Peripheral doses outside the electron applicators in an Elekta Versa HD Linac

doi:10.21203/rs.3.rs-3747339/v1

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Peripheral doses outside the electron applicators in an Elekta Versa HD Linac

https://doi.org/10.21203/rs.3.rs-3747339/v1

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Background

Radiotherapy is an essential component of cancer treatment, but healthy tissues can be exposed to out-of-field doses, potentially causing adverse effects and secondary cancers. This study investigates peripheral doses outside the electron beam applicator in an Elekta Versa HD linear accelerator.

Methods

Peripheral doses outside the electron applicator were measured using 6, 9, and 12 MeV beams at their respective maximum dose depths while maintaining a 100 cm source-to-surface distance. Measurements employed EBT3 films within solid water phantoms. The influence of field size on penumbra width and peripheral doses were examined using various cutouts (6 x 6 cm², 10 x 10 cm², and a 5 cm diameter circle) within a 10 × 10 cm² applicator, with gantry and collimator angles set to 0 degrees. Additionally, the impact of collimator angles on penumbra width and peripheral doses was explored, enhancing the understanding of dose distribution. Measured profiles were also compared with those calculated using Monaco treatment planning system.

Results

Findings showed that both penumbra width and peripheral dose values increased with energy across different field sizes and collimator angles. Root Mean Square Deviation (RMSD) analysis indicated deviations of 1.8 mm for penumbra and 1.1% for peripheral doses between measured profiles and Treatment Planning System (TPS) predictions for all field sizes. Notably, peripheral doses remained below 5% of the maximum dose at distances ranging from 10 to 15 mm away from the field edges, suggesting the potential for implementation of supplementary shielding strategies.

Conclusion

This study highlights the critical importance of considering peripheral doses in radiotherapy, emphasizing the need to evaluate the impact on healthy tissues outside the primary treatment area to ensure patient safety and mitigate long-term treatment-related side effects. The findings underscore the necessity of implementing appropriate measures to minimize peripheral doses.

Peripheral Dose

Electron Therapy

Applicator

Elekta Versa HD

Linac

Out-of-field

The introduction of modern radiotherapy technologies has resulted in substantial improvement of the long-term survival rates for cancer patients. However, individuals who have successfully battled cancer still face an increased risk of developing subsequent malignancies. Current statistics indicate that approximately 17–19% of these survivors eventually experience a second form of cancer[1]. This phenomenon can be attributed to a combination of factors, including ongoing lifestyle choices, genetic predisposition, and prior treatments such as radiotherapy and chemotherapy. While radiotherapy accounts for about 5% of these treatment-related secondary malignancies, it is challenging to isolate its specific impact due to the numerous contributing factors at play[1]. According to data from the U.S. National Cancer Institute's SEER program, the incidence of second malignancies has doubled from 1975 to 2009[1]. Consequently, given that stochastic effects can induce cancer without a minimum dose threshold, it is imperative not to underestimate the significance of peripheral radiation dose and penumbra width in radiation therapy, particularly when it comes to the proximity of critical organs, such as the eyes. According to the International Committee for Radiobiological Protection, a harmful opacity threshold dose for the eye lens is set at 0.5 Gy[2].

Linear accelerators are the most frequently used equipment in the field of radiation therapy for producing high-energy electrons and photon beams. Electron beam therapy, with a history spanning over half a century, has been widely used in radiation treatment, providing acceptable and consistent radiation doses. It proves particularly effective for treating superficial cancers located within approximately 5 cm of the skin[3]. This approach effectively minimizes damage to deeper tissues, a feat often unattainable with high-energy X-rays.

In electron beam therapy, the electron beam goes through a scattering foil and significantly interacts with various components in the accelerator head and the air between the exit window and the patient, resulting in an undesirably wide penumbra and contaminant photons and scattered electrons which can cause increase dose to depth in tissue[4]. The collimator jaws of the linear accelerator (linac) are inadequate for shaping the electron field since they are positioned at a considerable distance from the patient[4]. To address this issue, electron applicators are commonly used in combination with Cerrobend cutouts. Electron applicators help shaping the electron beam to conform to the contours of the treatment area, typically the tumor[5], and aid in maintaining or improving the beam flatness at the required depth [6]. Applicators serve to confine the electron beam and minimize its lateral dispersion from the accelerator head to the patient's skin, contributing to the accuracy of treatment delivery[5]. Applicator and cutout options are available in various sizes to accommodate different treatment areas. However, placing anything in an electron pathway, whether air or Cerrobend significantly increases contamination X-rays and scattered electrons. Considering the short mean free path of electrons, scattering occurs both within and outside the applicator, a unique consideration compared to photon-based techniques[7]. Studies examining scattered radiation from different types of electron applicators and various vendors have been documented in the literature[5, 7–20]. However, it is worth noting that a significant portion of these studies are quite dated, with most of them conducted over 20 to 35 years ago often utilizing outdated applicators and linear accelerators possibly due to gradual decrease in the use of electron treatment over the years.

Despite the lower number of cases for electron treatments, considering the progress in electron applicator technology and the introduction of new generations of linear accelerators, there is merit in conducting investigations to explore out-of-field scatter in more contemporary machines. Modern external beam radiotherapy dose determination based on measurements, TPSs, and Monte Carlo simulations are generally well-established for in-field assessments but challenging for out-of-field regions. TPSs are not commissioned for out-of-field calculations, leading to significant uncertainties beyond treatment field borders[21].

Quantifying this peripheral dose can help with evaluating potential tissue damage. The objective of this study is to examine the peripheral dose associated with an Elekta Versa HD linear accelerator and compare the measured data with Monaco treatment planning system (TPS). This comparison has not been reported for this linac generation .

Measurements were performed using an Elekta Versa HD machine (Elekta AB, Stockholm, Sweden) operating at 6, 9, and 12 MeV. When the selected electron applicator is connected to the accessory mount within the gantry head, the collimator jaws automatically open to a predefined field size.

2.1. Dosimetry

Gafchromic EBT3 films provided by Ashland^™ (NJ, USA), were used for dosimetry measurements due to their ability to provide high-resolution two-dimensional data with minimal energy dependence[22]. These films possess a composition that closely resembles human tissue[23].

For film calibration, EBT3 films from the same batch were carefully cut into 4 × 4 cm² pieces using a Beambox-FLUX 40WCO2 laser cutter (Koenig Machinery, VIC, Australia) in a dark room to prevent potential unwanted darkening of the film. Each film piece was affixed to the phantom using adhesive tape and then irradiated individually with 6 MeV electron beams in a solid water phantom (SWP) at a depth of 1.3 cm and a SSD of 100 cm. Before placing the films, the central beam axis was marked on the solid water phantom to ensure accurate alignment. The standard 10 x 10 cm² applicator and cutout were used to expose the films from 1 to 250 MUs.

Twenty hours after irradiation, each film was scanned using an Epson 12000XL (Nagano, Japan). Each film underwent four scans, and the average of the last three scans was used to account for minor inconsistencies in scanner response. MATLAB software (R2022b, Natick, Massachusetts, USA) was used for image processing and an analytical equation was derived to fit a double exponential curve through results, allowing for the conversion of pixel values from the red channel image (Fig. 1–3) into dose (cGy). This analytical equation was subsequently used to calculate the out-of-field doses. The measurement setup is shown in Fig. <link rid="fig1">1</link>–1.

2.2. Profile Measurements

For the purpose of conducting profile measurements, a ruler-like structure (Fig. 1–2) was created using Fusion 360 software (Autodesk, San Francisco, CA, USA) and used to cut films with a laser cutter. Subsequently, each film was exposed to 6, 9, and 12 MeV electron beams at the depth of maximum dose (D_max) for each energy, maintaining a SSD of 100 cm.

To evaluate the impact of field sizes on peripheral dose and penumbra, exposures were conducted with both the gantry and collimator at 0 degrees, at D_max for each energy in solid water, and the SSD set to 100 cm. 6 x 6 cm² and 10 x 10 cm² cut outs and a 5 cm diameter circular cutout were used in 10 x 10 cm² applicators. To investigate the effect of collimator angle, measurements were carried out at collimator angles of 0 and 180 degrees with the standard 10 x 10 cm² applicator and cutout. The rest of parameters were similar to the above measurements.

A 10 cm solid water block was used to provide backscatter. The suggested 5 cm air gap between the applicator end and the phantom surface was consistently maintained in each setup. Dose profiles were then acquired in both the cross-plane and in-plane directions for each configuration.

In-plane and cross-plane profiles were calculated for the same setups using Monaco® treatment planning system (TPS) version 6.1.3.0 (Elekta, Stockholm, Sweden). This version of TPS uses Monte Carlo (MC) technique, which is a statistical method based on random numbers, to estimate the dose. The calculation grid spacing was 0.15 cm (isotropic) and the number of histories per cm² was 500,000. The phantom was forced to be treated as water.

2.3. Calculation of Physical Parameters

Before undertaking an evaluation of peripheral doses, beam symmetry was measured following the the International Electrotechnical Commission (IEC) protocol [26]. Physical penumbra was determined based on the International Commission on Radiation Units and Measurements (ICRU) Report 62 guidelines which defines it as the distance between the 20% and 80% isodose curves (Fig. 1–6) at the depth of the maximum dose (D_max) [27].

In the context of quantifying peripheral doses during radiation therapy, specific attention was directed towards the region below the 20% isodose threshold. This choice was rooted in the observation of an exponential decay in radiation dose as the distance increased from the field edge. This rapid dose falloff hinted at underlying physical processes, potentially involving electron scattering and the presence of contaminating photons. Understanding the distribution and magnitude of these sub-threshold peripheral doses is useful, as nearby tissues could be impacted while the primary target area remains unaffected. Collectively, these doses, consisting of scattered electrons and contaminating photons, are commonly referred to as "peripheral doses". Recognizing the spatial distribution and extent of these peripheral doses is beneficial for the optimization of treatment plans and the minimization of unintended radiation exposure to non-targeted anatomical structures.

$$\varvec{P}\varvec{e}\varvec{r}\varvec{i}\varvec{p}\varvec{h}\varvec{e}\varvec{r}\varvec{a}\varvec{l} \varvec{D}\varvec{o}\varvec{s}\varvec{e} \left(\varvec{\%}\right)= \left(\frac{\varvec{D}\varvec{a}\varvec{r}\varvec{k} \varvec{B}\varvec{l}\varvec{u}\varvec{e} \varvec{A}\varvec{r}\varvec{e}\varvec{a}}{\varvec{C}\varvec{e}\varvec{n}\varvec{t}\varvec{r}\varvec{a}\varvec{l} \varvec{L}\varvec{i}\varvec{g}\varvec{h}\varvec{t} \varvec{B}\varvec{l}\varvec{u}\varvec{e} \varvec{A}\varvec{r}\varvec{e}\varvec{a}}\right)$$

3.1. Effect of field sizes on penumbra width and peripheral doses

The influence of field size on penumbra width and peripheral doses across various energy levels is shown in Fig. 3, which compares the measured and TPS-calculated profiles for the 6 ⋅ 6, and 10 ⋅ 10 cm² cutouts, and the 5 cm diameter circular cutout.

3.2. Effect of collimator angle on penumbra width and peripheral dose

The effect of the collimator angle on penumbra width and peripheral dose is shown in Fig. 6 for consistent field and applicator sizes across all tested energy levels. Measured profiles at D_max with collimator angles of 0 and 180 degrees for a 10 ⋅ 10 cm² cutout were compared with TPS predictions.

Peripheral dose may be resulted from scattered photon or electron components. Bremsstrahlung photons are generated upon the interaction between a high-energy electron beam and parts of the applicator. The electron component contributes to the peripheral dose through three primary mechanisms: (1) scattering out of the applicator, (2) penetration through collimating structures within the applicator, and (3) direct exit into the surrounding air without interacting with applicator components[7]. Notably, the latter two mechanisms—penetration and direct escape—are more common with higher-energy beams, while scattering is characteristic of lower-energy beams which includes scattered electrons and bremsstrahlung photons[8, 24]. The energy spectrum of scattered photons and electrons depends on the field size, distance from the applicator, and material of the applicator, which can be approximated using Monte Carlo simulations[7, 25]. The main source of the peripheral dose is known to be the photons produced in the Bremsstrahlung effect[13].

4.1. Symmetry

The symmetry of the measured and TPS-calculated profiles was found within the range of ±3% aligning with the acceptable standards stipulated by the IEC protocol. This investigation confirmed the uniform delivery of the dose across the treatment field. It was essential to ensure that the radiation hit the center of the film, as it contributed to the symmetrical profile and facilitated the calculation of the peripheral dose.

4.2. Effect of field sizes on penumbra width and peripheral doses

4.2.1. Penumbra Width

Figure 3 (a) indicated that on an average, Elekta displayed a penumbra value of 11.3 ± 1.4 mm for the measured in-line and cross-line profiles across all field sizes and energies. Furthermore, the penumbra value demonstrated an increasing trend with energy. Specifically, for a field size measured in cross-line and in-line at 6 MeV, the average penumbra width was 10.0 ± 0.5 mm, while at 12 MeV, it was 12.7 ± 0.1 mm. Interestingly, TPS-predicted values were slightly higher than measured. The root mean square deviation (RMSD) of the measured and TPS-calculated values across all fields and energies for both cross-line and in-line, was 1.8 mm on an average.

4.2.2. Peripheral Dose

4.3. Effect of Collimator Angles on penumbra width and peripheral doses

4.3.1. Penumbra Width

In Fig. 4 (a), the average penumbra value for collimator angles 0 and 180 degrees is 10.4 ± 0.2 mm in 6 MeV, 11.7 ± 0.2 mm in 9 MeV, and 13.1 ± 0.4 mm in 12 MeV. The penumbra width increased with energy. In both cross-line and in-line, the penumbra width for each collimator angle remained notably consistent in 6 and 9 MeV beams (as shown in Fig. 7, where the profiles for both collimator angles are perfectly aligned), while the 180-degree angle at 12 MeV exhibited a slight difference. Comparing the average differences in in-line and cross-line penumbra widths across both collimator angles, there is an increasing trend in differences with increasing energy, as shown in Fig. 7, which shows the absolute difference in penumbra width for each energy’s collimator angles for the averaged crossline and inline measurements.

4.3.2. Peripheral Dose

Overall, in the out-of-field region, doses were higher near the field edge, as expected. With increasing beam energy, the curves bulged outward, which means higher peripheral doses. It was noted that from 10 to 15 mm from the field edge up to about 110 mm, peripheral dose values were less than 5% of maximum dose (Fig. 9). These results are in agreement with the findings of Cardenas et al. [26] and Haghparast et al.[16].

In certain electron treatments, sensitive organs like the eyes and thyroid may lie near the treatment field edge, making it important to evaluate the out-of-field dose. The International Committee for Radiobiological Protection (ICRP 103) specifies a threshold dose of 0.5 Gy for the eye lens[2]. Electron beams have limited penetration compared to photons, potentially resulting in increased dose to skin. Skin, a sensitive tissue, is consistently affected during electron treatments, underscoring the significance of out-field doses.

While out-of-field electron doses may seem less significant than photon beams, they should be considered in electron therapy due to the absence of a secondary cancer threshold, with dose values dependent on energy, applicator dimensions, and depth[27].

This study investigated the dependence of the peripheral dose and penumbra width of electron beams on the cutout size, collimator angle, and gantry angle. On average, both the penumbra width and peripheral dose increased with beam energy across all field sizes and collimator angles. For each energy, peripheral doses and penumbra widths did not exhibit significant differences across varying collimator angles. The TPS showed higher values for penumbra width in comparison to the measured values. However, the measured and TPS-calculated peripheral doses were quite similar (RMSD = 1.8 mm). Approximately 10 to 15 mm away from the field edge, peripheral doses reduced to less than 5% of the maximum dose, suggesting that they could be easily removed by implementing additional shielding for patients, where needed. Understanding peripheral dose values can assist the radiotherapy team in safeguarding critical organs outside the applicator.

Ethical Approval and Consent to participate

Not applicable

Consent for publication

Not applicable

Availability of supporting data

Not applicable

Author’s contribution

Conception and design of study: KDM, PR, SG

Library searches: KDM

Writing the manuscript: KDM, PR, SG, MS, JD, MI, TM

Critical review and supervision: PR, MS, SG, JD

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication.

Acknowledgments

The authors would like to express their sincere gratitude to the Medical Physics Department at Genesis Care Wembley, Western Australia, for providing the opportunity to conduct measurements using the Elekta Versa HD linear accelerator.

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You are reading this latest preprint version

Peripheral doses outside the electron applicators in an Elekta Versa HD Linac

Status:

Version 1

Abstract

Background

Methods

Results

Conclusion

Figures

1. Introduction

2. Material and Methods

2.1. Dosimetry

2.2. Profile Measurements

2.3. Calculation of Physical Parameters

3. Results

3.1. Effect of field sizes on penumbra width and peripheral doses

3.2. Effect of collimator angle on penumbra width and peripheral dose

4. Discussion

4.1. Symmetry

4.2. Effect of field sizes on penumbra width and peripheral doses

4.2.1. Penumbra Width

4.2.2. Peripheral Dose

4.3. Effect of Collimator Angles on penumbra width and peripheral doses

4.3.1. Penumbra Width

4.3.2. Peripheral Dose

5. Conclusion

Declarations

References

Status:

Version 1