Dosimetry of Large Field Valencia applicators for Cobalt-60-based brachytherapy

BACKGROUND

Non-melanoma skin cancer is one of the most common types of cancer and one of the main approaches is brachytherapy. For small lesions, the treatment of this cancer with brachytherapy can be done with two commercial applicators, one of these is the Large Field Valencia Applicators (LFVA).

PURPOSE

The aim of this study is to test the capabilities of the LFVA to use clinically 60Co sources instead of the 192Ir ones. This study was designed for the same dwell positions and weights for both sources.

METHODS

The Penelope Monte Carlo code was used to evaluate dose distribution in a water phantom when a 60Co source is considered. The LFVA design and the optimized dwell weights reported for the case of 192Ir are maintained with the only exception of the dwell weight of the central position, that was increased. 2D dose distributions, field flatness, symmetry and the leakage dose distribution around the applicator were calculated.

RESULTS

When comparing the dose distributions of both sources, field flatness and symmetry remain unchanged. The only evident difference is an increase of the penumbra regions for all depths when using the 60Co source. Regarding leakage, the maximum dose within the air volume surrounding the applicator is in the order of 20% of the prescription dose for the 60Co source, but it decreases to less than 5% at about 1 cm distance.

CONCLUSIONS

Flatness and symmetry remains unaltered as compared with 192Ir sources, while an increase in leakage has been observed. This proves the feasibility of using the LFVA in a larger range of clinical applications.

Reproducibility and air gap pockets of 3D-printed brachytherapy applicator placement in high-dose-rate skin cancer

BACKGROUND AND PURPOSE

This study aimed to investigate the reproducibility of a novel approach using 3D printed brachytherapy applicators for the treatment of skin cancer. Specifically, we aimed to assess the accuracy of applicator placement and to minimize the existence of air gap pockets between the applicator and the patient's skin.

MATERIALS AND METHODS

A total of 20 patients plans diagnosed with skin cancer were enrolled in this study. All patients underwent high dose rate (HDR) brachytherapy. To ensure precise applicator placement, patient-specific 3D printed applicators were designed based on individual body and tumor topography, utilizing data obtained from computer tomography (CT) scans. All applicators were fabricated using fused deposition modeling technology.

RESULTS

The error in applicator placement was measured and found to be less than 1.0 mm on average, with a standard deviation of 0.9 mm. Additionally, the average error in air gap pockets between the applicator and the patient's skin was 0.4 mm (standard deviation was 0.5 mm). The study demonstrated that the personalized approach of 3D printed brachytherapy applicator placement in skin cancer treatment yielded highly accurate results. The average error of less than 1.0 mm in applicator positioning and the minimal air gap pockets demonstrated the reproducibility and precision of this technique.

CONCLUSION

Our study establishes the reproducibility and accuracy of 3D-printed brachytherapy applicator placement in the treatment of skin cancer. This personalized treatment approach offers a highly precise method for delivering radiation therapy, minimizing the risk to adjacent healthy tissues, and enhancing overall patient outcomes.

A formalism for traceable dosimetry in superficial electronic brachytherapy (eBT)

Objective. Despite the number of treatments performed with electronic brachytherapy (eBT) there is no uniform methodology for reference dosimetry for international traceability to primary dosimetry standards in different eBT systems. The objective of this study is to propose a formalism for traceability reference dosimetry in superficial eBT, that is easy to apply in the clinic. This method was investigated for an Elekta Esteya with one applicator.Approach. The calibration x-ray spectrum at the primary standards dosimetry laboratory was matched to the measured eBT photon spectrum. Subsequently, two ionization chambers of different types were calibrated at the primary standard dosimetry laboratory (PSDL) in terms of air kerma against a primary standard. The chambers were used to measure ionization chamber reading ratios in-air at different distances from the applicator. Monte Carlo based air kerma ratios were calculated at different positions from the eBT applicator as well as backscatter factors in water and average mass energy absorption ratios in water and in air. Relative measurements with radiochromic films were performed in a water phantom to determine the ratio of absorbed dose to water,Dw, at the surface toDwat 1 cm depth in water. These were compared with Monte Carlo calculations.Main results. Calculations and measurements were combined to estimate theDwat the surface and at 1 cm depth in water. Ionization chamber agreement of the surface dose was 1.7%, within an uncertainty of 6.8% (k= 2). They agreed with the manufacturer dosimetry within 1.8%, with an uncertainty of 5.0% (k= 2). The feasibility of the formalism and methodology for the Esteya system was demonstrated.Significance. This study proposes a method for harmonization of traceable reference dosimetry for eBT contact treatments which does not involve a detailed simulation of the ionization chamber. The method demonstrated feasibility for one eBT system using one surface applicator. In the future the method could be applied for different eBT systems.

Use of Thermoluminescence Dosimetry for QA in High-Dose-Rate Skin Surface Brachytherapy with Custom-Flap Applicator

Surface brachytherapy (BT) lacks standard quality assurance (QA) protocols. Commercially available treatment planning systems (TPSs) are based on a dose calculation formalism that assumes the patient is made of water, resulting in potential deviations between planned and delivered doses. Here, a method for treatment plan verification for skin surface BT is reported. Chips of thermoluminescent dosimeters (TLDs) were used for dose point measurements. High-dose-rate treatments were simulated and delivered through a custom-flap applicator provided with four fixed catheters to guide the Iridium-192 (Ir-192) source by way of a remote afterloading system. A flat water-equivalent phantom was used to simulate patient skin. Elekta TPS Oncentra Brachy was used for planning. TLDs were calibrated to Ir-192 through an indirect method of linear interpolation between calibration factors (CFs) measured for 250 kV X-rays, Cesium-137, and Cobalt-60. Subsequently, plans were designed and delivered to test the reproducibility of the irradiation set-up and to make comparisons between planned and delivered dose. The obtained CF for Ir-192 was (4.96 ± 0.25) μC/Gy. Deviations between measured and TPS calculated doses for multi-catheter treatment configuration ranged from -8.4% to 13.3% with an average of 0.6%. TLDs could be included in clinical practice for QA in skin BT with a customized flap applicator.

Investigation of Radiochromic Film Use for Source Position Verification through a LINAC On-Board Imager (OBI)

Background and Objectives: Quality assurance is an integral part of brachytherapy. Traditionally, radiographic films have been used for source position verification, however, in many clinics, computerized tomography simulators have replaced conventional simulators, and computerized radiography systems have replaced radiographic film processing units. With these advances, the problem of controlling source position verification without traditional radiographic films and conventional simulators has appeared. Materials and Methods: In this study, we investigated an alternative method for source position verification for brachytherapy applications. Source positions were evaluated using Gafchromic™ RTQA2 and EBT3 film and visually compared to exposed RTQA radiochromic film when using a Nucletron Oldelft Simulix HP conventional simulator and a Gammamed 12-i brachytherapy device for performance evaluation. Gafchromic film autoradiography was performed with a linear accelerator (LINAC) on-board imager (OBI). Radiochromic films are very suitable for evaluation by visual inspection with a LINAC OBI. Results: The results showed that this type of low-cost, easy-to-find material can be used for verification purposes under clinical conditions. Conclusions: It can be concluded that source-position quality assurance may be performed through a LINAC OBI device.

75Se - A promising alternative to 192Ir for potential use in the skin cancer brachytherapy: A Monte Carlo simulation study using FLUKA code

This study aimed to evaluate the possibility of utilizing the HDR ⁷⁵Se source for skin cancer brachytherapy. In this work, based on the BVH-20 skin applicator, two cup-shaped applicators, without and with the flattening filter, were modeled. To obtain the optimal flattening filter shape, an approach based on the MC simulation in combination with an analytical estimation was used. Then, the dose distributions for ⁷⁵Se-applicators were generated using MC simulations in water, and their dosimetric characterizations such as flatness, symmetry, and penumbra were evaluated. Furthermore, the radiation leakage in the backside of the applicators was estimated by additional MC simulation. Finally, to evaluate the treatment times, calculations were performed for two ⁷⁵Se-applicators assuming 5 Gy per fraction. The flatness, symmetry, and penumbra values for the ⁷⁵Se-applicator without the flattening filter were estimated to be 13.7%, 1.05, and 0.41 cm respectively. The corresponding values for ⁷⁵Se-applicator with the flattening filter were estimated to be 1.6%, 1.06, and 0.10 cm respectively. The radiation leakage value at a distance of 2 cm from the applicator surface was calculated to be 0.2% and 0.4% for the ⁷⁵Se-applicator without and with the flattening filter respectively. Our results showed that the treatment time for the ⁷⁵Se-applicator is comparable with that of the ¹⁹²Ir-Leipzig applicator. The findings revealed that the dosimetric parameters of the ⁷⁵Se applicator are comparable with the ¹⁹²Ir skin applicator. Overall, the ⁷⁵Se source can be an alternative to ¹⁹²Ir sources for HDR brachytherapy of skin cancer.

AAPM medical physics practice guideline 13.a: HDR brachytherapy, part A

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines (MPPGs) will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines: (1) Must and must not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. (2) Should and should not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances. Approved by AAPM's Executive Committee April 28, 2022.

Implementation of superficial radiation therapy (SRT) using SRT-100 Vision™ for non-melanoma skin cancer in a Radiation Oncology clinic

PURPOSE

This article describes our experience in implementation of superficial radiation therapy (SRT) using SRT-100 Vision™ for non-melanoma skin cancer.

METHODS

Following the American Association of Physicists in Medicine Task Group-61 protocol, absolute output (absorbed dose to water at surface (cGy/min)) was measured for three energies (50, 70, and 100 kV) and for six applicators (1.5-5.0 cm in diameter). Percent depth dose (PDD) and profiles were also measured. Timer testing and ultrasound testing were performed. A treatment time calculation worksheet was created. Quality assurance (QA) of SRT-100 Vision was implemented. After treatment workflow for our clinic was developed, end-to-end (E2E) testing was performed using a Rando phantom. Considerations for treatment using SRT-100 Vision were made.

RESULTS

Absolute output (cGy/min) decreases as energy increases and applicator size decreases. Due to scatter from the applicator, PDD at depths ≤5 mm does not follow conventional trends but PDD at depths ≥15 mm increases with increasing applicator size. Profiles for the 5 cm applicator do not have strong dependence on depth except profiles at 5 mm for 50 kV. Timer/end errors are negligible for all three energies. Ultrasound images confirm allowed field of view and depth as well as no image artifacts and spatial integrity. Daily, monthly and annual QA of SRT-100 Vision implemented in our clinic is listed in a table format. E2E testing results (<1%) demonstrate the functionality and performance of our treatment workflow. Our considerations for SRT treatment include patient, applicator size and energy selections, patient setup, and shields.

CONCLUSIONS

This article is expected to serve as guidance for Radiation Oncology and/or Dermatology clinics aspiring to initiate an SRT program in their clinics.

Risk and Quality in Brachytherapy From a Technical Perspective

AIMS

To provide an overview of the history of incidents in brachytherapy and to describe the pillars in place to ensure that medical physicists deliver high-quality brachytherapy.

MATERIALS AND METHODS

A review of the literature was carried out to identify reported incidents in brachytherapy, together with an evaluation of the structures and processes in place to ensure that medical physicists deliver high-quality brachytherapy. In particular, the role of education and training, the use of process and technical quality assurance and the role of international guidelines are discussed.

RESULTS

There are many human factors in brachytherapy procedures that introduce additional risks into the process. Most of the reported incidents in the literature are related to human factors. Brachytherapy-related education and training initiatives are in place at the societal and departmental level for medical physicists. Additionally, medical physicists have developed process and technical quality assurance procedures, together with international guidelines and protocols. Education and training initiatives, together with quality assurance procedures and international guidelines may reduce the risk of human factors in brachytherapy.

CONCLUSION

Through application of the three pillars (education and training; process control and technical quality assurance; international guidelines), medical physicists will continue to minimise risk and deliver high-quality brachytherapy treatments.

Impact of detector selection on commissioning of Leipzig surface applicators with improving immobilization in high-dose-rate brachytherapy

PURPOSE

Commission and treatment setup of Leipzig surface applicators, because of the steep dose gradient and lack of robust immobilization, is challenging. We aim to improve commissioning reliability by investigating the impact of detector choice on percentage depth dose (PDD) verifications, and to enhance accuracy and reproducibility in calibration/treatment setup through a simple and novel immobilization device.

METHODS AND MATERIALS

PDD distributions were measured with radiochromic films, optically stimulated luminescent dosimeters (OSLDs), a diode detector, and both cylindrical and parallel plate ionization chambers. The films were aligned to the applicators in parallel and transverse orientations. PDD data from a benchmarking Monte Carlo (MC) study were compared with the measured results, where surface doses were acquired from extrapolation. To improve setup accuracy and reproducibility, a custom-designed immobilization prototype device was made with cost-effective materials using a 3D printer.

RESULTS

The measured PDD data with different detectors had an overall good agreement (<±10%). The parallel plate ionization chamber reported unreliable doses for the smallest applicator. There was no remarkable dose difference between the two film setups. The two-in-one prototype device provided a rigid immobilization and a flexible positioning of the applicator. It enhanced accuracy and reproducibility in calibration and treatment setup.

CONCLUSION

We recommend using radiochromic films in the transverse orientation for a reliable and efficient PDD verification. The applicator's clinical applicability has been limited by a lack of robust immobilization. We expect this economical, easy-to-use prototype device can promote the use of Leipzig applicators in surface brachytherapy.

Feasibility of electronic brachytherapy in cervix cancer-A dosimetric comparison of different brachytherapy techniques

INTRODUCTION

This study analyzes cases in which electronic brachytherapy (eBT) led to acceptable treatment plans in cervical cancer. Findings were compared with dosimetry values obtained in ¹⁹²Ir-based treatments according to the high-risk clinical target volume (HR-CTV) and the disease stage.

MATERIAL AND METHODS

We retrospectively analyzed 48 patients with cervical cancer from two centers. The patients were treated with ¹⁹²Ir based on MRI. It was possible to use interstitial needles via an Utrecht-type applicator. Dosimetry was simulated using eBT and the parameters D90 and D98 (HR-CTV) and D2cc, D1cc, and D0.1cc (bladder, rectum, and sigmoid colon) were evaluated. The Mann-Whitney U test was used for comparison. The overall cohort of patients was analyzed, as were the sub-cohorts based on stage (FIGO stages I+IIA, IIB and III-IV). Finally, the dosimetry of the eBT plans was evaluated, and the plans obtained were classified as "good", "acceptable", or "poor".

RESULTS

Statistically significant differences were found between the eBT and ¹⁹²Ir plans for D98 (HR-CTV), D1cc and D0.1cc (bladder), and D1cc and D0.1cc (sigmoid colon). A total of 31 cases (64.6%) were considered good, seven (14.6%) were considered acceptable, and 10 (20.8%) were considered poor. For volumes <30 cc, all the plans were good or acceptable; for volumes >30 cc, 54.3% were good, and 71.4% were good or acceptable. By stage, eBT plans for patients with stage IB-IIA disease were good in 100%, whereas those for patients with stage IIB were good in 70.6% and III-IV disease were good in 50%.

CONCLUSIONS

eBT provides appropriate dosimetry for treatment of cervical cancer in selected cases.

Feasibility of magnetic resonance-only high-dose-rate surface brachytherapy for clinical application

PURPOSE

In this article, we investigate the feasibility of magnetic resonance (MR)-only imaging for high-dose-rate (HDR) surface brachytherapy (SABT). We examined whether a standard CT-based planning can be replaced with an MR-only planning. For this purpose, the MRI digitization and plan quality check processes were compared against the standard CT-based processes. A prospective clinical implementation of the MR-only planning was evaluated on a clinical data set.

METHODS

A pointwise encoding time reduction with radial acquisition (PETRA) sequence was optimized for visualization of Freiburg flap (FF) on MR images. MR and conventional CT images were acquired with a FF applicator (Elekta, Stockholm, Sweden) placed on the following phantoms: (1) flat styrofoam (FST), FF locked-in placed with supporting structure; (2) cast-made facemask, and (3) porcine leg (PL). Catheters were digitized and activated with 10 mm step size on Oncentra Brachy 4.5.3 Treatment Planning System. The CT-only and MR-only treatment plans were generated by optimizing the dose to the target defined as volume at 3 mm skin depth. To compare the plans, the MRI-to-CT alignment was performed via rigid registration. Positional displacements of dwell positions between CT and MR plans were compared on the FST phantom and the relative percent dose difference in 2210 different points from CT or MR-only plans was compared. For all three phantoms, the comparabilities between CT and MR-only plans were assessed by calculating dice similarity coefficient (DSC) for volumes enclosing 150%, 125%, 100%, 95%, 90%, 80%, and 65% isodose lines (V₁₅₀  -V₆₅ ). The MR images of FF placed on the forearm of a healthy subject were acquired with this optimized PETRA sequence and used for treatment planning. The relative percent dose was calculated on 140 representative points placed at 3 mm skin depth to evaluate the dose to the skin.

RESULTS

Using the optimized PETRA sequence, MRTP digitization accuracy was < 1 mm in each dimension and on three-dimensional (3D) displacement for the FST phantom. In each phantom and clinical data set, it was possible to generate MR-only treatment plans with the 3 mm skin depth prescription. In the FST phantom, the mean relative dose at the points was not significantly different (< 0.1% difference) for CT or MR-based plans. The assessment of similarities in dose profiles between CT and MR-only plans' provided DSC values greater than 0.96, 0.92, and 0.73 for all volumes enclosing up to 100%, 125%, and 150% isodose lines, respectively.

CONCLUSION

The feasibility of generating a HDR treatment plan with FF using MR-only has been evaluated in phantoms with varying geometry and for a clinical data set. The optimization of a standard MRI sequence-PETRA-implemented in this study showed that FF-based catheters can be digitized and a plan can be generated using only MRI. The resulting MR-only plans were comparable to the conventional CT-based plans, suggesting that MRI alone can generate clinically acceptable plans for FF in phantoms and on a clinical data set. Reliable MR-only treatment planning could improve treatment prescription through more accurate characterization of soft tissue targets.

Additive manufacturing (3D printing) in superficial brachytherapy

The aim of this work is to provide an overview of the current state of additive manufacturing (AM), commonly known as 3D printing, within superficial brachytherapy (BT). Several comprehensive database searches were performed to find publications linked to AM in superficial BT. Twenty-eight core publications were found, which can be grouped under general categories of clinical cases, physical and dosimetric evaluations, proof-of-concept cases, design process assessments, and economic feasibility studies. Each study demonstrated a success regarding AM implementation and collectively, they provided benefits over traditional applicator fabrication techniques. Publications of AM in superficial BT have increased significantly in the last 5 years. This is likely due to associated efficiency and consistency benefits; though, more evidences are needed to determine the true extent of these benefits.

Non-melanoma Skin Cancer Treated by Contact High-dose-rate Radiotherapy (Brachytherapy): A Mono-institutional Series and Literature Review

BACKGROUND/AIM

Non-melanoma skin cancers (NMSC) are the most common neoplasms worldwide and their incidence has been proven to increase in recent years and their treatment should aim at cancer cure as well as cosmetic and functional results. The aim of the study was to report the results of our mono-institutional series of high-dose-rate radiotherapy (cHDR-RT) in NMSC, based on a homogenous technique and two different treatment schedules.

PATIENTS AND METHODS

All patients affected by NMSC who were consecutively evaluated and treated at our Interventional Oncology Center from October 2018 to August 2020, were included. Patients underwent cHDR-RT using flap applicators and remotely afterloaded Ir-192 sources.

RESULTS

Overall, 51 patients were treated for a total of 67 lesions. Local control (LC) and disease-specific survival (DSS) were 94.0% and 100%, respectively. Grade 1, grade 2, grade 3 and grade 4 acute toxicity rates were 24.6%, 3.5%, 3.5%, and 0.0%, respectively. The cosmetic results were graded as excellent/good, fair, and poor in 73.7%, 19.3%, and 7.0%.

CONCLUSION

cHDR-RT of NMSC is an effective alternative to surgery due to excellent outcomes both in terms of local control and aesthetic results especially in the face.

A study of Type B uncertainties associated with the photoelectric effect in low-energy Monte Carlo simulations

Purpose.To estimate Type B uncertainties in absorbed-dose calculations arising from the different implementations in current state-of-the-art Monte Carlo (MC) codes of low-energy photon cross-sections (<200 keV).Methods.MC simulations are carried out using three codes widely used in the low-energy domain: PENELOPE-2018, EGSnrc, and MCNP. Three dosimetry-relevant quantities are considered: mass energy-absorption coefficients for water, air, graphite, and their respective ratios; absorbed dose; and photon-fluence spectra. The absorbed dose and the photon-fluence spectra are scored in a spherical water phantom of 15 cm radius. Benchmark simulations using similar cross-sections have been performed. The differences observed between these quantities when different cross-sections are considered are taken to be a good estimator for the corresponding Type B uncertainties.Results.A conservative Type B uncertainty for the absorbed dose (k = 2) of 1.2%-1.7% (<50 keV), 0.6%-1.2% (50-100 keV), and 0.3% (100-200 keV) is estimated. The photon-fluence spectrum does not present clinically relevant differences that merit considering additional Type B uncertainties except for energies below 25 keV, where a Type B uncertainty of 0.5% is obtained. Below 30 keV, mass energy-absorption coefficients show Type B uncertainties (k = 2) of about 1.5% (water and air), and 2% (graphite), diminishing in all materials for larger energies and reaching values about 1% (40-50 keV) and 0.5% (50-75 keV). With respect to their ratios, the only significant Type B uncertainties are observed in the case of the water-to-graphite ratio for energies below 30 keV, being about 0.7% (k = 2).Conclusions.In contrast with the intermediate (about 500 keV) or high (about 1 MeV) energy domains, Type B uncertainties due to the different cross-sections implementation cannot be considered subdominant with respect to Type A uncertainties or even to other sources of Type B uncertainties (tally volume averaging, manufacturing tolerances, etc). Therefore, the values reported here should be accommodated within the uncertainty budget in low-energy photon dosimetry studies.

Individualized mould-based high-dose-rate brachytherapy for perinasal skin tumors: technique evaluation from a dosimetric point of view

Purpose

Dosimetric treatment planning evaluations concerning patient-adapted moulds for iridium-192 high-dose-rate brachytherapy are presented in this report.

Material and methods

Six patients with perinasal skin tumors were treated with individual moulds made of biocompatible epithetic materials with embedded plastic applicators. Treatment plans were optimized with regard to clinical requirements, and dose was calculated using standard water-based TG-43 formalism. In addition, retrospective material-dependent collapsed cone calculations according to TG-186 protocol were evaluated to quantify the limitations of TG-43 protocol for this superficial brachytherapy technique.

Results

The dose-volume parameters D₉₀, V₁₀₀, and V₁₅₀ of the planning target volumes (PTVs) for TG-43 dose calculations yielded 92.2% to 102.5%, 75.1% to 93.1%, and 7.4% to 41.7% of the prescribed dose, respectively. The max- imum overall dose to the ipsilateral eyeball as the most affected organ at risk (OAR) varied between 8.9 and 36.4 Gy. TG-186 calculations with Hounsfield unit-based density allocation resulted in down by –6.4%, –16.7%, and –30.0% lower average D₉₀, V₁₀₀, and V₁₅₀ of the PTVs, with respect to the TG-43 data. The corresponding calculated OAR doses were also lower. The model-based TG-186 dose calculations have considered reduced backscattering due to environmental air as well as the dose-to-medium influenced by the mould materials and tissue composition. The median PTV dose was robust within 0.5% for simulated variations of mould material densities in the range of 1.0 g/cm³ to 1.26 g/cm³ up to 7 mm total mould thickness.

Conclusions

HDR contact BT with individual moulds is a safe modality for routine treatment of perinasal skin tumors. The technique provides good target coverage and OARs’ protection, while being robust against small variances in mould material density. Model-based dose calculations (TG-186) should complement TG-43 dose calculations for verification purpose and quality improvement.

Depth-dose measurement corrections for the surface electronic brachytherapy beams of an Esteya® unit: a Monte Carlo study

Three different correction factors for measurements with the parallel-plate ionization chamber PTW T34013 on the Esteya electronic brachytherapy unit have been investigated. This chamber type is recommended by AAPM TG-253 for depth-dose measurements in the 69.5 kV x-ray beam generated by the Esteya unit.

Monte Carlo simulations using the PENELOPE-2018 system were performed to determine the absorbed dose deposited in water and in the chamber sensitive volume at different depths with a Type A uncertainty smaller than 0.1%. Chamber-to-chamber differences have been explored performing measurements using three different chambers. The range of conical applicators available, from 10 to 30 mm in diameter, has been explored.

Using a depth-independent global chamber perturbation correction factor without a shift of the effective point of measurement yielded differences between the absorbed dose to water and the corrected absorbed dose in the sensitive volume of the chamber of up to 1% and 0.6% for the 10 mm and 30 mm applicators, respectively. Calculations using a depth-dependent perturbation factor, including or excluding a shift of the effective point of measurement, resulted in depth-dose differences of about ± 0.5% or less for both applicators. The smallest depth-dose differences were obtained when a shift of the effective point of measurement was implemented, being displaced 0.4 mm towards the center of the sensitive volume of the chamber. The correction factors were obtained with combined uncertainties of 0.4% (k = 2). Uncertainties due to chamber-to-chamber differences are found to be lower than 2%.

The results emphasize the relevance of carrying out detailed Monte Carlo studies for each electronic brachytherapy device and ionization chamber used for its dosimetry.