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Inter-fractional entrance dose monitoring as quality assurance using Gafchromic EBT3 film

1 Department of Bio-convergence Engineering, Korea University, Seoul; Proton Therapy Center, National Cancer Center, Goyang, Korea
2 Department of Bio-convergence Engineering, Korea University, Seoul, Korea
3 Proton Therapy Center, National Cancer Center, Goyang, Korea

Date of Submission02-Jan-2020
Date of Decision27-May-2020
Date of Acceptance16-Jul-2020
Date of Web Publication24-Jul-2021

Correspondence Address:
Dongho Shin,
Ilsan-Ro 323, Ilsandong-gu, Goyang-si
Myonggeun Yoon,
Anam-Ro 145, Seongbuk-Gu, Seoul
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcrt.JCRT_8_20

 > Abstract 

Introduction: This study describes a simple method of inter-fractional photon beam monitoring to measure the entrance dose of radiation treatment using Gafchromic EBT3 film.
Materials and Methods: The film was placed at the center of a 1-cm thick phantom shaped like a block tray and fixed on the accessory tray of the gantry. The entrance dose was measured following the placement of the film in the accessory tray. The dose distribution calculated with the treatment planning system was compared with the dose distribution on the irradiated EBT3 films. The effectiveness of this methodology, as determined by gamma passing rates, was evaluated for the 22 fields of eight three-dimensional conformal radiotherapy (3D-CRT) plans and the 41 fields of nine intensity-modulated radiotherapy (RT) plans. The plans for three-dimensional conformal RT included treatments of the rectum, liver, breast, and head and neck, whereas the plans for intensity-modulated RT included treatments of the liver, brain, and lung.
Results: The gamma passing rates for 3D-CRT ranged from 96.4% to 99.5%, with the mean gamma passing rate for 22 fields being 98.0%. The gamma passing rate for intensity-modulated RT ranged from 96.1% to 98.9%, with the mean gamma passing rate for 41 fields being 97.7%. All gamma indices were over the 95% tolerance level.
Conclusions: The methodology described in this study, based on Gafchromic EBT3 film, can be utilized for inter-fractional entrance dose monitoring as quality assurance during RT. Clinical application of this method to patients can verify the accuracy of beam delivery in the treatment room.

Keywords: Gafchromic EBT3 film, intensity-modulated radiotherapy, inter-fractional dosimetry, three-dimensional conformal radiotherapy, treatment quality assurance

How to cite this URL:
Moon SY, Jo Y, Seo J, Shin D, Yoon M. Inter-fractional entrance dose monitoring as quality assurance using Gafchromic EBT3 film. J Can Res Ther [Epub ahead of print] [cited 2021 Dec 5]. Available from: https://www.cancerjournal.net/preprintarticle.asp?id=322272

 > Introduction Top

Radiotherapy (RT) aims to accurately deliver prescribed doses of radiation to complex-shaped tumors while minimizing doses delivered to organs at risk. Although current techniques utilize sophisticated dose delivery sequences, the risk of errors leading to serious consequences for patients is not negligible. The New York Times reported in 2010 that a computer for a treatment planning system saved plans for tongue cancer patients with multileaf collimators (MLC) inaccurately positioned, an error not detected by medical staff, an error not detected by medical staff. This resulted in overdoses of many fractions, with the patient dying several weeks later.[1] Furthermore, there are many errors in radiation delivery caused to machine error such as field parameters or subfield incorrect or omitted, MLC malfunction, and so on.[2],[3],[4],[5] Random errors can occur, such as a missing MLC dynalog file caused by transfer error. Before the treatment, machine quality assurance (QA) and patient-specific QA are performed to determine the status of the machine and to verify the treatment plan. Despite multi-layered QA procedures, however, many dosimetric accidents have occurred, suggesting flaws in conventional QA procedures.[6],[7] Furthermore, the overall process of RT is complex, requiring understanding of the principles of medical physics, radiobiology, radiation safety, dosimetry, RT planning, simulation, and interaction of RT with other treatment modalities.[8] Although monitor chambers within the linear accelerator (LINAC) can assess dosimetric characteristics, including dose, dose rate, and field symmetry, these chambers may function incorrectly or cease to function. The occurrence of many adverse events not caught by a series of QA procedures suggests that an independent QA methodology may be needed to prevent serious accidents.

Several methods are available to verify each fraction, including in vivo measures of dosimetry at specific points using detectors such as thermoluminescence detectors, metal–oxide–semiconductor field-effect transistors,[9] diodes,[10] electronic portal imaging devices (EPID),[9],[11] and transmission detectors. Except for two-dimensional (2D) systems, however, these detectors require the application of correction factors, depending on field size, target to surface distance, and the use of accessories such as wedges.[10],[12],[13],[14],[15] Since EPIDs measure transit dose passing through the patient, these devices are affected by patient position, movements, and anatomical changes. Transmission detectors positioned between the LINAC gantry and the patient, along with several measurement strategies, are therefore required. Commercially available systems include the DAVID system (PTW-Freiburg, Freiburg, Germany),[16] the Dolphin detector (IBA Dosimetry, Uppsala, Sweden),[17] and an integral quality monitoring (IQM) system (iRT Systems, Koblenz, Germany),[18] which is an ionization chamber. These systems have intrinsic limited resolution due to the characteristics of their design: limited number of wires and plane-parallel ionization chambers. Furthermore, IQM has a limitation that spatial sensitivity on the detector works only in one direction, which is the direction of MLC movement.[19] Radiation signals in these systems are displayed by digitized electronics, located on boards close to an active exposure area. Measurement errors during this process may be due to the leakage of current caused by interference. Another drawback to these systems is beam interference. These devices are inserted into the beam path, increasing the surface dose for large field sizes and small solid-state drives (SSDs). Thus, in treating obese or other selected patients, it may be prudent to remove these devices or modify treatment plans to minimize surface doses.[20],[21] Furthermore, it is cumbersome to measure transmission factors several times during treatment, depending on energy and field size, as these measurements consume both time and labor, as well as being costly.

Gafchromic EBT3 film (International Specialty Products, Wayne, New Jersey, USA) is a commercially available 2D dosimeter, with a response nearly independent of radiation energy and dose rate used in treatment. This film also shows high spatial resolution allowing very precise field measurements to millimeter levels.[22],[23] It is used in evaluating beam profiles and symmetry for RT[24],[25] and the QA for stereotactic radiosurgery and stereotactic body RT.[26],[27],[28] Furthermore, 2D dose distribution has been confirmed for patient-specific QA.[23],[29],[30] The present study therefore assessed the ability of inter-fractional photon beam monitoring to evaluate the entrance dose distribution under the gantry using Gafchromic EBT3 film.

 > Materials and Methods Top

System configuration

To evaluate the accuracy of an irradiated beam, the entrance dose distribution of each plan was measured by inter-fractional monitoring using Gafchromic EBT3 film. The film was placed at the center of a 1-cm thick phantom shaped like a block tray and fixed on the accessory tray of the gantry. A 5-mm thick phantom was placed above the film for buildup and a second 5-mm thick phantom was placed below the film to allow for the effects of back scattering [Figure 1].
Figure 1: System configuration

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Film calibration

The films were cut to several pieces of 5 × 5 size and the orientation on each film was marked. The pieces were irradiated on a Varian Trilogy under condition of 10, 25, 50, 75, 100, 150, 200, 250, 300, 350, 400, 450, and 500 cGy. The films were placed horizontally inside a solid water (Gammex RMI, Middleton, WI, USA) slab phantom of 30 × 30 size and placed on the central axis at 10 cm depth. The irradiated film pieces were scanned after 24 h post exposure using a commercial flatbed scanner EPSON Expression 10000XL. Then, a calibration curve is calculated through the optical density value of the film relative to the dose.


To test the reproducibility of Gafchromic EBT3 film in the linac tray, one field was exposed to three overlapped films together. All three films were exposed by the same beam fluence and compared using gamma index (GI) analysis. A comparison of the three films would, therefore, indicate the degree of the inherent uncertainty in EBT3-film-based dosimetry.

Then, the film under the gantry measured the dose distribution in air of the photon beam used to treat patients and a feasibility test was performed. To verify the treatment by measuring the full plan using one film, the film size should cover all field size within the plan. Moreover, if the film is used by cutting according to the individual patient's field size, the setup is cumbersome to match the center of field and film placed on the phantom shaped like a block tray. Furthermore, there may be measurement error resulted from damage of the cutting part. Hence, in this study, the film was used without a cut (i.e., original size) for all patients to simplify the procedure and improve accuracy. Eight treatment plans for 3D conformal radiotherapy (3D-CRT) and nine plans for intensity-modulated radiotherapy (IMRT), all of which had been clinically approved to treat patients, were randomly selected. The plans for 3D-CRT included treatments of the rectum, liver, breast, and head and neck cancers, whereas the plans for IMRT included treatments of the liver, brain, and lung cancers. To assess the 2D dose distributions at the measurement position, the patient plans were modified using ECLIPSE software (Varian Medical Systems, Palo Alto, CA, USA) for the TPS of the Varian Trilogy LINAC. Verification plans were based on computed tomography images of the solid water phantom. The block tray coupled with the film was added to the condition of each field and the dose distribution was recalculated for an SSD of 65.4 cm, the position of the block tray. The dose distribution 5 mm below the surface was exported and compared with the dose distribution measured by the EBT3 film using RIT software (Radiological Imaging Technology, Inc., Colorado Springs, CO, USA).

Each field was measured to all plans of 3D-CRT and IMRT, and the composite plan that merges all fields within one fraction was measured to randomly selected plans according to the organs in 3D-CRT and IMRT.

Comparison method

Calculated and measured dose distributions were compared using the gamma evaluation method, a quantitative 2D dose verification method.[31] The films were scanned using the flatbed scanner EPSON and the red channel response, which provides the best contrast, was analyzed using RIT 113 software. The global GI was used and the points with doses <30% of the maximum dose were ignored to reduce the effect of noise. The criteria of 3%/3 mm were applied and the percentage of pixels with a GI <1 was evaluated.

 > Results Top

Reproducibility of Gafchromic EBT3 film

GI analysis with the same criteria of 3%/3 mm is shown in [Table 1] and [Figure 2]. Three dose distributions show very high passing rates ≥99.64% and ≥99.34% on 3D-CRT and IMRT, respectively. It indicates that EBT3-film-based dosimetry yields reproducible results.
Table 1: Gamma index passing rates of three films at three-dimensional conformal radiotherapy and intensity-modulated radiotherapy

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Figure 2: Gamma maps for reproducibility of film

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Each field of three-dimensional conformal radiotherapy

[Table 2] shows the GI passing rates of the dose distributions calculated by the TPS and the dose distributions measured using the EBT3 film during 3D-CRT. All 22 fields of the eight patients had GI passing rates ≥96.13%. The mean GI passing rate for all fields was 98.07% ± 1.08%. [Figure 3] shows the gamma distributions by the field for a rectum cancer patient who underwent 3D-CRT.
Table 2: Gamma indices of treatment planning system-Gafchromic EBT3 film for three-dimensional conformal radiotherapy (units: %)

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Figure 3: Gamma maps for every field of a three-dimensional conformal radiotherapy plan for a patient

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Each field of intensity-modulated radiotherapy

[Table 3] shows the GI passing rates of the dose distributions calculated by the TPS and the dose distributions measured using EBT3 film during IMRT. All 41 fields of the nine patients had GI passing rates ≥95%. The mean GI passing rate for all fields was 97.45% ± 2.60%. [Figure 4] shows the gamma distributions by the field for a liver cancer patient who underwent IMRT.
Table 3: Gamma indices of treatment planning system-Gafchromic EBT3 film for intensity-modulated radiotherapy (units: %)

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Figure 4: Gamma maps for every field of an intensity-modulated radiotherapy plan for a patient

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Composite field of three-dimensional conformal radiotherapy and intensity-modulated radiotherapy

For a breast cancer patient undergoing 3D-CRT, the mean GI passing rate of all fields was 98.65% ± 0.93%, and the GI passing rate of the composite plan was 99.20%. For a liver cancer patient undergoing IMRT, the mean GI passing rate of all fields was 96.81% ± 1.19%, and the GI passing rate of the composite plan was 96.81% [Figure 5]. In the same way, [Table 4] shows the GI passing rate of the composite plan and three plans were randomly selected by the organ at 3D-CRT and IMRT. [Figure 6] shows the gamma distributions related to these results.
Table 4: Gamma index passing rates of the composite plan and all fields at three-dimensional conformal radiotherapy and intensity-modulated radiotherapy

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Figure 5: Gamma maps for each field and for a composite field for three-dimensional conformal radiotherapy and intensity-modulated radiotherapy

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Figure 6: Gamma maps for a composite field for three-dimensional conformal radiotherapy and intensity-modulated radiotherapy

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These mentioned GI passing rates indicate good agreement for 3D-CRT and IMRT.

 > Discussion Top

The GI passing rates were found to be lower in patients undergoing IMRT than in those undergoing 3D-CRT. In IMRT, the dose distribution is nonuniform, modulating the radiation intensity within a field. The lower GI passing rates are caused by the low agreement in regions of low doses, as EBT3 film yields inaccurate measurements at low doses.[32] Nevertheless, all fields had GI passing rates >95%, indicating that EBT3 film can be used to monitor inter-fractional entrance doses during RT. This study confirmed that the EBT3 film yielded accurate measurements at each field for 3D-CRT and IMRT.

This methodology utilizes EBT3 film as a dosimeter, the block tray for measurement accuracy and the accessory tray for mounting to the gantry. The effect of the block tray-shaped 10 mm phantom with the film on the dose distribution in the patient was assessed using MatriXX (IBA Dosimetry, Schwarzenbruck, Germany) and MultiCube to measure relative dosimetry in the presence and absence of it. Gamma calculations were performed using OmniPro I'mRT software with the same criteria of 3%/3 mm. There is no significant dose distribution interference due to the monitoring device by showing 99.17% and 99.85% of passing rate for 3D-CRT and IMRT, respectively [Figure 7]. The beam attenuation by the phantom with the film was also verified using an ionization chamber, 0.6 cc in size (Farmer Chamber, IBA Dosimetry, Schwarzenbruck, Germany). It was located in the 3D water phantom (SSD = 100 cm, depth = 10 cm, effective depth = 1.8 mm, with a field size of 10 cm × 10 cm) and current was measured with an electrometer. The output of a beam passing through the phantom of thickness 10 mm was compared to the output with nothing in the beam path. The output signal decreased 4.77% with the monitoring device (1.298 nC) compared to the output signals (1.363 nC) measured with no hindrance in the beam path. Therefore, the output factor should be applied when calculating dose distribution in TPS. In general, the influence of the block tray is easily calculated and counted in TPS. Since the thickness and material of the phantom we used are similar to the block tray in photon therapy, the changed output factor can be easily implanted by the TPS system too.
Figure 7: Dose maps measured by MatriXX in the presence and absence of the block tray-shaped phantom

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If all fields are measured for every fraction, a large quantity of film and an enormous workload would be required, and this method would no longer be cost-efficient. Furthermore, additional workforce, time, and cost would be required to calibrate the film before treatment, to change it for every field during treatment, and to analyze the results after treatment. To reduce workforce, time, and cost, we propose an efficient method checking whether a composite plan that merges all fields within one fraction has a GI passing rate >95%. Composite plans also show GI passing rates of 95% or more in 3D-CRT and IMRT. These findings indicate the ability to verify the accuracy of one fraction using one sheet of EBT3 film. Moreover, the accuracy of the EBT3 film is improved since all fields are included and the absolute dose within the distribution is increased. Furthermore, if a patient is prescribed 20 fractions, the wasted resources, such as workforce, time, and cost, can be reduced markedly by verifying one fraction once a week rather than every fraction and this way is efficient in clinics treating many patients. However, this can reduce the possibility of detecting unexpected delivery errors. One should accept trade-off between possibility detecting the unexpected errors and efficiency for clinical practice. Hence, the verification cycle may be subject to department policy depending on the performance of treatment equipment, number of the treated patients, and other conditions.

This methodology can be easily adapted to use with any type of LINAC. Furthermore, there are almost no introductory barriers if a block tray is a default option for LINAC in use. Despite the ability to manage treatment quality after every fraction, one drawback of this method was the cumbersome procedures in the TPS, recalculating the dose at film position and exporting the DICOM file. IN general, prior to treatment using LINAC, patient-specific QA based on the patient treatment plan is implemented using MatriXX to verify whether the beam is delivered to the patient accurately as intended. We propose another simple and efficient way to apply the proposed methodology. If a film is located in block tray during pretreatment QA using Matrixx and the result of pretreatment QA is acceptable, then the dose distribution measured using the film serves as a reference (i.e., QA-determined reference) for subsequent film-based measurement during treatment. If the measured dose distribution is matched with the QA-determined reference, it can be verified that the accurate beam has been delivered to the patient.

 > Conclusion Top

Error detection and management in radiation oncology is a major topic of interest in medical physics research.[33],[34],[35] Along with controlling critical factors that can harm patients, monitoring inter-fractional beam delivery can prevent dosimetric incidents. The existing transmission systems, such as Dolphin and PerFraction, are not used universally caused by high price, necessity, etc., in the majority of the institutions using the LINAC. These problems may be resolved by monitoring inter-fractional entrance doses using EBT3 film. Our results demonstrated the feasibility of this methodology. Clinical application of this method to patients can verify the accuracy of beam delivery using film-based measurement in the treatment room.

Financial support and sponsorship

This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea, funded by the Ministry of Education (NRF-2017R1D1A1B03031056 and NRF-2018R1D1A1B07047770).

Conflicts of interest

There are no conflicts of interest.

 > References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]

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


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