thickness and sample composition. These preliminary results suggest that the analysed materials can be used as absorbers or shields in different applications in radiotherapy and radioprotection.

Dosimetric shield evaluation with tungsten sheet in 4, 6, and 9MeV electron beams

In electron radiotherapy, shielding material is required to attenuate beam and scatter. A newly introduced shielding material, tungsten functional paper (TFP), has been anticipated to become a very useful device that is lead-free, light, flexible, and easily processed, containing very fine tungsten powder at as much as 80% by weight. The purpose of this study was to investigate the dosimetric changes due to TFP shielding for electron beams. TFP (thickness 0-15mm) was placed on water or a water-equivalent phantom. Percentage depth ionization and transmission were measured for 4, 6, and 9MeV electron beams. Off-center ratio was also measured using film dosimetry at depth of dose maximum under similar conditions. Then, beam profiles and transmission with two shielding materials, TFP and lead, were evaluated. Reductions of 95% by using TFP at 0.5cm depth occurred at 4, 9, and 15mm with 4, 6, and 9MeV electron beams, respectively. It is found that the dose tend to increase at the field edge shaped with TFP, which might be influenced by the thickness. TFP has several unique features and is very promising as a useful tool for radiation protection for electron beams, among others.

Application of a dummy eye shield for electron treatment planning

Metallic eye shields have been widely used for near-eye treatments to protect critical regions, but have never been incorporated into treatment plans because of the unwanted appearance of the metal artifacts on CT images. The purpose of this work was to test the use of an acrylic dummy eye shield as a substitute for a metallic eye shield during CT scans. An acrylic dummy shield of the same size as the tungsten eye shield was machined and CT scanned. The BEAMnrc and the DOSXYZnrc were used for the Monte Carlo (MC) simulation, with the appropriate material information and density for the aluminum cover, steel knob and tungsten body of the eye shield. The Pinnacle adopting the Hogstrom electron pencil-beam algorithm was used for the one-port 6-MeV beam plan after delineation and density override of the metallic parts. The results were confirmed with the metal oxide semiconductor field effect transistor (MOSFET) detectors and the Gafchromic EBT2 film measurements. For both the maximum eyelid dose over the shield and the maximum dose under the shield, the MC results agreed with the EBT2 measurements within 1.7%. For the Pinnacle plan, the maximum dose under the shield agreed with the MC within 0.3%; however, the eyelid dose differed by -19.3%. The adoption of the acrylic dummy eye shield was successful for the treatment plan. However, the Pinnacle pencil-beam algorithm was not sufficient to predict the eyelid dose on the tungsten shield, and more accurate algorithms like MC should be considered for a treatment plan.

Recommendations for clinical electron beam dosimetry: supplement to the recommendations of Task Group 25

The goal of Task Group 25 (TG-25) of the Radiation Therapy Committee of the American Association of.Physicists in Medicine (AAPM) was to provide a methodology and set of procedures for a medical physicist performing clinical electron beam dosimetry in the nominal energy range of 5-25 MeV. Specifically, the task group recommended procedures for acquiring basic information required for acceptance testing and treatment planning of new accelerators with therapeutic electron beams. Since the publication of the TG-25 report, significant advances have taken place in the field of electron beam dosimetry, the most significant being that primary standards laboratories around the world have shifted from calibration standards based on exposure or air kerma to standards based on absorbed dose to water. The AAPM has published a new calibration protocol, TG-51, for the calibration of high-energy photon and electron beams. The formalism and dosimetry procedures recommended in this protocol are based on the absorbed dose to water calibration coefficient of an ionization chamber at 60Co energy, N60Co(D,w), together with the theoretical beam quality conversion coefficient k(Q) for the determination of absorbed dose to water in high-energy photon and electron beams. Task Group 70 was charged to reassess and update the recommendations in TG-25 to bring them into alignment with report TG-51 and to recommend new methodologies and procedures that would allow the practicing medical physicist to initiate and continue a high quality program in clinical electron beam dosimetry. This TG-70 report is a supplement to the TG-25 report and enhances the TG-25 report by including new topics and topics that were not covered in depth in the TG-25 report. These topics include procedures for obtaining data to commission a treatment planning computer, determining dose in irregularly shaped electron fields, and commissioning of sophisticated special procedures using high-energy electron beams. The use of radiochromic film for electrons is addressed, and radiographic film that is no longer available has been replaced by film that is available. Realistic stopping-power data are incorporated when appropriate along with enhanced tables of electron fluence data. A larger list of clinical applications of electron beams is included in the full TG-70 report available at http://www.aapm.org/pubs/reports. Descriptions of the techniques in the clinical sections are not exhaustive but do describe key elements of the procedures and how to initiate these programs in the clinic. There have been no major changes since the TG-25 report relating to flatness and symmetry, surface dose, use of thermoluminescent dosimeters or diodes, virtual source position designation, air gap corrections, oblique incidence, or corrections for inhomogeneities. Thus these topics are not addressed in the TG-70 report.

Determining superficial dosimetry for the internal canthus from the Monte Carlo simulation of kV photon and MeV electron beams

This paper presents the findings of an investigation into the Monte Carlo simulation of superficial cancer treatments of an internal canthus site using both kilovoltage photons and megavoltage electrons. The EGSnrc system of codes for the Monte Carlo simulation of the transport of electrons and photons through a phantom representative of either a water phantom or treatment site in a patient is utilised. Two clinical treatment units are simulated: the Varian Medical Systems Clinac 2100C accelerator for 6 MeV electron fields and the Pantak Therapax SXT 150 X-ray unit for 100 kVp photon fields. Depth dose, profile and isodose curves for these simulated units are compared against those measured by ion chamber in a PTW Freiburg MP3 water phantom. Good agreement was achieved away from the surface of the phantom between simulated and measured data. Dose distributions are determined for both kV photon and MeV electron fields in the internal canthus site containing lead and tungsten shielding, rapidly sloping surfaces and different density interfaces. There is a relatively high level of deposition of dose in tissue-bone and tissue-cartilage interfaces in the kV photon fields in contrast to the MeV electron fields. This is reflected in the maximum doses in the PTV of the internal canthus field being 12 Gy for kV photons and 4.8 Gy for MeV electrons. From the dose distributions, DVH and dose comparators are used to assess the simulated treatment fields. Any indication as to which modality is preferable to treat the internal canthus requires careful consideration of many different factors, this investigation provides further perspective in being able to assess which modality is appropriate.

Solid water as phantom material for dosimetry of electron backscatter using low-energy electron beams: a Monte Carlo evaluation

This study evaluated the dosimetry of electron backscatter when Solid Water is used to substitute water as phantom in electron radiotherapy. Monte Carlo simulation (EGSnrc-based code) was employed to predict electron energy spectra and depth doses for the 0.5 and 1 cm of Solid Water and water slabs above 3 mm of lead (Pb) layers using electron beams with energies of 4 and 6 MeV. For comparison, Monte Carlo simulations were repeated with Pb layers taken out from the phantoms using the same experimental configuration. Analyses on electron energy spectra for the 4 and 6 MeV electron beams showed that deviations of electron energy distributions between the Solid Water and water phantom were more significant in the high-energy range (i.e., close to the maximal electron energy) than the lower range corresponding to the electron backscatter. These deviations of electron energy spectra varied with depth and were mainly due to the electron fluence or beam attenuation. Dosimetry results from Monte Carlo simulations showed that the Solid Water phantom had lower depth dose compared to water with the same experimental setup. For the 4 MeV electron beams with 0.5 cm of Solid Water, depth doses were 1.8%-3.9% and 2.3%-4.4% lower than those in water, with and without the Pb layer underneath, respectively. Thicker Solid Water of 1 cm resulted in different decreases in depth doses of 1.8%-4.6% (with Pb) and 2.3%-4.4% (without Pb) compared to water. For higher nominal electron beam energy of 6 MeV with 0.5 cm of Solid Water, depth doses decreased 1.7%-2.9% (with Pb) and 1.6%-2.1% (without Pb) compared to water. These decreases in depth doses changed to 1.7%-3.7% (with Pb) and 1.7%-3% (without Pb) when the thickness of Solid Water was increased to 1 cm. The dosimetry data in this study are useful in determining the correction factor when using Solid Water to substitute water for the electron backscatter measurement in electron radiotherapy.

Dosimetric dependence of the dimensional characteristics on a lead shield in electron radiotherapy: a Monte Carlo study

This study investigates the dosimetric dependence of the dimension of a lead (Pb) layer for shielding using clinical electron beams with different energies. Monte Carlo simulations were used to generate phase space files for the 4, 9 and 16 MeV electron beams produced by a Varian 21 EX linear accelerator using the EGSnrc‐based BEAMnrc code, and validated by measurements using films. Pb layers with different thicknesses (2, 4, 6 and 8 mm) and diameters (2.5, 3, 3.5 and 4 cm) were placed at the center of an electron field on a solid water phantom. Beam profiles were determined at the depth of maximum dose (dm) using Monte Carlo simulations. The dose profiles under the Pb layer at dm, including the penumbra at the edge of the layer and relative dose at the central beam axis (CAX), were studied with varying thicknesses and diameters of Pb. It is found that 2 mm of Pb is adequate to provide 5 half value layer (HVL) attenuation for the 4 MeV electron beams, and the beam profiles at dm are dependent on the diameter but not the thickness of the Pb. However, for the 9 and 16 MeV electron beams, the relative dose at the CAX and dm depends on both the thickness and diameter of the Pb layer. For 8 mm thickness of Pb, 4 and 5 HVL attenuation of electron beams with energies of 9 and 16 MeV can be achieved at dm, respectively. Moreover, the beam profile under the Pb layer at dm depends on: (1) the penumbra region at the edge of the Pb layer; (2) the beam attenuation varying with the thickness of the Pb layer; (3) the electron side scatter contributing to the CAX under the Pb layer; and (4) the photon contamination produced by the Pb layer. A parameter called “shielding area factor” (defined as the ratio of the length between two points of 50% relative doses in the beam profile at dm to the diameter of the Pb layer) is suggested to predict the required size and thickness of Pb for shielding a target with known dimension at dm. The dosimetric data calculated by Monte Carlo simulations in this study are useful to select the suitable thickness and size of Pb for the protection of critical tissue in electron radiotherapy.

PACS number: 87.53.Bn; 87.55.kh and 87.55.km.

Electron therapy for orbital and periorbital lesions using customized lead eye shields

PURPOSE

To establish the protective efficacy against late complications of electron therapy using customized lead eye shields in cases with orbital and periorbital lesions.

METHODS

Between 1982 and 2006, 16 patients with 22 orbital and periorbital lesions were treated by electron therapy. Customized lead eye shields were prepared and placed in the respective patients' eyes during each fraction of electron therapy. The toxicity and local control rates were analyzed.

RESULTS

The preparation period for the customized lead eye shields was 2 days. The shields could be used throughout the treatment period in all the patients. No evidence of radiation cataract was observed in 15 of the 16 patients. None of the patients developed corneal ulceration or evidence of lead poisoning.

CONCLUSION

Customized lead eye shields could be made relatively quickly, and electron therapy for orbital and periorbital lesions could be undertaken safely without any late complication.

Transmission and dose perturbations with high-Z materials in clinical electron beams

High density and atomic number (Z) materials used in various prostheses, eye shielding, and beam modifiers produce dose enhancements on the backscatter side in electron beams and is well documented. However, on the transmission side the dose perturbation is given very little clinical importance, which is investigated in this study. A simple and accurate method for dose perturbation at metallic interfaces with soft tissues and transmission through these materials is required for all clinical electron beams. Measurements were taken with thin-window parallel plate ion chambers for various high-Z materials (Al, Ti, Cu, and Pb) on a Varian and a Siemens accelerator in the energy range of 5-20 MeV. The dose enhancement on both sides of the metallic sheet is due to increased electron fluence that is dependent on the beam energy and Z. On the transmission side, the magnitude of dose enhancement depends on the thickness of the high-Z material. With increasing thickness, dose perturbation reduces to the electron transmission. The thickness of material to reduce 100% (range of dose perturbation), 50% and 10% transmission is linear with the beam energy. The slope (mm/MeV) of the transmission curve varies exponentially with Z. A nonlinear regression expression (t=E[alpha+beta exp(-0.1Z)]) is derived to calculate the thickness at a given transmission, namely 100%, 50%, and 10% for electron energy, E, which is simple, accurate and well suited for a quick estimation in clinical use. Caution should be given to clinicians for the selection of thickness of high-Z materials when used to shield critical structures as small thickness increases dose significantly at interfaces.

Three-year retrospective review of superficial radiotherapy for skin conditions in a Perth radiotherapy unit

Western Australia has only two superficial radiotherapy units, one of which is located at Fremantle Hospital, and run by the radiation oncologists of Perth Radiation Oncology Centre. A 3-year retrospective review was undertaken of all patients who underwent treatment at this unit from 1999 to 2001. Patients were identified from the unit's log book, and data was collected from their files. For malignant skin conditions, 369 lesions were treated in 259 patients over the study period. The patients' median age was 76 years. A wide variety of conditions were treated, but the most common diagnoses were basal cell carcinoma (237 lesions) and squamous cell carcinoma (92 lesions), most commonly located in the head region. The most frequently used treatment schedule was 36 Gy in six fractions over a 3-week period. Where radiotherapy was administered as primary treatment, the diagnoses had been biopsy-proven in only 53% of cases. Fifty-four patients underwent treatment of benign skin disease over the study period; most commonly keloid scars (41 patients) followed by warts (six patients). We conclude that superficial radiotherapy has a distinct role in dermatology, particularly for skin carcinomas around the nose and eyes, which cannot presently be superseded by electron beam therapy.