Abstract
Background/Aim: We aimed to clarify the TomoTherapy irradiation method for accurate dose delivery to the postoperative ear keloid with minimal exposure. Materials and Methods: An electron beam of Elekta synergy and static and helical photon beams of TomoTherapy were delivered to the auricle and lobe of an anthropomorphic phantom compensated using a soft rubber bolus. The doses to the ear surface and the eyeballs and thyroid were measured using radiochromic film and glass dosimeters, respectively. Results: Using static, helical, and electron beams, the respective doses to the ear surface were 97.9%, 103.0%, and 91.7% of the prescribed dose; the respective doses to the thyroid were 0.6, 0.8, and 2.4 cGy; the respective doses to the left eyeball were 3.3, 6.9, and 2.7 cGy. Conclusion: The static beam of the TomoTherapy can be safely used for treating ear keloids, while ensuring target dose. The helical photon beam spreads out the low-dose exposure.
An increasing number of institutions are installing a single TomoTherapy (Accuray, Sunnyvale, CA, USA) radiotherapy system, which employs only a fixed-energy 6 MV photon beam. Using the intensity-modulated radiation therapy technique, the photon beam of the TomoTherapy system has been applied to superficial targets, such as keloid (1), total scalp (2), and total skin (3), as an alternative option in case an electron beam is unavailable. For ear keloid, the local control rate is expected to be more than 80% after irradiation of the postoperative scar using an electron beam (4, 5). Electron beam irradiation has a serious complication rate of less than 1% (6) and results in a smaller out-of-field radiation dose than a photon beam delivered by a C-arm linear accelerator used for intensity-modulated radiation therapy (7). In contrast, the exposure doses to normal tissues in keloid treatment using helical beam of TomoTherapy have not been compared to those of conventional electron beam.
When applied to irregular surfaces with large slope variations, the helical beam of TomoTherapy provides a better local control rate of keloid than a conventional electron beam (1). However, the exposure dose to normal tissues resulting from both target scattering and collimator leakage dose during photon beam irradiation of TomoTherapy may be higher than that from the electron beam because TomoTherapy tends to require high radiation outputs, measured in monitor units (MUs). Moreover, it is expected to overcome disadvantages such as the inhomogeneity of the dose distribution in electron beam by compensating with a customized bolus in large slope and irregular surface region (8). Therefore, the advantages of using TomoTherapy to treat postoperative keloid scars are not clear, and a feasible TomoTherapy method that achieves an out-of-field dose comparable with that of the electron beam remains to be developed.
The purpose of this study was to clarify an optimal TomoTherapy beam delivery method for accurate dose delivery to the target volume in the ear with minimal exposure of eyeballs and thyroid. Regarding the dose to normal tissues, the photon beam of TomoTherapy was compared with conventional electron beam as a reference.
Materials and Methods
Treatment planning. An anthropomorphic phantom (RANDO; The Phantom Laboratory, Salem, NY, USA) with the irregular surface of the left ear compensated using a soft rubber bolus (HM bolus; Hayakawa Rubber, Hiroshima, Japan) (8) was used, and images of the phantom were acquired using a computed tomography scanner (Aquilion LB; Canon, Japan) (Figure 1). Based on the mean size of ear keloids, we assumed there were 2 cm of postoperative scars (9), with one scar on the auricle and another on the lobe. The clinical target volume was defined as a region encompassing a 0.5 cm expansion around the scar perimeter and having the thickness of the ear (10). The planning target volume (PTV) was defined as a 0.5 cm expansion around the clinical target volume (1). Electron beams from an Elekta Synergy linear accelerator (Elekta AB, Stockholm, Sweden) and photon beams from a TomoTherapy system delivered a total dose of 15 Gy in 3 fractions to the auricle only and to both the auricle and lobe (auricle+lobe). For the electron beam, the Pinnacle (v9.10, Philips Radiation Oncology Systems, Fitchburg, WI, USA) treatment planning system (TPS) was used. The International Commission on Radiation Units reference point was placed at the center of the PTV, and 12 MeV energy was used to deliver more than 90% of the prescribed dose to at least 99% of the PTV at a 0° gantry angle (11). For the photon beam, the Precision TPS (v2.0.1.1, Accuray, Sunnyvale, CA, USA) was used, and a mean dose of 5 Gy per fraction was prescribed to the PTV. We optimized the dose distribution to deliver more than 90% of the prescribed dose to at least 99% of the PTV and more than 95% of the prescribed dose to at least 95% of the PTV (Table I). Photons were delivered to the PTV using either helical or static beams from the tangential directions. To create a helical beam tangentially incident to the PTV, we added dose constraints to avoid exposure of the whole brain to the beam (1, 12). The planned dose of the PTV and organs at risk (eyeballs and thyroid) are shown in Table II and Table III. Conformity of the PTV was defined as V95%/VPTV, where V95% is the volume covered by 95% of the prescribed dose and VPTV is the PTV volume (13).
Appearance and computed tomography image of a soft rubber bolus combined with an anthropomorphic phantom.
Planning goals of the planning target volume (PTV) using electron and photon beams.
Planned radiation dose for the auricle.
Planned radiation dose for both auricle and lobe.
Dosimetric comparison of electron and photon beams. A piece of radiochromic film (Gafchromic EBT3; Ashland, Covington, KY, USA) was placed on the ear surface of the anthropomorphic phantom to measure the dose to the PTV. The images of radiochromic films were digitized using a scanner (ES-10000G; Epson, Nagano, Japan) after irradiation for 24 h (14), then analyzed using the DD-System software (v10.3; R-TECH, Tokyo, Japan).
Glass dosimeters (Dose Ace, GD-302 M; Asahi Techno Glass, Tokyo, Japan) were used to measure the dose received by normal tissues because we confirmed in advance that the Pinnacle TPS could not detect the electron dose outside the PTV. Glass dosimeters were inserted into the anthropomorphic phantom to measure the dose to the eyeballs and thyroid. The measurements were repeated three times at each point and the mean doses delivered by the electron beam and static and helical photon beams were compared.
Results
Dosimetric comparison of electron and photon beams. The radiation doses at the ear surface in the PTV during electron and photon irradiation, measured using radiochromic film, are shown in Table IV. When the auricle+lobe was irradiated with the electron beam, the mean surface dose was 91.7% of the prescribed dose. The mean dose at the ear surface for the other treatment plans was 97.0-103.0% of the prescribed dose. In addition, 89% (16/18) of all measurements in PTV were within 95-105% of the prescribed dose.
Radiation doses (mean±standard deviation) by the surface of the ear in the planning target volume (PTV) irradiated by the electron beam and static and helical photon beams.
The radiation doses absorbed by each organ at risk, measured using glass dosimeters, are shown in Table V. During both auricle and auricle+lobe irradiation, the respective doses absorbed by the thyroid using static and helical beams were 25% and 33% of that absorbed using the electron beam. The respective doses absorbed by the left eyeball using static and helical photon beams were approximately 1.3- and 3.4-times that absorbed by the auricle alone during electron beam irradiation and 1.2- and 2.6-times that absorbed by the auricle+lobe during electron beam irradiation. Compared with the electron beam, the static photon beam delivered a similar dose to the eyeballs in the same plane with the PTV and a lower dose to the thyroid outside the PTV.
Exposure doses (mean±standard deviation) of risk organs irradiated by the electron beam and static and helical photon beams.
Discussion
We clarified the exposure dose to normal tissue during treatment of ear keloid and showed that using the static or helical photon beam of the TomoTherapy system can reduce the respective doses to the thyroid by 75-77% or 67-69% compared with that using the 12 MeV electron beam of the Elekta Synergy system (Table V). Using the customized bolus, the photon beam can deliver 97.0-103.0% of the prescribed dose to the ear surface (Table IV). However, the output of the helical beam was 5166.4 MU and 8005.4 MU for irradiation of the auricle and auricle+lobe, respectively. Therefore, it is recommended to use the static beam of the TomoTherapy system when an electron beam is unavailable.
The exposure dose to the left eyeball using the static photon beam was less than half of that using the helical photon beam (Table V) because the low-dose region of the latter is more spatially dispersed (Figure 2). Our planned doses for the left eyeball using the anthropomorphic phantom were 5 cGy (auricle) and 7 cGy (auricle+lobe) using the helical beam, which are comparable with the clinical plan using the helical beam of the TomoTherapy system (1). The static beam and electron beam resulted in respective doses to left eyeball of 2.0 cGy and 1.6 cGy (auricle) and 3.3 cGy and 2.7 cGy (auricle+lobe) (Table V), indicating that the static beam and electron beam can provide comparable exposure doses. Using TomoTherapy, the out-of-field dose at a caudal distance from the isocenter of more than 10 cm is reported to be less than 1.0% of the isocenter dose (15). In this study, the thyroid was approximately 12 cm away from the isocenter, and the thyroid doses were 0.4 cGy (0.08% of prescribed dose) for irradiation of the auricle only and 0.8 cGy (0.16% of prescribed dose) for irradiation of the auricle+lobe (Table V).
Dose distributions for (A) static and (B) helical photon beams. For the helical beam, the low-dose region is spread out in the same plane as the irradiation field. The dashed curves indicate low-dose distribution, which are 10% of the prescribed dose.
There is a difference in the response of the glass dosimeter between electron and photon beams, with the corresponding values of the glass dosimeter using the 6-20 MeV electron beam being 4% lower than that using the 6 MV photon beam (16). Thus, the exposure doses of the thyroid using the 6 MV photon beam with TomoTherapy remain lower than that using the 12 MeV electron beams in this study. The dose conformities to the PTV for auricle+lobe using static and helical beams were 1.41 and 1.29, respectively. The helical beam provided better dose coverage for a complicated PTV, similar to observations in a previous study (1). However, the static beam had better conformity to the PTV than the conventional electron beam (Table III). Compared with the calculated dose using combined radiation with a gel bolus for keloid treatment, the accuracy of the delivered dose in the PTV was 8% higher using the photon beam and 5% lower using the electron beam (1). In the present study, 89% (16/18) of all measured doses were within 95-105% of the prescribed dose, and there was no difference in the accuracy of the dose delivered using electron and photon beams. The reason for this likely stem from the use of a soft rubber bolus that can shape and maintain the contours of the ear and reproduce the structure of the ear. Irradiation of the scar should be started 2-74 h after surgery (17). A three-dimensional printer takes more than 3-4 h to create a customized bolus following the contours of the ear (18). The soft rubber bolus can be shaped during the computed tomography simulation and used with both electron and photon beams (8). Therefore, the soft rubber bolus may be effective in the treatment of keloids.
Patient setup methods and the alignment of the beam and ear vary among institutions (1, 10, 19). A traditional gel bolus should be replaced by a customized bolus with high dose reproducibility. A limitation of this study was the use of a 12 MeV electron beam, required because the thickness of the anthropomorphic phantom (10-13 mm) was greater than the average thickness of a human ear (20). However, a 4-12 MeV electron beam is used for keloid treatment in clinical practice (21). And then, our soft rubber bolus can be made any thickness, and the energy settings will vary depending on the bolus thickness. The advantages of TomoTherapy include its added shielding design to reduce the out-of-field radiation dose (22) and ability to limit the exposure dose to the thyroid to less than 1 cGy. Keloids in patients below 18 years of age are treated conservatively with steroids, which may take more than two years to effect a cure, because of concerns about possible radiation-induced stochastic effects on the thyroid (23). In such cases, TomoTherapy may provide a viable alternative.
Conclusion
The static photon beam of the TomoTherapy system can be used with a customized bolus as a safe alternative to conventional electron beam for treating ear keloid, while ensuring PTV dose coverage. In contrast, the helical photon beam should be used carefully because the low-dose region spreads out to normal tissues.
Acknowledgements
This work was supported partly by Japan Society for the Promotion of Science KAKENHI [grant number 19K08211]. The Authors thank Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.
Footnotes
Authors’ Contributions
KO, HM, KK and KN designed the study and the analysis. KO and KK measured and analyzed the data. KO, HM and YY prepared the manuscript. All Authors read and approved the final manuscript.
Conflicts of Interest
HM received a research donation from Hayakawa Rubber Co., Ltd., Tokyo, Japan.
- Received June 2, 2022.
- Revision received June 25, 2022.
- Accepted June 25, 2022.
- Copyright © 2022 International Institute of Anticancer Research (Dr. George J. Delinasios), All rights reserved.
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