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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 14  |  Issue : 12  |  Page : 1110-1116

Evaluation of electron dose calculations accuracy of a treatment planning system in radiotherapy of breast cancer with photon-electron technique


1 Medical Physics Research Center, Mashhad University of Medical Sciences, Mashhad, Iran
2 Department of Medical Physics and Medical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran
3 Comprehensive Cancer Centers of Nevada, Las Vegas, Nevada, USA

Date of Web Publication11-Dec-2018

Correspondence Address:
Ashraf Farkhari
Department of Medical Physics, Faculty of Medicine, Mashhad University of Medical Sciences, Pardis-e-Daneshgah, Vakil Abad Boulevard, Mashhad
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.199457

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 > Abstract 


Aim: The aim of this study was to assess the accuracy of electron dose calculations of Prowess Panther treatment planning system (TPS) for abutting photon-electron (PE) technique. In this work, we have assessed the accuracy of electron dose calculations in a simulated internal mammary field because this field is irradiated with electron in PE technique.
Materials and Methods: In this study, regions of in-field, under electron shield, and outside the internal mammary field were evaluated. Thermoluminescent dosimeter (TLD-700) chips were used within RANDO phantom for dose measurement. Prowess Panther TPS was also applied for dose calculation. Finally, confidence limit values were obtained to quantify the TPS electron dose calculation accuracy of an internal mammary field.
Results: The results show that for outside of field and under shield regions, Prowess Panther TPS underestimated the dose compared to the measured doses by TLD-700, whereas for in-field regions, the calculated doses by Prowess Panther TPS compared to the measured doses by TLD-700, for some points are overestimated and other points are underestimated. Finally, the confidence limit values were obtained for various regions of the internal mammary field. Confidence limits for in-field, outside of field, and under shield regions were 54.23, 108.19, and 80.51, respectively.
Conclusions: It is concluded that the accuracy of electron dose calculations of Prowess Panther TPS is not adequate for internal mammary field treatment. Therefore, it is recommended that for fields with electron beams Prowess Panther TPS calculations should not be entirely relied upon.

Keywords: confidence limit, dose calculation accuracy, photon-electron technique, radiotherapy, treatment planning system


How to cite this article:
Toossi MT, Soleymanifard S, Farhood B, Farkhari A, Knaup C. Evaluation of electron dose calculations accuracy of a treatment planning system in radiotherapy of breast cancer with photon-electron technique. J Can Res Ther 2018;14, Suppl S5:1110-6

How to cite this URL:
Toossi MT, Soleymanifard S, Farhood B, Farkhari A, Knaup C. Evaluation of electron dose calculations accuracy of a treatment planning system in radiotherapy of breast cancer with photon-electron technique. J Can Res Ther [serial online] 2018 [cited 2019 Oct 23];14:1110-6. Available from: http://www.cancerjournal.net/text.asp?2018/14/12/1110/199457




 > Introduction Top


Breast cancer is the most common malignancy and after lung cancer is the second leading cause of cancer death among women in the United States.[1] As a study indicated, on average, one in eight women will develop breast cancer during their lifetime.[2] Breast cancer may be treated with surgery, radiation therapy, chemotherapy, and hormone therapy. The first treatment choice for many patients is surgery (except in advanced diseases).[1] Radiation therapy is carried out on around 50% of all patients with localized cancer.[3],[4] For radiation therapy of breast cancer, various techniques can be performed because of the anatomical variations of the region, and depending on whether internal mammary nodes are included in the target volume. Some of the breast cancer radiotherapy techniques with inclusion internal mammary nodes are wide-area (WA) technique, photon-electron (PE) techniques, and oblique electron (OE) technique. In WA technique, tangential photon areas are applied in a way that internal mammary is included. The PE techniques can be carried out in several different ways such as: Oblique PE (OPE) technique and vertical PE (VPE) technique.[5] Furthermore, there are some other ways to do the PE technique in the literatures.[6],[7],[8] In OPE technique, PE beams are irradiated to the internal mammary area obliquely in parallel with the tangential field. In VPE technique, PE beams are irradiated vertically to the chest wall, as the gantry angle is 0° (IEC scale). In the OE technique, the electron beam is irradiated in parallel with the tangential field including the lateral point of the breast.[5]

As reported in many publications, cardiac tissue is exposed to some degree of radiation in radiation therapy of the left chest wall and breast, and consequently, a small increase in cardiac deaths is seen in the long term.[9],[10],[11] In the other hand, irradiation to the internal mammary nodes remains controversial among radiation oncologists due to the concern of potential lung and heart toxicities.[12] Use of collimation system, wedge filters, three-dimensional (3D) tomography-based planning system, and dynamic multileaf collimator is recommended to provide a homogeneous dose distribution in the target volume, decreasing the dose administered to the lung and heart as well as decreasing cutaneous reactions.[13],[14],[15],[16] Other than these radiation therapy techniques mentioned above, better dose reductions for critical organs are provided by proton beam therapy, intensity-modulated radiotherapy, and helical tomotherapy treatment.[17] In addition, delivering an inaccurate dose to the target or around normal tissues can cause tumor recurrence and adverse effect to normal tissues. Hence, it is necessary to deliver adequate dose to the tumor while keeping the dose to organs at risk as low as possible. To achieve this goal, treatment planning systems (TPSs) should calculate the dose exactly; therefore, quality assurance (QA) in the radiation therapy treatment planning process is necessary to minimize the possibility of accidental exposure and to ensure accurate dose delivery to the patient. Reduction of uncertainties and errors plays a significant role in the outcome of radiation therapy treatment.[18]

There are several studies relevant to the dose calculation accuracy of different algorithms/TPSs in radiation therapy.[19],[20],[21],[22],[23],[24],[25] However, few studies have assessed the accuracy of electron dose calculation, especially in breast radiotherapy.[26],[27],[28],[29]

Remoto and Corpuz[26] carried out QA of Pinnacle TPS for external beam radiation therapy. They concluded that dose calculation accuracy of the TPS for electrons and photons, in most calculation points, was acceptable, although significant differences from measured dose were observed in some points. Ding et al.[27] compared the accuracy of two commercial electron beam TPSs for dose calculations. They showed that the pencil beam model has some serious limitations in predicting cold and hot spots in inhomogeneous phantoms for small low- or high-density inhomogeneities. Furthermore, the Monte Carlo calculated results generally agree better with measurements.

To the best of our knowledge, there is no investigation on electron dose calculation accuracy of Prowess Panther TPS in breast radiotherapy, with PE technique. Therefore, the aim of this study was to assess the accuracy of electron dose calculations of the TPS for PE technique. In fact, we assessed the accuracy of the electron dose calculations in a typical internal mammary field because only this field is irradiated using electron in PE technique.


 > Materials and Methods Top


Treatment planning and irradiation of the phantom

A computed tomography scan of a RANDO phantom (the Phantom Laboratory, NY, USA) was taken to produce a treatment plan. The images were transferred to Prowess Panther version 5.2 (Prowess Inc., USA). This TPS has two algorithms: One is Collapsed Cone Convolution Superposition that calculates dose based on full heterogeneity correction and is a full 3D dose calculation. Another model is a fast photon effective model and also fast electron effective (FEE) model that calculates dose based on measured data where all the tissue is assumed to be water with no tissue heterogeneity or effective path length through tissue is taken into. In this study, FEE model was used in electron dose calculation.

The treatment plan used an anterior supraclavicular 15 MV photon field, a posterior axillary 15 MV photon field, two opposed tangential 6 MV photon beams, and an anterior oblique 15 MeV electron field abutted to two tangential beams. To protect the heart and lung, a shield with 7.5 mm thickness adjusted for OE field, as this shield was a lead cutout for the electron applicator cone. [Figure 1] showed irradiation setup for anterior oblique 15 MeV electron field irradiation. Furthermore, treatment area of the internal mammary field on the RANDO phantom is showed in [Figure 2].
Figure 1: Phantom setup for anterior oblique 15 MeV electron field irradiation

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Figure 2: Treatment area of internal mammary field on the RANDO phantom

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In this study, we investigated the accuracy of electron dose calculation in the anterior oblique 15 MeV electron field.

Calibration of applied dosimeters and dosimetric method

In this study, thermoluminescent dosimeter (TLD-700) chips were used for electron dose measurement, as this type of TLDs was used for electron dose measurement by researchers in previous literature.[30],[31] Readout and analysis of these TLDs were performed in the Medical Physics Research Center (Mashhad, Iran), which has a specific protocol for TLD analysis. TLD-700 is produced by thermoluminescent dosimeter company and made of LiF, Mg, and Ti with the size of 3.2 mm × 3.2 mm and thickness of 0.9 mm. These TLDs have a reproducibility of approximately ± 1.5% (1 standard deviation [SD]). Twenty TLD-700 chips were placed in a Perspex holder which was located on a 30 cm × 30 cm × 20 cm water equivalent phantom. A 25 mm water equivalent slab was placed on the holder to make a build-up region. Moreover, an applicator with size of 20 cm × 20 cm was used. First, they were irradiated to determine their individual efficiency correction coefficient (ECC). Then, they were irradiated with 50 cGy and readout by Harshaw reader to determine reader calibration factor. Finally, all of the TLD-700 chips were irradiated with 50 cGy, and their individual ECC was determined. Forty dosimeters were placed in different regions of slices No. 12–16 of the RANDO phantom. Two dosimeters were used to measure the background radiation. To increase the accuracy of dosimetry results, radiations were repeated four times.


 > Analysis Of Results Top


For analysis of the results, TRS 430,[32] The Nederlandse Commissie voor Stralingsdosimetrie (NCS, Netherlands Commission on Radiation Dosimetry),[33] and TECDOC 1540[34] protocols were applied. These protocols provide information on QA of TPSs. According to these protocols, the difference between the measured and the calculated dose is defined based on equation 1:

δ (%) = 100 × (DcalcDmeas)/Dmeas (1)

Where, Dmeas and Dcalc are the measured dose by TLD-700 chips and the calculated dose by TPS, respectively. Therefore, the confidence limit is determined based on equation 2:

Δ = |average deviation| + 1.5 × SD (2)

The confidence limit is obtained by calculation of the average deviation between the measured and the calculated dose values for several data points in comparable positions and the SD of the differences (1 SD of the average).

Finally, the confidence limit values were obtained for in-field, under shield, and outside field regions and were compared to the tolerance limit suggested in NCS protocol.


 > Results Top


Slices No. 12–16 of RANDO phantom were selected to assess the accuracy of electron dose calculations in the internal mammary field, as in these slices in-field, outside field, and under shield regions were evaluated.

In these slices, doses measured by TLD-700 chips were compared with corresponding values calculated by Prowess Panther TPS. These results are illustrated in the following tables.

[Figure 3] shows the location of TLDs in slice No. 12–16 of RANDO phantom (Part [A]) and image of slice No. 12–16 of RANDO phantom and the location of points corresponding to the measured points in Prowess Panther TPS (Part [B]).
Figure 3: (a) Slices No. 12–16 of RANDO phantom showing placement of thermoluminescent dosimeter chips and (b) Slices No. 12–16 of RANDO phantom by the treatment planning system showing placement of dose calculations

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In the figure, according to No. TLD chips, each of in-field, outside field, penumbra, under shield regions were specified.

[Table 1] lists the measured dose (Dmeas), the calculated dose (Dcalc), and the difference between Dmeas and Dcalc in the selected points for in-field regions. It is noteworthy that the bolded digits in this table are the measured/calculated doses obtained for the points close to the interfaces of inhomogeneities. Hence, data related to these regions were not used to achieve the confidence limit values.
Table 1: The measured dose (Dmeas), the calculated dose (Dcalc) and the difference between Dmeas and Dcalc in the selected points for in-field regions

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[Table 2] lists the measured dose (Dmeas), the calculated dose (Dcalc), and the difference between Dmeas and Dcalc in the selected points for outside field regions. It is noteworthy that the bolded digits in this table are the measured/calculated doses obtained for the points close to the interfaces of inhomogeneities. Hence, data related to these regions were not used to achieve the confidence limit values.
Table 2: The measured dose (Dmeas), the calculated dose (Dcalc) and the difference between Dmeas and Dcalc in the selected points for outside field regions

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[Table 3] lists the measured dose (Dmeas), the calculated dose (Dcalc), and the difference between Dmeas and Dcalc in the selected points for under shield regions. It is noteworthy that the bolded digits in this table are the measured/calculated doses obtained for the penumbra region. Hence, data related to these regions were not used to achieve the confidence limit values.
Table 3: The measured dose (Dmeas), the calculated dose (Dcalc) and the difference between Dmeas and Dcalc in the selected points for under shield regions

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Finally, the confidence limit values were obtained for selected points in internal mammary field, as for in-field, outside field, and under shield regions were 54.23, 108.19, and 80.51, respectively.


 > Discussion Top


In this study, using TLD-700, the accuracy of electron dose calculation in the internal mammary field for Prowess Panther TPS (version 5.2) was quantified so that regions of in-field, outside field, and under shield were evaluated.

The results show that for outside field and under shield regions, the calculated doses by Prowess Panther TPS compared to the measured doses by TLD-700 are underestimated, whereas for in-field regions, the calculated doses by Prowess Panther TPS compared to the measured doses by TLD-700, for some points are overestimated and other points are underestimated.

According to the NCS protocol that provides information on QA of TPSs, tolerance limit for accuracy of electron dose calculation in complex geometry (i.e. inhomogeneous phantom) and in-field regions (i.e. high dose-small dose gradient regions) is three or four, and in outside field regions (i.e. low dose-small dose gradient regions), it is four (if differences normalized to the dose at a point at the same depth on the central beam axis) or forty (if differences normalized to local dose).[33] The results of this study showed that the confidence limit values for the in-field, under shield, outside field, and regions were not within the NCS recommended tolerance. Several studies have investigated the accuracy of electron dose calculation of different TPSs/algorithms. Remoto and Corpuz[26] carried out QA of Pinnacle TPS for external beam radiation therapy. They used CIRS phantom for dose calculation accuracy of TPS. In their study, for assessment of the accuracy of electron dose calculation, manual and Pinnacle calculations were compared. Furthermore, for assessment of the accuracy of photon dose calculation, dose calculated by the TPS was verified against doses measured with a Farmer chamber. They concluded that dose calculation accuracy of the TPS for electrons and photons, in most calculation points, was acceptable, although significant differences from measured dose were observed in some points. Pemler et al.[28] evaluated a commercial electron beam TPS against Monte Carlo algorithm. In this study, various tests have been performed for evaluation of the Monte Carlo treatment planning algorithm. Their results showed that the algorithm has satisfactory results for all of the basic tests as well as in the presence of inhomogeneities. Deviations were observed in the high dose region and off-axis regions for high (18 and 22 MeV) and very low (6 MeV) energies. Chan et al.[29] assessed the accuracy of a commercial electron Monte Carlo (eMC) dose calculation algorithm by Gafchromic EBT3 films. They investigated the effects of oblique incidence, small field size, and inhomogeneous media on the electron dose distribution and to compare calculated and measured results. Their results showed agreement between EBT3 and XiO eMC was within 3% or 3 mm for the nonstandard fields and 2% or 2 mm for most standard fields. Larger differences were found in the buildup region were XiO eMC underestimates the dose for small circular fields by up to 5% and overestimates the dose by up to 10% for obliquely incident fields compared to measurement. Furthermore, calculations for inhomogeneities of lung, ribs, and skull tissue agreed with measurement to within 3% or 3 mm. Ding et al.[27] compared the accuracy of two commercial electron beam TPSs for dose calculations: One uses a Monte Carlo algorithm and another uses a pencil beam model. They showed that the pencil beam model has some serious limitations in predicting cold and hot spots in inhomogeneous phantoms for small low- or high-density inhomogeneities, especially for low energy electron beams, such as 9 MeV. Errors >10% were seen in predicting high- and low-dose variations for 3D inhomogeneous phantoms. For the TPS which uses the Monte Carlo algorithm, the calculated results generally agreed better with measurements.

It is notable that TLD is an acceptable tool for QA in radiation therapy. TLDs have a number of relevant advantages such as tissue equivalence, small physical size, and high sensitivity.[35] Monte Carlo simulation can also be applied for evaluation of the accuracy of electron dose calculation of TPSs in radiation therapy. As a subject for future study, evaluation of the accuracy of electron dose calculation of Prowess Panther TPSs in electron beam techniques using Monte Carlo simulation will be interesting.

As a subject for future studies, we will evaluate (1) the electron dose calculation accuracy for different TPSs, (2) the electron dose calculation accuracy of Prowess Panther TPSs using Monte Carlo simulation, and (3) the electron dose calculation accuracy for other areas such as head and neck.


 > Conclusion Top


In this study, using TLD-700, the accuracy of electron dose calculations in the internal mammary field for Prowess Panther TPS (version 5.2) was assessed. The results show that for outside field and under shield regions, Prowess Panther TPS underestimates the dose compared to the measured doses by TLD-700, whereas for in-field regions, the calculated doses by Prowess Panther TPS compared to the measured doses by TLD-700, for some points are overestimated and other points are underestimated. Finally, it is conceded that the accuracy of electron dose calculations of Prowess Panther TPS is not enough in the internal mammary field. Therefore, it is commended that physicists should not rely on Prowess Panther TPS calculation for the electron beam.

Acknowledgment

This article is based on the data extracted from the M. Sc., desecration Code No. A-882 presented to the Medical Physics Department of Mashhad University of Medical Sciences. The authors would like to thank Reza Radiation Oncology Center for their sincere co-operation without which completion of this work was not easily possible.

Financial support and sponsorship

Mashhad University of Medical Sciences (Mashhad, Iran) has financially supported the work, and this is stated in the acknowledgment section of the article.

Conflicts of interest

There are no conflicts of interest.

 
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