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.

End-to-end dosimetry audit for three-dimensional image-guided brachytherapy for cervical cancer

BACKGROUND

End-to-end dosimetry audit for brachytherapy is challenging due to the steep dose gradient. However, it is an efficient method to detect unintended errors in actual clinical practice.

PURPOSE

We aimed to develop an on-site end-to-end test phantom for three-dimensional image-guided brachytherapy (IGBT) for cervical cancer.

METHODS

The test phantom we developed consisted of a water tank with an applicator/detector holder. The holder was designed to accommodate the applicator and insert an ionization chamber (PinPoint; PTW, Freiburg, Germany) to measure the dose at point A. Imaging and reconstruction were performed in the same way as performed for a patient. The feasibility of our test phantom was assessed in two different hospitals using tandem and ovoid (made of either metal or carbon) applicators that the hospitals provided.

RESULTS

The measured and calculated doses at point A were compared for each applicator. We observed that the values obtained using metal applicators were consistently lower, on an average by -2.3%, than the calculated values, while those obtained using carbon applicators were comparable to the calculated values. This difference can be attributed to the attenuation of the dose by the metal applicators, resulting in a lower dose at point A. The majority of treatment planning system, including the one used in this study, do not account for the material of applicator.

CONCLUSIONS

An end-to-end test phantom for IGBT was developed, tested, and applied in a dosimetry audit in hospitals and showed favorable results for evaluating the point A dose.

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.

Personalized Superficial HDR Brachytherapy—Dosimetric Verification of Dose Distribution with Lead Shielding of Critical Organs in the Head and Neck Region

Background: Surface brachytherapy, usually characterized by a high dose gradient, allows the dose to be precisely deposited in the irradiated area while protecting critical organs. When the lesion is located in the nasal or ocular region, the organ of vision must be protected. The aim of this study was to verify the dose distributions near critical organs in the head and neck region during a brachytherapy procedure using lead shielding of the eye. Methods: Anthropomorphic phantom using 3D-printing technology was prepared. The doses deposited at a point in the lens of the eye and on the surface of the eyelid, directly under the lead shield were calculated and measured using EBT3 radiochromic films. Comparison of doses planned in the treatment planning system using the TG-43 formalism, TG-186 formalism, and measured were also performed. Results: Comparing the planned and calculated doses with TG186 formalism it can be assumed that the use of lead shields is a method for protecting the organ of vision from the adverse effects of ionizing radiation. Conclusions: The decision to use a lead shield during facial surface brachytherapy procedures should be made on a patient-by-patient basis and based on model-based calculation methods recommended by TG186.

Design and characterization of flattening filter for high dose rate 192Ir and 60Co Leipzig applicators used in skin cancer brachytherapy: A Monte Carlo study

PURPOSE

This study aimed to design optimal flattening filters for high dose rate (HDR) ¹⁹²Ir and ⁶⁰Co Leipzig applicators which are used to treat skin cancer.

MATERIALS AND METHODS

MCNPX Monte Carlo code was used to design flattening filters for Leipzig applicators with inner diameters of 1, 2 and 3 cm. Then, their dosimetric characterizations such as dose distribution, dose profile, percentage depth dose, flatness, symmetry and homogeneity were evaluated in a 20 × 20 × 20 cm³ water phantom and compared with those without the flattening filter.

RESULTS

The flattening filter thickness varied from 0 mm (at the edge) to the maximum values of 0.30, 1.18, and 2.41 mm for the ¹⁹²Ir Leipzig applicators of H1, H2, and H3 type, respectively. This quantity has maximum values of 0.96, 6.27, and 12.31 mm for the ⁶⁰Co double wall applicators of D1, D2, and D3 type, respectively. The dose profile flatness values for the H1, H2, and H3 ¹⁹²Ir Leipzig applicators with the optimal flattening filters were 0.76, 1.26, and 1.85%, respectively. Furthermore, the dose profile flatness values for the D1, D2, and D3 ⁶⁰Co double wall applicators with the optimal flattening filters were 1.11, 2.10 and 3.12%, respectively. The dose profile symmetry values obtained from various source-applicator combinations were less than 1.02. Compared to the applicators without flattening filter, the homogeneity values for the H1, H2, and H3 ¹⁹²Ir Leipzig applicators with the optimal flattening filters were improved 1.68, 6.51, and 13.17 times, respectively, and for the D1, D2, and D3 ⁶⁰Co double wall applicators were improved 1.23, 6.21 and 9.54 times, respectively.

CONCLUSION

The findings revealed that the inhomogeneous dose distribution resulted from the Leipzig applicators without the optimal flattening filter at the treatment surface could be improved by insertion of optimal lead flattening filters between the sources and treatment surface.

Dosimetric characterization of a novel 90Y source for use in the conformal superficial brachytherapy device

PURPOSE

To characterize the dose distribution in water of a novel beta-emitting brachytherapy source for use in a Conformal Superficial Brachytherapy (CSBT) device.

METHODS AND MATERIALS

Yttrium-90 (⁹⁰Y) sources were designed for use with a uniquely designed CSBT device. Depth dose and planar dose measurements were performed for bare sources and sources housed within a 3D printed source holder. Monte Carlo simulated dose rate distributions were compared to film-based measurements. Gamma analysis was performed to compare simulated and measured dose rates from seven ⁹⁰Y sources placed simultaneously using the CSBT device.

RESULTS

The film-based maximum measured surface dose rate for a bare source in contact with the surface was 3.35 × 10^-7 cGy s^-1 Bq^-1. When placed in the source holder, the maximum measured dose rate was 1.41 × 10^-7 cGy s^-1 Bq^-1. The Monte Carlo simulated depth dose rates were within 10% or 0.02 cm of the measured dose rates for each depth of measurement. The maximum film surface dose rate measured using a seven-source configuration within the CSBT device was 1.78 × 10^-7 cGy s^-1 Bq^-1. Measured and simulated dose rate distribution of the seven-source configuration were compared by gamma analysis and yielded a passing rate of 94.08%. The gamma criteria were 3% for dose-difference and 0.07056 cm for distance-to-agreement. The estimated measured dose rate uncertainty was 5.34%.

CONCLUSIONS

⁹⁰Y is a unique source that can be optimally designed for a customized CSBT device. The rapid dose falloff provided a high dose gradient, ideal for treatment of superficial lesions. The dose rate uncertainty of the ⁹⁰Y-based CSBT device was within acceptable brachytherapy standards and warrants further investigation.

Correction factors for ionization chamber measurements with the 'Valencia' and 'large field Valencia' brachytherapy applicators

Treatment of small skin lesions using HDR brachytherapy applicators is a widely used technique. The shielded applicators currently available in clinical practice are based on a tungsten-alloy cup that collimates the source-emitted radiation into a small region, hence protecting nearby tissues. The goal of this manuscript is to evaluate the correction factors required for dose measurements with a plane-parallel ionization chamber typically used in clinical brachytherapy for the 'Valencia' and 'large field Valencia' shielded applicators. Monte Carlo simulations have been performed using the PENELOPE-2014 system to determine the absorbed dose deposited in a water phantom and in the chamber active volume with a Type A uncertainty of the order of 0.1%. The average energies of the photon spectra arriving at the surface of the water phantom differ by approximately 10%, being 384 keV for the 'Valencia' and 343 keV for the 'large field Valencia'. The ionization chamber correction factors have been obtained for both applicators using three methods, their values depending on the applicator being considered. Using a depth-independent global chamber perturbation correction factor and no shift of the effective point of measurement yields depth-dose differences of up to 1% for the 'Valencia' applicator. Calculations using a depth-dependent global perturbation factor, or a shift of the effective point of measurement combined with a constant partial perturbation factor, result in differences of about 0.1% for both applicators. The results emphasize the relevance of carrying out detailed Monte Carlo studies for each shielded brachytherapy applicator and ionization chamber.

Commissioning and quality assurance procedures for the HDR Valencia skin applicators

The Valencia applicators (Nucletron, an Elekta company, Elekta AB, Stockholm, Sweden) are cup-shaped tungsten applicators with a flattening filter used to collimate the radiation produced by a high-dose-rate (HDR) ¹⁹²Ir source, and provide a homogeneous absorbed dose at a given depth. This beam quality provides a good option for the treatment of skin lesions at shallow depth (3-4 mm). The user must perform commissioning and periodic testing of these applicators to guarantee the proper and safe delivery of the intended absorbed dose, as recommended in the standards in radiation oncology.

In this study, based on AAPM and GEC-ESTRO guidelines for brachytherapy units and our experience, a set of tests for the commissioning and periodic testing of the Valencia applicators is proposed. These include general considerations, verification of the manufacturer documentation and physical integrity, evaluation of the source-to-indexer distance and reproducibility, setting the library plan in the treatment planning system, evaluation of flatness and symmetry, absolute output and percentage depth dose verification, independent calculation of the treatment time, and visual inspection of the applicator before each treatment. For each test, the proposed methodology, equipment, frequency, expected results, and tolerance levels (when applicable) are provided.

Novel simple templates for reproducible positioning of skin applicators in brachytherapy

PURPOSE

Esteya and Valencia surface applicators are designed to treat skin tumors using brachytherapy. In clinical practice, in order to avoid errors that may affect the treatment outcome, there are two issues that need to be carefully addressed. First, the selected applicator for the treatment should provide adequate margin for the target, and second, the applicator has to be precisely positioned before each treatment fraction. In this work, we describe the development and use of a new acrylic templates named Template La Fe-ITIC. They have been designed specifically to help the clinical user in the selection of the correct applicator, and to assist the medical staff in reproducing the positioning of the applicator. These templates are freely available upon request.

MATERIAL AND METHODS

Templates that were developed by University and Polytechnic Hospital La Fe (La Fe) and Hospital Clínica Benidorm (ITIC) in cooperation with Elekta, consist of a thin sheet made of transparent acrylic. For each applicator, a crosshair and two different circles are drawn on these templates: the inner one corresponds to the useful beam, while the outer one represents the external perimeter of the applicator. The outer circle contains slits that facilitate to draw a circle on the skin of the patient for exact positioning of the applicator. In addition, there are two perpendicular rulers to define the adequate margin. For each applicator size, a specific template was developed.

RESULTS

The templates have been used successfully in our institutions for more than 50 patients' brachytherapy treatments. They are currently being used for Esteya and Valencia applicators.

CONCLUSIONS

The template La Fe-ITIC is simple and practical. It improves both the set-up time and reproducibility. It helps to establish the adequate margins, an essential point in the clinical outcome.