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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 15  |  Issue : 3  |  Page : 544-549

Measurement of eye and lens doses in the presence and absence of shield during whole brain irradiation


1 Medical Physics and Medical Engineering Department, Faculty of Medicine; Radiation Oncology Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran
2 Iran Gamma Knife Center, Tehran, Iran
3 Medical Physics and Medical Engineering Department, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran

Date of Web Publication29-May-2019

Correspondence Address:
Dr. Ghazale Geraily
Medical Physics and Medical Engineering Department, Faculty of Medicine, Tehran University of Medical Sciences, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.191056

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 > Abstract 


Aim: The aim was to measure doses of eyes and lenses in the presence and absence of shield during whole brain irradiation (WBI). In addition, the dose calculations accuracy of radiation therapy dose plan treatment planning system (TPS) in dose calculation of the eyes and lenses in WBI was evaluated.
Materials and Methods: To measure the eyes and lenses doses during WBI, an inhomogeneous phantom of human head was used. Then, the eyes and lenses doses in the presence and absence of shield were measured by EBT3 film.
Results: In single fraction with 200 cGy to reference point, average doses received by the left and right eyes in the absence of shield were 20 ± 1.5 and 22 ± 1.0 cGy, respectively, and for the left and right eyes in the presence of shield were 18 ± 2.2 and 21 ± 2 cGy, respectively. In addition, the average dose received by the left and right lenses in the absence of shield were 19.5 ± 0.5 and 18.5 ± 0.5 cGy, respectively, and for the left and right lenses in the presence of shield were 20.5 ± 1.5 and 19.5 ± 1.5 cGy, respectively. The results showed the TPS compared to the film underestimates doses for the eyes and lenses.
Conclusion: The average dose received by the eyes and lenses during WBI were estimated around 9–11% of prescribed dose. According to the results, there is probability of radiation-induced cataractogenesis during WBI. By investigating the effect of shield on the lenses and eyes doses, using shield during WBI is not recommended. In addition, the results showed dose calculation accuracy of the TPS for the estimation of doses received by the eyes and lenses during WBI is not acceptable.

Keywords: Eye dose, lens dose, shield, treatment planning system, whole brain irradiation


How to cite this article:
Farhood B, Geraily G, Goodarzi AH, Zia A, Najafi M, Farahani S. Measurement of eye and lens doses in the presence and absence of shield during whole brain irradiation. J Can Res Ther 2019;15:544-9

How to cite this URL:
Farhood B, Geraily G, Goodarzi AH, Zia A, Najafi M, Farahani S. Measurement of eye and lens doses in the presence and absence of shield during whole brain irradiation. J Can Res Ther [serial online] 2019 [cited 2019 Aug 22];15:544-9. Available from: http://www.cancerjournal.net/text.asp?2019/15/3/544/191056




 > Introduction Top


According to estimates provided by Ferlay et al. in 2010, the incidence of primary central nerve system (CNS) tumors is as 3.2 and 3.9 per 100,000 person-year worldwide in females and males, respectively.[1] However, the incidence of primary CNS tumors is more in developed countries.[2]

Treatment modality for CNS tumors routinely is surgery for durable decompression and to obtain a tissue diagnosis. Patients who are medically inoperable, have unresectable and/or multiple lesions or refuse surgery mostly receive whole brain irradiation (WBI) for palliation of their symptoms.[3] WBI is effectively applied in the treatment of several brain tumors, including germ cell tumors, medulloblastoma,[4],[5] primitive neuroectodermal tumors, and ependymoma.[6] It is also used for prophylactic purposes in patients with small cell lung cancer and acute lymphoblastic leukemia.[7],[8] In addition, WBI was used for the treatment of brain metastasis.[9]

There are different techniques that can be used for WBI. Usually, patients are treated with two opposed lateral fields. A clinical setup for WBI is simple collimation technique that includes “a rectangular treatment field over the cranial vault with the collimator angled so as to place the inferior border 1–2 cm below a line drawn from the medial canthus to the mastoid tips.” The anterior, superior, and posterior borders are simply adjusted with a light border flashing over the skull.[3]

With regard to the sensitive organs such as eyes and lenses around the field of whole brain, the chance of reaching dose to the eyes and lenses is due to the beam divergence and scattered radiation. In the human body, one of the most radiosensitive tissues is the lens of the eye.[10],[11],[12] The only actual pathology of the lens is opacity called cataract.[13] Several studies on radiation-induced cataract showed an average latency period for the development of cataract of 2–3 years that it depends on the dose to the eye.[14],[15] It should also be noted that in recent years, cataract is considered as a deterministic effect not stochastic,[16] so dose to the lens of the eye should be reduced as much as possible.

There are several studies that have measured the dose received by the eye and the lens as well as evaluated impact shield on dose reaching to the lens and eye.[17],[18],[19],[20]

Patil et al.[18] evaluated the eyes and lenses doses and coverage dose of planning target volume (PTV) in WBI. They concluded in the absence of volumetric planning techniques, French Society of Pediatric Oncology (SFOP) guidelines lead to insufficient coverage and the in-house method is recommended. Although the dose to the lens in conformal and in-house plans was more than the tolerance limits but with the advancement in cataract surgery, the fear of the complication should not cause inadequate coverage of PTV. Andic et al.[17] compared the dosimetric data from conventional two-dimensional (2D) helmet-field with those from 3D conformal radiotherapy (3D-CRT) in WBI. They concluded in comparison with conventional 2D helmet-field planning, 3D-CRT planning remarkably improved the dose homogeneity during WBI, and the dose coverage of retro-orbital (RO) areas while protecting ocular lenses.

To the best of our knowledge, there is no measurement of the eye and the lens doses in the presence and absence of shield during radiotherapy of whole brain field (the simple collimation technique of the above mentioned). However, there are several studies in the field of measurement of the eyes and doses in helmet and collimation multi leaf colimator (MLC)- based technique during radiotherapy of whole brain field. Thus, in this research, we measured the eyes and lenses doses in the presence and absence of shield during radiotherapy of whole brain field and whether the shield reduces the eyes and lenses dose or not? Other aim of this study was to assess the dose calculations accuracy of radiation therapy (RT) dose plan treatment planning system (TPS) in dose calculation of the eyes and lenses in WBI.


 > Materials and Methods Top


To measure the eyes and lenses doses during WBI, an inhomogeneous phantom of human head with three types of heterogeneity as bone, soft tissue, and air was used. [Figure 1] shows the inhomogeneous phantom of human head. Then treatment planning was carried out for the phantom, and it was irradiated. Finally, the eyes and lenses doses in the presence and absence of the shield were measured by EBT3 film. In addition to evaluating the dose calculations accuracy of RT dose plan, TPS doses measured by the film were compared with doses calculated by the TPS.
Figure 1: Head phantom made to measure the eye and the lens doses

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Treatment planning and irradiation of the head phantom

A computed tomography scan of the head phantom was taken to produce a treatment plan. The images were transported to RT dose plan TPS. [Figure 2] shows the head phantom plotted by the TPS.
Figure 2: Head phantom plotted by treatment planning system

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In this study, two lateral parallel opposed fields were planned. Gantry angles were set to 275° and 85° so that the removing the beam divergence was considered for reducing the dose reaching to the eyes and lenses. Furthermore, collimator angles were set to 315° and 45°. A source axis distance technique was applied to deliver 200 cGy dose to the selected point (the center of the head phantom). A rectangular treatment field over the cranial vault with the collimator angled was used so that the inferior border was placed 1–2 cm below a line drawn from the medial canthus to the mastoid tips. The anterior, superior, and posterior borders are simply adjusted with a light border flashing over the skull. In addition, to assess the impact shield on the eyes and lenses dose, a lead shield was designed in accordance with the treatment plan. The entire process of setup and irradiation were repeated for this technique similar to the technique including no shield.

Before irradiation, the piece films of EBT3 Gafchromic were placed on the surface of the left and right eyes and also left and right lenses of the head phantom. The head phantom was irradiated based on the treatment plan with 6 MV X-rays produced by a Siemens Primus Accelerator (Siemens AG, Erlangen, Germany). Finally, doses received by the film pieces were measured. In addition, measured doses by the film compared with doses calculated by the TPS. It is noteworthy that 5 mm depth from the surface of eye was considered to measure lenses dose.

Calibration of applied dosimeters and dosimetric method

In this study, EBT3 Gafchromic film (ISP, Wayne, NJ, USA) was used for dose measurement of the eyes and lenses. The film is made of an active layer of 30 μm in the center and two polyester layers of 125 μm on the sides of the active layer. Due to the symmetry of the film, there is not any front or back side for the film, and both sides have similar scanning light conditions.[21] To calibrate the film, several pieces of the film 5 cm × 5 cm were cut from a single film sheet. To avoid any mistake in set up of the films orientation in the scanning process, top right of the film pieces was marked. For measurement, the background optical density (OD) (pixel value before irradiation), these films were scanned by a Microtek ScanMaker 9800XL (Microtek Inc. Santa Fe Spring, CA) before irradiation. The films were scanned in transmission mode with a resolution of 150 dpi. Images obtained from scanning were saved nonzipped with tagged image file format. Each piece of film, three times were scanned to reduce the noise effects of the scanner. This scanner uses Microtek ScanWizard Pro software (version V7.041, Microtek Inc. Santa Fe Spring, CA) for scanning. In addition, ImageJ software (National Institutes of Health, Bethesda, Maryland) software was used for analyzing and obtaining the pixel values in each film. After scanning, each film was placed in the center of 10 cm × 10 cm radiation field on a 30 cm × 30 cm × 20 cm water equivalent phantom. A 15 mm water equivalent slab was placed on the film for to create buildup condition. The films were irradiated with 6 MV X-rays produced by a Siemens Primus accelerator (Siemens AG, Erlangen, Germany) in radiotherapy center of Imam Hospital (Tehran, Iran) with 0–40 Gy doses at regular intervals. Forty-eight hours after irradiation, all film pieces were scanned by the Microtek 9800XL scanner. Net optical density (NOD) was calculated according to the following formula given by Eq. 1:

NOD = ODCal − ODBack= −(log10 [PCal] − log10 [PBack]) (1)

where ODCal is the calibration OD, ODBack is background OD, PBack is background pixel value, and PCal is calibration pixel value. The average pixel value related to each scan was calculated in the red channel by using MATLAB (version 7.11.0.584, The MathWorks Inc., Natick, MA, USA) software so that in the range of 0–10 Gy, this channel has the highest sensitivity to dose. Pixels related to near the edges were excluded, and the central film part was used in the calculation of the average pixel value. For each piece of film, the average pixel value was calculated from three separate scans. Dose value (Gy) versus NOD was plotted and to fit power and exponential functions of EBT3 films; MATLAB software was used.

For high accuracy of dosimetry results, measuring the doses were repeated three times and average doses received by the eyes and lenses were obtained by three times radiation.

Analysis of results

For analysis of the results, TRS 430[22] and TECDOC 1540[23] protocols were applied. These protocols provide information on quality assurance (QA) of TPSs. According to these protocols, the difference between the calculated and the measured dose is defined based on Formula 2:

δ (%) =100 × (DcalDmeas)/Dmeas (2)

where Dmeas and Dcalc are the measured dose by the film and the calculated dose by the TPS, respectively.


 > Results Top


Calibration of EBT3 Gafchromic film

[Figure 3] shows the calibration curve of EBT3 Gafchromic film.
Figure 3: Calibration curve of EBT3 Gafchromic film

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The measured dose values (Gy) can be calculated through measured NOD values in red channel by following Formula 3:

D = 6.584 × OD + 201.2 × OD 3.4 R2 = 0.996 (3)

The eye and lens doses in the presence and absence of shield

It should be noted that the doses given in [Table 1] and [Table 2] are in relation to a single fraction with 200 cGy to reference point. The average dose of the surface of the left and right eyes in the presence and absence of shield during WBI is presented in [Table 1].
Table 1: The average dose of surface of the left and right eyes in the presence and absence of shield during whole brain irradiation

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Table 2: The average dose of the left and right lenses in presence and absence of shield during whole brain irradiation

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The average dose of the left and right lenses in the presence and absence of shield during WBI is presented in [Table 2].

Comparison between measured and calculated doses

In this study, measured doses by EBT3 film were compared with calculated dose by the TPS. [Table 3] shows the average measured dose and the calculated dose of the surface of the left and right eyes in the presence and absence of shield during WBI.
Table 3: The average measured dose and the calculated dose of surface of the left and right eyes in the presence and absence of shield during whole brain irradiation

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[Table 4] shows the average measured dose and the calculated dose of the left and right lenses in the presence and absence of shield during WBI.
Table 4: The average measured dose and the calculated dose of the left and right lenses in the presence and absence of shield during whole brain irradiation

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 > Discussion Top


In this study, we measured the eyes and lenses doses during WBI. In addition, effect of shield on the received dose by eyes and the lenses was evaluated. In addition, the accuracy of RT dose plan TPS in the dose calculation of the eyes and lenses during WBI was investigated.

Our results show that average dose received by the eyes and lenses during WBI for single fraction with 200 cGy were 18–22 cGy which were around 9–11% of prescribed dose. Since total prescribed dose in WBI is about 3000–6000 cGy;[24],[25],[26] therefore, there is likely to reach around 300–600 cGy (about 10% of prescribed dose) to the eyes and lenses. In recent years, the International Commission on Radiological Protection published a new declaration that drastically reduced the threshold values for radiation-induced cataractogenesis to 500 mGy from the previous values of 2000–8000 cGy.[16] It should also be noted that in recent years, cataract considered as deterministic effect not stochastic,[16] so dose to the lens of eye should be reduced as much as possible. As a result of this study, there is a probability of radiation-induced cataractogenesis. Our results related to effect of shield on the lens and the eye doses showed that it reduced negligibly doses of the surface of the eye and increased the negligibly dose of the lens that it can be due to beam hardening of the shield. Hence, using the shield during WBI for reducing dose of the lens is not recommended. Gondi et al.[20] reported the eye and lens doses during WBI using intensity-modulated radiotherapy (IMRT) and helical tomotherapy. Their results showed the mean eye dose in IMRT, and helical tomotherapy was 5.2 Gy and the mean lens dose in IMRT, and helical tomotherapy were 3.8 and 3.4 Gy, respectively, which is in good agreement with our results. Yavas et al.[19] evaluated two different radiotherapy techniques (helmet-field and collimation MLC-based technique) with respect to the doses reaching to the organs at risk during WBI in patients with brain metastasis. Their results showed the mean right lens dose in helmet and collimation MLC-based techniques were 286 and 251 cGy, respectively. In addition, the mean left lens dose in helmet and collimation MLC-based techniques were 268 and 250 cGy, respectively. They concluded collimation MLC-based techniques be beneficial, due to the lens doses as well as the dose received by the eye-ball. Our results were in accordance with their results. Also in relation to reducing the lens dose, Andic et al.[17] showed 3D-CRT planning in comparison with conventional 2D helmet-field planning, remarkably improved the dose homogeneity during WBI, and the dose coverage of RO areas while protecting ocular lenses.

As it is evident from [Table 1] and [Table 2], dose, different dose values for the right and left lenses as well as for right and left eyes is different that it may be a due to symmetric geometry of the head phantom.

In relation to investigating the accuracy of RT dose plan TPS in dose calculation of the eyes and lenses, the results showed the TPS underestimates doses for the eyes and lenses. Therefore, underestimation of the dose received by a radiosensitive organ such as the lens using the TPS-calculated dose leads to underestimation in the probability of radiation-induced cataractogenesis. In addition, the average dose differences (%) between the measured doses by the film and the calculated doses by TPS (δ) were 68.93. According to TRS 430[22] and TECDOC 1540[23] protocols that provide information on QA of TPSs, tolerance limit for inhomogeneous field (i.e., complex geometry) in outside of the field (i.e., low dose-small dose gradient) is 40. The results of this study showed that the dose calculation accuracy of the RT dose plan TPS to estimate doses of the eyes and lenses is insufficient. Our results were in agreement with other studies [27],[28],[29] which show the accuracy of TPSs for outside field region is not enough.

As a subject for future study, measuring the eye and lens doses during WBI using Monte Carlo simulation will be interesting.


 > Conclusion Top


In this study, doses received by the eyes and lenses in around 9–11% of prescribed dose were estimated; therefore, there is likely to reach around 300–600 cGy (about 10% of prescribed dose) to the lenses and eyes which increase probability of radiation-induced cataractogenesis. By investigating the effect of shield on the lens and the eye doses, the results showed that it reduced negligibly doses of surface of the eyes and increased negligibly dose of the lenses. In addition, the results showed the TPS underestimates doses for the eyes and lenses, and its dose calculation accuracy is not acceptable.

Financial support and sponsorship

Tehran University of Medical Sciences (Tehran, Iran) has financially supported the work. Aso, this article is based on the data extracted from research work with code no. 95.02.207.32505.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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