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ORIGINAL ARTICLE
Year : 2018  |  Volume : 14  |  Issue : 6  |  Page : 1245-1250

Evaluating the effect of the vacuum bag on the dose distribution in radiation therapy


Department of Medical Physics, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran

Date of Web Publication28-Nov-2018

Correspondence Address:
Tinoosh Almasi
Department of Medical Physics, School of Medicine, Isfahan University of Medical Sciences, Isfahan
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.188431

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 > Abstract 


Introduction: Immobilization of patients in radiation therapy can be performed with a vacuum bag (VB). The aim of this study is to measure the effect of the VB in the surface and depth dose of patients in radiation therapy.
Materials and Methods: The effect of the VB on the surface dose and depth dose is measured in clinical conditions. Various dosimeters were used in following measurements: parallel plate chamber for depth dose, farmer ionization chamber for various gantry angles, and Mapcheck2 dosimeter for various thicknesses of VB. The effect of VB cap, which may be placed in the beam path, is also measured using EDR2 film. The measurements were performed for 6 MV and 18 MV photons with an Oncor linac.
Results: The increase of 30% and 25% in the surface dose with VB was observed for 6 MV and 18 MV, respectively. Though due to the use of VB, the reduction of the absorbed dose at a 5 cm depth is under 1% and can be ignored in MU calculation. For various thicknesses of VB, 8–14 cm, the attenuation of the primary beam were up to 2.5% for 6 MV and 1.2% for 18 MV photon. The presence of VB cap in the path of radiation reduced the depth dose up to 15% and 11% for 6 MV and 18 MV, respectively.
Conclusion: The use of VB can increase the surface dose of the patient up to 30% and this fact should be considered in treatment planning. For some lateral fields the cap of the VB may interfere with radiation field. If the cap of VB is placed in the beam path, it can cause a cold spot in tumor.
Discussion: The use of VB can increase the surface dose of the patient up to 30%. For some lateral fields, the cap of the VB might interfere with the radiation field. If the cap of VB is placed in the beam path, it can cause a cold spot in the tumor volume.

Keywords: Dosimetry, immobilization, radiation therapy, vacuum bag


How to cite this article:
Jabbari K, Almasi T, Rostampour N, Tavakoli MB, Amouheidari A. Evaluating the effect of the vacuum bag on the dose distribution in radiation therapy. J Can Res Ther 2018;14:1245-50

How to cite this URL:
Jabbari K, Almasi T, Rostampour N, Tavakoli MB, Amouheidari A. Evaluating the effect of the vacuum bag on the dose distribution in radiation therapy. J Can Res Ther [serial online] 2018 [cited 2019 Sep 21];14:1245-50. Available from: http://www.cancerjournal.net/text.asp?2018/14/6/1245/188431




 > Introduction Top


Most of the radiation therapy treatments require immobilization of patients during the treatment to maintain a fixed position throughout the course of treatment.[1],[2] There are several techniques for the fixation of the patient during a radiation therapy session. Currently, a most common technique for the patient fixation is the use of thermoplastic masks. However, for some cases, such as patients with surface lesions, unstable patients, and patients who do not cooperate during treatment, such as children and very old patients, and patients who require sedation or general anesthesia, the use of vacuum bag (VB) can be very useful.[1],[3],[4]

VBs are in the form of a thick uniform bag which is used instead of thermoplastic masks. VB is initially open to the air, it is placed beneath the patient, and it is formed around patient's body. Then, it gets a rigid form with a connection to a vacuum motor through a vacuum cap. The VB application at the time of treatment can have an influence on dose distribution and physical parameters of the radiation field.

Many studies about the accuracy and repeatability of patients setting during treatment have been reported.[5],[6],[7],[8],[9],[10],[11] and some studies have shown treatment improvements resulting from the use of these positioning devices.[5],[12],[13],[14]

Graham et al. performed a comparison between two methods to fix armrest and VB in order to immobilize breast cancer patients during radiotherapy. They concluded that in patients who used VB, the radiation dose to the lung was lower than who used armrest.[15] In another study, Cheung et al. evaluated the buildup dose from 6 MV X-rays under pelvic and abdominal patient immobilization devices. They showed that there was an enhancement in the dose delivered to the skin by interactions of the X-rays within the VB material. They concluded that the basal layer doses increased from 16% of the maximum dose for an open field up to 52% with a bag thickness of 2.5 cm for a 10 cm × 10 cm field at 6 MV energy.[16]

The motivation of this project was the observation of uncommon burns on the patients back who used VB in a radiotherapy center. Since in these cases, the VB was in the beam path; one of the issues that evaluated in this project was to determine the precise amount of change of skin dose during the use of VB. In this study, the dosimetric effect of VB in various angles, thicknesses, and depths are studied for 6 MV and 18 MV photon beam energies.


 > Materials and Methods Top


This study was performed with an Oncor linac from Siemens for 6 and 18 MV photons. [Figure 1] illustrates the FQZ 58A model of VB (from ORTIF) with dimensions of 85 cm × 100 cm. The PTW farmer and parallel palate ion chambers, EDR2 film, PTW uniDos electrometer, and REF 1177MapCheck were used for dosimetry of various parameters.
Figure 1: FQZ 58A model of vacuum bag with dimensions of 85 cm × 100 cm which was used in the study. The vacuum cap of the bag is in the right corner of the figure, and it is open before connection to vacuum motor

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In the first step, the effect of VB on percentage depth dose (PDD) was evaluated. This measurement was performed in several stages using slabs of RW3 phantoms containing multiple layers. To evaluate the effect of VB on depth dose, the parallel plate Roos chamber was inserted in a special RW3 slab designed to fit the Roos chamber. The irradiation was performed for 6 MV and 18 MV energies in the presence and absence of the VB. The depth of measurement was changed from 1 mm to 10 cm using RW3slabs with various thicknesses. For different depths, the slab containing dosimeter was moved between the slices of the phantom. For each depth, 100 MU was delivered. The field size for this experiment was 10 × 10 cm2 and source-to-surfaces distance was (SSDs) =100 cm. It should be noted that such an experiment is impossible to be performed in a water phantom since it is almost impossible to place and hold a VB on the water surface.

In the second step, a uniform VB was placed on the couch and it was connected to a vacuum pump to get a relatively uniform rigid shape. In this case, the entire surface of the VB has a uniform thickness. To evaluate the effect of gantry angle on the absorbed dose in the presence of VB, the irradiation was performed in the gantry angles of 0, 20, 40, 80, 340, 320, 300, and 280°. The field size in this experiment for 0° gantry was 10 cm × 10 cm and SSD = 100 cm. For 6 MV and 18 MV energies, 1.5 and 3 cm of RW3 slabs were used, respectively, as a build-up on the slab containing the farmer ion chamber. All the measurements were repeated with and without VB. The total thickness of RW3 phantom was 30 cm to have a full scattering condition.

To evaluate the effect of various VB thicknesses on surface dose, the MapCheck2 dosimeter manufactured by Sun Nuclear was used.[17],[18],[19] This device is mainly used for quality assurance in intensity modulated radiation therapy; however, it can be used for the evaluation of the planar dose with high accuracy.

The advantage of this device for two-dimensional dosimetry over the film dosimetry is that it directly uses small detectors for dose measurement. This device contains 1040 detectors at 5 mm distances from each other in a grid. Since the MapCheck2 would achieve the surface dose (two-dimensional) distribution directly via an array of detectors, its results are very accurate.[20],[21],[22] For this work, VB was shaped like a steep slope surface [Figure 2] and connected to a vacuum pump. Then, the VB was placed on the surface of MapCheck2 and exposed by 100 MU with the 30 cm × 30 cm field size (the size that covers the entire MapCheck2 surface) at SSD = 100 cm. The minimum and maximum thickness of inclined VB was 8 cm and 16 cm, respectively. This experiment evaluates the dosimetric effect of various thicknesses of the VB in contact with the patient. All the process was also repeated in the absence of VB. All the measured data from the detectors are saved with related software of the Mapcheck2. It should be noted that the MapCheck has 2 cm buildup in itself; therefore, it is not possible to measure the surface dose with this device.
Figure 2: The vacuum bag with various thicknesses from 8 to 16 cm to evaluate the effect of it on absorbed dose. The MapCheck was placed under the vacuum bag

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There is a plastic cap on the corner of VB which is connected to a vacuum pump, and it may be placed in the beam path, especially in lateral fields. This case was observed for few patients in the clinic during the regular check-up of the treatment setup, for example, in pelvis case when the VB is placed under lower part of the body for irradiation of the pelvis and four field box. Therefore, the effect of the cap on depth dose was measured. For this purpose, the area of VB containing the cap was irradiated for dosimetry. An RW3 phantom with 5 cm thickness and an EDR2 film were placed under the cap of VB as it is illustrated in [Figure 3].
Figure 3: How to place the cap on the beam path

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This assembly was irradiated with 100 MU and 20 cm × 13 cm field size in SSD = 100 cm. In this experiment, the film was calibrated according to the film dosimetry protocol by Pai et al.[23] The film was developed and scanned with a special film scanner (Microtech). The absorbed dose in all points of the film was calculated using the calibration curve.


 > Results Top


The effect of the VB on the depth dose for 6 MV and 18 MV photons is illustrated in [Figure 4] and [Figure 5], respectively. Figures illustrate the percentage depth dose in the water like a phantom in two cases. In one case, the VB is placed on the phantom surface and in another case, the PDD is related to uniform phantom without the VB.
Figure 4: Effect of vacuum bag on percentage depth dose for beam energy of 6 MV in the presence and absence of vacuum bag

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Figure 5: Effect of vacuum bag on percentage depth dose for beam energy of 18 MV in the presence and absence of vacuum bag

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[Figure 6] and [Figure 7] also illustrate the effect of VB on absorbed dose in various gantry angles. As illustrated in figures, the angle of the gantry is changed between 80 and 280° with respect to vertical position of the gantry. It should be noted that in 90 and 279° the central axis of the beam is parallel to the surface of the VB.
Figure 6: Effect of vacuum bag on absorbed dose in various gantry angles for 6 MV beam energy in the presence and absence of vacuum bag. The parameter M/Mmax is the ratio of readings from the cylindrical ionization chamber

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Figure 7: Effect of vacuum bag on absorbed dose in various gantry angles for 18 MV beam energy in the presence and absence of vacuum bag. The parameter M/Mmax is the ratio of readings of the detector

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[Figure 8] and [Figure 9] illustrate the effect of VB on absorbed dose in various thicknesses of VB for 6 MV and 18 MV photons, respectively. D1 and D2 are related to the absorbed dose in the presence and absence of the VB, respectively. The horizontal axis shows the thickness of VB, which is irradiated by the photons. The measured data were obtained by MapCheck2 dosimeter. This device mainly illustrated the isodose distribution in various formats. However, the reading for the array of detectors in the MapCheck device is also available in a simple word document. As it is mentioned, the results were imported to an in-house software MATLAB R2014a (Mathworks, Massachusetts, Natick, USA).
Figure 8: Effect of vacuum bag on absorbed dose in various thicknesses for 6 MV photon. D1 and D2 are related to the absorbed dose in the presence and absence of the vacuum bag, respectively. X (cm) is related to the thickness of vacuum bag

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Figure 9: Effect of vacuum bag on absorbed dose in various thicknesses for 18 MV photon. D1 and D2 are related to the absorbed dose in the presence and absence of the vacuum bag, respectively. X (cm) is related to the thickness of vacuum bag

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The effect of VB cap on absorbed dose is illustrated in [Figure 10], [Figure 11], [Figure 12], [Figure 13] for 6 MV and 18 MV, respectively. The areas related to dose reduction due to the presence of the cap are marked in these figures. These figures were plotted with a MATLAB software for film dosimetry after determination of the absorbed dose for all points of the irradiated film. This software according to the calibration curve of the film converts the optical density of each point to an absorbed dose. After calculation of the dose for all points, the strong image processing tool box of the MATLAB is used to generate the isodose curves and three-dimensional illustration of the surface dose.
Figure 10: (a) The results of the film dosimetry for the case in which the cap of the vacuum bag is placed in the 6 MV radiation beam. The profile of the along the dashed line in figure (a) is plotted in figure (b)

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Figure 11: (a) The isodose curves of the film in Figure 10 illustrating the effect of the vacuum bag cap on absorbed dose for 6 MV beam energy, (b) the color map of dose distribution for effect of the vacuum bag cap on absorbed dose

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Figure 12: (a) The results of the film dosimetry for the case in which the cap of the vacuum bag is placed in the 18 MV radiation beam. The profile of the along the dashed line in figure (a) is plotted in figure (b)

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Figure 13: (a) The isodose curves of the film in Figure 12 illustrating the effect of the vacuum bag cap on absorbed dose for 18 MV beam energy, (b) the color map of dose distribution for effect of the vacuum bag cap on absorbed dose

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 > Discussion Top


One of the main factors affecting the accuracy of treated volume is the careful positioning of patient. As [Figure 4] and [Figure 5] illustrate, the effect of VB on PDDs is considerable in the first few centimeters of the phantom for both 6 MV and 18 MV energies. As illustrated, in first few millimeters of the phantom, the presence of VB increases the surface dose significantly. The relative increases in surface dose due to the use of VB were up to 30% and 25% for 6 MV and 18 MV energies, respectively. After the depth of 2 cm approximately, no significant differences were observed in the PDD curves. These differences of PDDs are decreased in large depths. In practice, this increase in surface dose is the reason of unusual burn on the back of legs of patients who used VB in PA field. The VB in posterior and lateral fields acts like a bolus.

Another important point is the amount of attenuation at various depths in a patient's body, for example, the depth of 5 cm. This factor has an influence on calculations of monitor units. In these experiments, the amount of attenuation, which is used correction factor, were 0.991 and 0.992 for the 6 MV and 18 MV, respectively. According to these numbers, the amount of depth dose attenuation was <:1% in each energy. Therefore, this factor can be ignored for MU calculation for the depth dose as it is the case for most of the clinical cases.

As it is illustrated in [Figure 6] and [Figure 7], the increase of gantry angle causes the decrease of depth dose in the presence of the VB. In various angles, the maximum difference of 4.9% between VB and no VB cases was observed for 6 MV. The amount of difference for 18 MV was 1.31%. This is due to the increase of VB thickness in the path of oblique beams which has a less amount for 18 MV because it has more penetrating power compared with 6 MV beams.

According to [Figure 8] and [Figure 9], one concludes that for various thicknesses of VB, 8–14 cm, it can attenuate the primary photon beam from 0.40% to 2.5% for 6 MV photon. It is unlikely to have 14 cm of the VB thickness on the real clinical usage, however just in case, 2.5% of the attenuation is not negligible. The magnitude of attenuation for 18 MV is 0.419% to 1.23% for 18 MV photon. This is due to the relatively less attenuation of high-energy photons of 18 MV.

The effect of the VB cap on the absorbed dose for 6 MV and 18 MV is illustrated in [Figure 10], [Figure 11], [Figure 12], [Figure 13]. The figures are the results of the film dosimetry which was placed under the VB cap at 5 cm depth to illustrate the impact of the cap on the depth dose. In this case, a cold spot up to %15 for 6 MV and %11 for 18 MV can happen in the treated volume. According to the results, the effect of VB cap is not negligible, and it can cause a considerable reduction of the dose inside the tumor. Therefore, in treatment planning, one should avoid to place the cap within the irradiated area. It usually occurs in the left and right lateral fields. Thus, the treatment fields should be designed in such a way that VB cap should be outside the field. Another possible solution is that at the time of the preparation of the VB around the patient, the cap should be far from the irradiated area.


 > Conclusion Top


This project evaluated the various aspects of dosimetry in the presence of the VB for fixation of the patient in radiation therapy. First of all, it is concluded that in the normal use of the VB and large depths, the effect of the VB in the reduction of the depth dose is <:1% for various energies and it can be neglected in the correction of the physics calculation. However, the effect of the VB in increasing the surface dose in quite considerable. The surface dose is increased up to 30%, and this was the reason for unusual burns of the patients in the back of the leg in contact with VB. All these patients had a posterior-anterior radiation field in which the VB was in the path of the radiation beam. Another important finding of this project was the effect of the cap of the VB. This is the cap through which the VB is connected to the vacuum pump. In some lateral fields, this cap can be in the path of the radiation. It is found that in this case one can have a cold spot in the deep-seated tumors. The simple solution for this problem is the movement of the patient in the VB before the connection of the vacuum pump.

Financial support and sponsorship

This paper was fully supported by a grant from the Isfahan University of Medical Sciences, Isfahan, Iran (Project No. 392404).

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]



 

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