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ORIGINAL ARTICLE
Year : 2022  |  Volume : 47  |  Issue : 4  |  Page : 362-366
 

In vivo Dosimetry for dose verification of total skin electron beam therapy using gafchromic® EBT3 film dosimetry


1 Department of Physics, Institute of Applied Sciences and Humanities, GLA University, Mathura, Uttar Pradesh; Department of Radiological Physics and B.E, Sher I Kashmir Institute of Medical Sciences, Srinagar, Jammu and Kashmir, India
2 Department of Physics, Institute of Applied Sciences and Humanities, GLA University, Mathura, Uttar Pradesh, India
3 Department of Radiation Oncology, Sher I Kashmir Institute of Medical Sciences, Srinagar, Jammu and Kashmir, India

Date of Submission05-Aug-2022
Date of Decision30-Sep-2022
Date of Acceptance16-Oct-2022
Date of Web Publication10-Jan-2023

Correspondence Address:
Dr. Misba Hamid Baba
Research Officer, Room No 255, Department of Radiological Physics & B.E, Sher I Kashmir Institute of Medical Sciences, Soura, Srinagar, Jammu and Kashmir
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jmp.jmp_72_22

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   Abstract 

Background and Purpose: Total skin electron beam therapy (TSEBT) is an important skin-directed radiotherapeutic procedure done in the treatment of cutaneous T-cell lymphomas, namely, mycosis fungoides (MF). This procedure is usually done at larger source-to-surface distances with the patient standing on a rotatory platform. As the patient has to stand in different positions without any rigid immobilization devices, there are chances that the total skin may not get uniformly irradiated which could lead to nonuniform dose distributions. Therefore, all the necessary arrangements should be made to evaluate the dose for different regions of the skin using suitable in vivo dosimeters at the radiotherapy centers offering these treatments. This study aimed to evaluate the consistency between the delivered and planned doses in vivo during TSEBT using Gafchromic EBT3 film dosimetry. Materials and Methods: The surface dose for the six MF patients treated for TSEBT at our hospital from 2018 to 2022 was measured and evaluated. 2 cm × 2 cm Gafchromic® EBT3 films were used to measure skin dose at reference body positions of clinical interest. All the patients were treated with the modified Stanford technique. The irradiated film strips were analyzed for the dose using the IMRT OmniPro software. The doses at respective positions were expressed as mean dose ± standard deviation and the deviation was calculated as the percentage of the prescribed dose. Results: One hundred and fifty-four Gafchromic® EBT3 film strips irradiated on six TSEBT patients showed a maximum dose variation of 2.00 ± 0.14 Gy, in the central body regions. The dose variation in the peripheral areas such as hands and ears was larger. A variation of 2 ± 0.32 Gy was observed on the hands and ears. The uniformity of the dose delivered to maximum body parts was within −7% and +16% for the peripheral areas like hands. The American Association of Physicists in Medicine recommends a dose uniformity of 8% and 4% in the vertical and horizontal patient plane for direct incident beam; however, for oblique incidences like in the modified Stanford technique, the dose variation is about 15%. Conclusion: In vivo dosimetry using Gafchromic EBT3 film dosimetry for TSEBT yields objective data to find the under or overdose regions. That can be useful to provide quality treatment, especially when treatments tend to be as complex as TSEBT.


Keywords: Dose uniformity, Gafchromic® EBT3 film, in vivo dosimetry, total skin electron beam therapy


How to cite this article:
Baba MH, Singh BK, Wani SQ. In vivo Dosimetry for dose verification of total skin electron beam therapy using gafchromic® EBT3 film dosimetry. J Med Phys 2022;47:362-6

How to cite this URL:
Baba MH, Singh BK, Wani SQ. In vivo Dosimetry for dose verification of total skin electron beam therapy using gafchromic® EBT3 film dosimetry. J Med Phys [serial online] 2022 [cited 2023 Feb 3];47:362-6. Available from: https://www.jmp.org.in/text.asp?2022/47/4/362/367428



   Introduction Top


Cutaneous T-cell lymphoma (CTCL) is a heterogeneous group of rare non-Hodgkin lymphoma characterized by uncontrolled clonal proliferation of malignant T-lymphocytes in the skin. Mycosis fungoides (MF) is the most common CTCL subset and accounts for >60% of all CTCL cases.[1],[2],[3],[4] Early-stage disease presentation occurs as patches limited to certain body areas, whereas disease in its late stage is characterized by lesions throughout the skin, which may be a tumor, or ulcerative with systemic involvement. Patients having early presentation have usually a good prognosis and long-term survival as compared to late-stage disease patients. Moreover, MF continues to be a disease that is not curable and needs lifelong treatment. Out of many skin-directed treatments available to treat MF, total skin electron beam therapy (TSEBT) is considered an effective treatment modality.[5],[6],[7],[8],[9] The recommended depth to be treated is <1 cm and therefore the energy sufficing this much penetration is a 4 MeV electron beam.[10] Among a variety of delivery techniques, the most common is the modified Stanford dual-field technique using a 6 MeV degraded electron beam from a linear accelerator at extended source-to-surface distances (SSD).[11] As the patient's whole skin needs to be irradiated, and the majority of the times, it is delivered to the patient in standing positions at extended SSD without any rigid immobilizations. Patients' skin receives radiation in the standing position; therefore, there are chances that the body areas which are not flat due to body contour variations may be under or over-irradiated. However, the dose uniformity should be maintained with ±8% in the vertical direction and ±4% in the horizontal axis.[10] Therefore, it becomes essential to evaluate the treatment to adjust the Monitor Units (MUs) for the dose variation. Usually, there is underdosing in certain body areas such as the inner thighs, perineum, soles, forehead, scalp, etc., due to body contour or self-shielding. Furthermore, a suitable dosimeter should be used to measure the delivered dose in vivo to adjust for any under/overdosing. The dose measurements made in vivo provide basic data for the boost fields, and consequently, the boost dose is to be planned. Our study aimed to demonstrate and standardize the use of Gafchromic EBT3 films in TSEBT in vivo dose measurements. This study unveils the first time, how Gafchromic EBT3 film can be used in TSEBT in vivo measurements easily with inexpensive tools. The reason for selecting EBT3 Gafchromic films is its availability in most of the radiotherapy centers, thus putting no extra financial burden on the clinics.


   Materials and Method Top


Film calibration

Seven 20 cm × 3 cm Gafchromic® EBT3 films kept in the SP34 phantom at a depth of 5 cm were irradiated to known doses of radiation (0.5, 1, 1.5, 2, 2.5, 3, and 4 Gy) in solid water phantom (SP34, IBA Dosimetry AB, Uppsala, Sweden) at SSD of 100 cm in 6 MV photon Clinac DHX linear accelerator (Varian Medical Systems, Palo Alto, CA). A standard graph was plotted between the delivered dose and the optical density of Gafchromic® EBT3 films [Figure 1] using Epson scanner and OnmiPro I'mRT + software after 24 h postexposure.[12],[13],[14],[15],[16],[17] For the dose estimation, 15 points were randomly selected in an Region of Interest (ROI) of 6 cm × 2 cm. The readout time was selected after 24 h to equilibrate the variations in the optical densities as this development time has shown negligible variations.[18],[19]
Figure 1: Standard plot for EBT3 Gafchromic films

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Patient irradiation and film location

The surface dose for the TSEBT patient was measured for the first cycle of irradiation using 26, 2 cm × 2 cm Gafchromic® EBT3 films placed on the different anatomical locations on the patient's body [Figure 2] to measure the delivered dose in all the six body positions for the TSEBT for irradiation by modified Stanford technique. The films were plastic wrapped (in cling film) to protect their surface from grease and dirt from patients' skin during the measurement. The films were carefully marked to ensure that all the films are irradiated on the same surface, and to decrease any uncertainty or errors arising due to the surface irradiations. Although the variation in dose for both surfaces of the film was negligible still care was taken to ensure that film is irradiated from the same side always. The same procedure of dose monitoring was repeated for the third and last cycles of the treatment.
Figure 2: Schematic representation of dose measurement points by EBT3 Gafchromic films

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Film dose evaluation/analysis

The irradiated film strips were analyzed for the dose using the same software after 24 h. Each film was read at 10 points within an ROI of 1.5 cm × 1.5 cm and the mean of that was read as the final dose. The dose recorded at respective positions was expressed as the mean dose in Gy ± standard deviation (SD) and pooled the data in Microsoft Excel. Further, at each location, the percentage of the prescribed dose received was calculated. Then, the percentage variation in dose (percent deviation from the prescribed dose), was calculated. Skin dose variation to various anatomical sites from the prescription dose was plotted using a scatter plot.


   Results Top


One hundred and fifty-four film strips were irradiated in different anatomical locations on the six TSEBT patients from 2018 to 2022. Five out of six measurements showed a maximum dose variation of −7% of the prescription dose, i.e., 2.00 ± 0.14 Gy, in the central body regions. The dose variation in the peripheral areas like hands was larger. A variation of + 16% of the prescribed dose, i.e., 2 ± 0.32 Gy was observed on the hands. The dose variation in the peripheral areas was larger. A maximum dose variation of −73% which corresponds to an underdosing by 1.46 Gy was observed in the perineum area. The dose variation was maximum in the shielded areas such as the eyes and nails. An underdosing of 96% was observed at the perineum, soles of the foot, and under eye shields, respectively. An underdosing of 11% of the prescribed dose was observed in the inner thighs and axilla. In vivo dose reported by patient number 2 was found 30% less than the prescribed in central as well as peripheral regions. [Table 1] shows the dose variation in the percentage of the prescribed dose, as well as in terms of mean percent variation ± SD from the prescribed dose of 2 Gy per cycle recorded at various anatomical locations on the patient's body surface. [Figure 3] shows the dose variation in the percentage of the prescribed dose at all anatomical sites.
Table 1: Dose variation in the percentage of the prescribed dose and mean±standard deviation for six total skin electron beam therapy patients

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Figure 3: Scatter plot of the percentage dose variation (values in the red box are the doses under lead shield, blue depicts the underdosed areas, violet the overdosed areas, and green the self-shielded areas)

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   Discussion Top


Our results of in vivo dosimetry (IVD) with Gafchromic EBT3 film showed that the variation in radiation dose to most of the body parts was uniform, with a variation of ±7% is well within the recommended values. Therefore, the majority of the body surfaces were irradiated to about 90% of the prescribed dose. A dose variation of ± 16% was observed on the hands. A maximum underdosing of 73% which corresponds to a dose of 1.46 Gy was observed in the perineum area. Therefore, a perennial boost was planned for the last 10–15 fractions of the treatment to account for this underdosing of 1.43 Gy per cycle. Desai et al. and Weaver et al. in their studies on IVD of TSEBT patients have reported similar variations in dose using a thermoluminescent dosimeter (TLD) as an in vivo dosimeter.[20],[21] Therefore, our results are consistent with the already published data. An underdosing of 96% of the prescribed dose was observed at the soles of the foot, due to the self-shielding of the soles in the standing position. This too was compensated with a boost dose of 10–12 Gy in a fraction size of 1 Gy per day toward the end of the treatment. Furthermore, the dorsum of the feet received 16% more dose, for which feet were shielded after 20 Gy to avoid adverse reactions. Weaver et al. and Antolak et al. have reported a similar overdosing of 140% to the dorsum of the foot using TLD which is consistent with our results.[21],[22] The dose variation in the shielded areas such as eyes and nails was – 96% since these areas were all time shielded with a 3 mm lead sheet dipped in wax. Doses to the inner thighs and axilla were also less as much as by 11% of the prescribed dose, likely due to the self-shielding. We, therefore, recommend that special attention should be given to the patient positioning during the treatment. The dose delivered to maximum body parts was within −7% and +16%. The American Association of Physicists in Medicine (AAPM) recommends a dose uniformity of 8% in the vertical plane and 4% in the horizontal plane at the calibration point by a directly incident beam, but with six dual fields practically it is always more than 15%.[10],[23],[24] As the maximum dose value was 16% which is almost consistent with AAPM recommendations for oblique incidence. For one patient, the IVD showed an underdosing of 30%. On inspecting the dosimetric calibration, it was found that the beam required tuning and calibration. Accordingly, the MU corrections were made for the accurate dose delivery for that patient. Furthermore, beam calibration was done to standardize the beam output at the patient plane. In vivo film dosimetry provides all the data for any necessary MU corrections and thereby increases the confidence of any radiotherapeutic procedure. Many authors have reported the use of different detectors to measure this variation. Elsayad K et al. in 2017 used TLD to monitor in vivo radiation dose in 73 TSEBT patients with two techniques and found that 18% of the patients required MU modifications after the first TLD measurement. Furthermore, the anatomical sites were identified for boost treatments, from the TLD measurements done during irradiation.[13] AAPM-TG-23 also recommends the IVD using TLD of the fact that the film used is to be packed in a lightproof jacket which may thereby decrease or hamper the skin dose. Moreover, TLD readout cost becomes a burden on the clinic. Kairn T et al.(in 2020) analyzed the results of optically stimulated luminescent dosimeters (OSLDs) used for in vivo dose measurements during total skin electron therapy (TSET) treatments of patients with MF and found that OSLDs can be used to obtain measurements of TSET dose that can inform monitor unit adjustments and identify regions of under and overdosage, while potentially informing continuous quality improvement in TSET treatment delivery suggesting that IVD is a must for TSEBT treatment. However, the OSLDs may not be affordable to all the radiotherapy clinics, again due to the extra economic burden, especially on low-income and source-limited clinical TSEBT setups.[25] The use of Gafchromic film EBT has been demonstrated by Bufacchi et al. to determine skin dose for patients undergone TSET using EBT films, as well as TLDs for comparison, and found that EBT results showed a reasonable agreement with TLDs data.[26] However, Brown et al. have demonstrated the energy dependence of EBT Gafchromic films is more than EBT3 films.[27] Ideally, the energy response for any dosimetric detector should be flat, i.e., the calibration system should be independent of energy over a certain range of radiation qualities. Although they found that EBT3 film was less sensitive than EBT and EBT2 films, its energy independence makes it a better in vivo dosimeter. We at our center have a standardized IVD system for all the special techniques like TSEBT and strongly recommend this method to all other radiotherapy centers offering special treatments like TSEBT. Although for busy centers, IVD may become cumbersome, once a batch of Gafchromic films is calibrated; it is quite easy and fast IVD method. Moreover, unlike TLD discs, film thickness being in millimeters (approximately = 0.28 mm) does not even alter or affect the skin dose making it superior to TLD systems.


   Conclusion Top


IVD measures must be provided to report complex treatments like TSEBT to provide a confidence limit to the efficiency of the treatment delivery. It is a must to do to provide objective data so a radiation oncologist may prescribe any supplementary fields if there is any underdosing to any part of the skin. It also helps in the assessment of possible toxicity due to overdosing which could have severe implications for patients' quality of life and patient follow-up. Therefore, we recommend that IVD must be followed for every patient to be irradiated for TSEBT in the radiotherapy centers offering the treatment. Furthermore, with a standardized film dosimetry system as described in our work, it is easy to follow the procedure and be confident of the dose delivery methods.

Limitations

The limitations of our study were that we only considered six patients, the reason being the less incidence of this type of T-cell lymphoma in our region. Furthermore, an average number of 26 anatomical sites were considered for in vivo dose determinations, which could be extended to more anatomical sites. Another limitation was that we measured the dose only for the first two cycles of the irradiation, which could also be elaborated to a few more cycles toward the end of the treatment that may show a different degree of variation as the patient tends to lose patience toward the end of the treatment.

Patient consent and ethics

Patients signed consent before treatment was initiated and data were collected. All procedures performed in studies involving human participants were following the ethical standards of the institutional and national research committee and with the 1964 Helsinki Declaration and comparable ethical standards.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
   References Top

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