Abstract
Purpose: The dose expansion methods as the skin flash and virtual bolus were used to solve intrafraction movement for breast planning due to breathing motion. We investigated the skin dose in each planning method by using optically stimulated luminescence on an in-house moving phantom for breast cancer treatment in tomotherapy. The impact of respiratory motion on skin dose between static and dynamic phantom's conditions was evaluated. Methods: A phantom was developed with movement controlled by the respirator for generating the respiratory waveforms to simulate respiratory motion. Five optically stimulated luminescence dosimeters were placed on the phantom surface to investigate the skin dose for the TomoDirect and TomoHelical under static and dynamic conditions. Eight treatment plans were generated with and without skin flash or virtual bolus by varying the thickness. The difference in skin dose between the two phantom conditions for each plan was explored. Results: All plans demonstrated a skin dose of more than 87% of the prescription dose under static conditions. However, the skin dose was reduced to 84.1% (TomoDirect) and 78.9% (TomoHelical) for dynamic conditions. The treatment plans without skin flash or virtual bolus showed significant skin dose differences under static and dynamic conditions by 4.83% (TomoDirect) and 9.43% (TomoHelical), whereas the skin flash with two leaves (TomoDirect 2L) or virtual bolus of at least 1.0 cm thickness (VB1.0) application compensated the skin dose in case of intrafraction movements by presenting a skin dose difference of less than 2% between the static and dynamic conditions. Conclusion: The skin dose was reduced under dynamic conditions due to breathing motion. The skin flash method with TomoDirect 2L or virtual bolus application with 1.0 cm thickness was useful for maintaining skin dose following the prescription by compensating for intrafraction movement due to respiratory motion for breast cancer in tomotherapy.
Keywords: skin dose, tomotherapy, breast cancer, OSL, movable phantom
Introduction
Radiotherapy (RT) has played an important role in the multimodality treatment of breast cancer. 1 Patients with locally advanced-stage post-mastectomy breast cancer have a complex target volume, generally consisting of the chest wall (CW) and regional lymph nodes. The challenge of treatment planning is that it covers a large, superficial surface, which is a thin area. 2 Moreover, the target volume extends up to the air-to-skin interface and is moving due to breathing. This may lead to a target volume lack of coverage in the case of inter- and/or intrafraction movements. 3 Previous studies have reported the dosimetric effects of intrafractional organ motion for breast irradiation.4–6
Nottrup et al 7 monitored the patients’ CW motions for RT during breathing using an external optical marker. The breathing variations over an entire treatment course, including intra- and interfraction variations, were 15.2 mm (median over the patient population), range 5.5 to 26.7 mm, with the variations in exhale level as the major contributing factor. Moreover, Lowanichkiattikul et al 8 determined the CW movement of each patient during deep inspiratory breath-hold (DIBH) and expiratory breath-hold in postoperative breast cancer patients. They recommended that the additional margins of 7 mm in the anteroposterior direction should adequately cover the extreme CW movement in 95% of the patients. For three-dimensional conformal RT, intrafraction movement issues are less problematic because the two radiation fields can be widened, so the area over the skin level also belongs to radiation field 3 as shown in Figure 1A, while the two kinds of dose intensity expansion methods, one is virtual bolus (VB), the other is skin flash (SF) were used for IMRT technique. 9
Figure 1.
Planning techniques to cover intrafraction movements in (A) an opened-up treatment field wider for three-dimensional conformal radiotherapy (3D-CRT), (B) skin flash methods for TomoDirect (fixed field IMRT), and (C) virtual bolus methods for helical tomotherapy.
The SF method, a field extension outside the skin surface, for fixed field IMRT as in Figure 1B, was used to deliver additional radiation on the space above the skin to cover the intrafractional movement caused by respiration. 1 The TomoDirect (TD) delivery mode which is a nonrotational treatment by coplanar static beams, with the couch moving at a constant speed through a fixed binary multileaf collimator that modulates the beam, was able to use SF methods to cover the movement due to respiration. 10 In contrast, a simple approach of SF is not applicable with Helical Tomotherapy (HT) which is a technique using continuous gantry rotations around the patient with thousands of narrow beamlets, which are individually optimized to cover the target. 10 Therefore, other solutions should be found for HT techniques, the use of artificial build-up material over the skin during optimization but not in the actual treatment, known as the VB method 11 as shown in Figure 1C. The VB method is a technique that forces the leaves to be positioned away from the external part of the breast by creating an artificial build-up material at the skin during optimization. It is used only during treatment planning and is not present in actual dose delivery. 1 Therefore, in clinical practice, the usual solutions for intrafraction movement issues can be taken into account using VB in HT.1,3 However, it is becoming a challenge to accurately evaluate the skin dose that patients receive during treatment delivery. There are a few reports related to demonstrating the difference of skin dose in static and dynamic conditions for investigating the skin dose due to intrafraction movement in HT treatment. Therefore, the aim of this study was to measure the skin dose using optically stimulated luminescence (OSL) on an in-house phantom to compare static and dynamic conditions with SF or VB application at various thicknesses for breast planning using TD and HT.
Materials and Methods
Moving Phantom
The moving phantom was developed from natural latex material, as shown in Figure 2A. The phantom was filled with polyurethane to be localized as the target area. The movement of the phantom was controlled by a respirator with an oxygen tank (gas volume 1950 L), as shown in Figure 2B. The respiration signal was observed using the Anzei belt system, a gating device manufactured by Siemens, which acquires the respiratory signal using a belt (AZ-733 V Anzai) equipped with a strain gauge attached directly to the phantom. This belt detects chest movements by measuring the pressure variations. The analog signal was digitized and sent to the monitoring station. 12 The respiratory waveforms of the moving phantom were adjusted to simulate the amplitude and respirations per minute (RPM) compare with human movement by setting RPM = 18.2, as shown in Figure 2C. For most people, the average number of breaths per minute typically ranges from 12 to 20, while at rest. 13
Figure 2.
The natural latex phantom (A) with the respiratory system, Anzai belt, respirator, oxygen tank (B), and the respiratory waveforms of human (C) and moving phantom (D).
Simulation and Treatment Planning
Computed tomography (CT) images of the phantom were acquired with a slice thickness of 3 mm under static conditions (with moving function turned off). Five OSL dosimeters were placed on the phantom surface during the CT image acquisition to localize the position of the skin dose measurement. The structure of the polyurethane in the phantom images represents the target volume. In this study, the right breast of the phantom was localized as the planning target volume (PTV), and the avoidance structure was localized under the PTV as the organ-at-risk structure as in Figure 3. The prescription dose for all the plans was 50 Gy in 25 fractions. The TomoTherapy treatment planning system, Accuray Precision version 2.0.1.1 (Accuray, Inc.), was used to create the following eight plans in both TD and TomoHelical (HT).
Figure 3.
The planning target volume (PTV) on the right breast of phantom (red), and the avoidance structure on the left breast (purple) and under the PTV (yellow).
TomoDirect
Three beams for the medial tangential direction and three beams for the lateral tangential direction were used in the discrete delivery mode. The TD plans without SF (TD0L) and with SF were applied by varying the retraction in one (TD1L), two (TD2L), and three (TD3L) leaves to compensate for intrafraction movement. All plans used a jaw width of 2.5 cm, pitch of 0.25, and modulation factor of 3.0.
TomoHelical
Four plans of the rotational delivery mode were created with and without a VB. HT treatment plans without VB (NoVB) were created following normal procedure for the reference plan using a jaw width of 2.5 cm, a pitch of 0.28, and a modulation factor of 3.0. The three HT plans for VB thickness of 1.0 cm (VB1.0), 1.5 cm (VB1.5), and 2.0 cm (VB2.0) were created by generating the pretended material on the breast surface in treatment planning but absent during irradiation to cover the patient's movement. In this case, the density of the VB was defined as 1.0 g/cm3. 1
Skin Dose Measurement
An Inlight OSL nanodot (Landauer, Inc.) dosimeters were used for skin dose measurements on the moving phantom surface. 14 The calibration of the nanoDot was performed on a linear accelerator under a 6MV photon beam using a 30 × 30 × 20 cm solid water phantom with a depth of 1.5 cm and field size of 10 × 10 cm. This allows for the concomitant use of a Farmer ionization chamber under the same calibration conditions. The OSL was read using a MicroStar Reader (Landauer, Inc.) at least 30 min after irradiation for a stable number of counts. 1 The correction factor for skin dose measurement due to intrinsic buildup of the OSL dosimeter was calculated by determining the relationship between the surface and buildup dose of the Markus ionization chamber and OSL. 15
The positions of five OSLs were placed at the nipple and 2.0 cm in the lateral, medial, superior, and inferior from the nipple on the phantom, as shown in Figure 4. The skin dose of 16 measurements was performed in two scenarios of the phantom for all eight plans, static and dynamic conditions, to compare the effect of intrafraction motion on the skin dose. Regarding the statistical analysis of the skin dose measurement, the paired sample t-test was used to compare static and dynamic conditions with and without a VB plan to determine the statistical significance. A P value <.05 was considered statistically significant with SPSS statistical software, version 17 (Chicago: SPSS Inc.).
Figure 4.
Locations of optically stimulated luminescence (OSL) for skin dose measurement: (1) upper breast, (2) outer breast, (3) center, (4) inner breast, and (5) lower breast.
Results
Skin Dose Measurement of TD
The skin dose at each measurement point for the TD in the static and dynamic conditions of the phantom are listed in Table 1. All plans demonstrated skin dose of more than 170 cGy (>85% of the prescription dose) under static conditions. The treatment plans showed statistically significant skin dose differences between static and dynamic conditions by 4.83% (without flash, P = .0049) and 2.63% (TD1L, P = .0485). However, the flash application as TD2L and TD3L showed not significant skin dose difference between the two phantom conditions by 0.27% (P = .4830) and 0.46% (P = .6024). Moreover, the TD2L showed less than a 2% difference in static and dynamic conditions for all OSL locations. The results of the skin dose measurement revealed the advantage of flash margin application in TD plans for compensating the intrafraction movement and maintaining the skin dose according to the prescription.
Table 1.
Skin Dose (cGy) for TomoDirect of Static and Moving Phantom Condition.
| TomoDirect | OSL | Position | Calculation | Measurement | % Dose difference (static-moving) | P value | |
|---|---|---|---|---|---|---|---|
| Static | Moving | ||||||
| Plans | (cGy) | (cGy) | (cGy) | ||||
| TD0L | 1 | Upper | 178.44 | 183.16 | 180.24 | −1.60 | |
| 2 | Outer | 169.96 | 168.91 | 156.95 | −7.08 | ||
| 3 | Center | 177.80 | 184.77 | 175.06 | −5.26 | ||
| 4 | Inner | 143.88 | 155.73 | 147.35 | −5.38 | ||
| 5 | Lower | 183.76 | 191.00 | 181.30 | −5.07 | ||
| Average (cGy ± SD) | 170.77 ± 14.15 | 176.71 ± 11.82 | 168.18 ± 13.38 | −4.83 | .0049 | ||
| TD1L | 1 | Upper | 177.44 | 189.00 | 178.49 | −5.56 | |
| 2 | Outer | 169.64 | 163.47 | 162.94 | −0.33 | ||
| 3 | Center | 176.44 | 186.58 | 182.89 | −1.98 | ||
| 4 | Inner | 144.40 | 152.94 | 149.61 | −2.18 | ||
| 5 | Lower | 183.48 | 184.09 | 179.12 | −2.70 | ||
| Average (cGy ± SD) | 170.28 ± 13.67 | 175.22 ± 11.09 | 170.61 ± 13.65 | −2.63 | .0485 | ||
| TD2L | 1 | Upper | 178.4 | 190.71 | 188.76 | −1.02 | |
| 2 | Outer | 170.64 | 165.17 | 166.83 | 1.00 | ||
| 3 | Center | 177.80 | 190.25 | 189.04 | −0.64 | ||
| 4 | Inner | 145.20 | 154.52 | 154.32 | −0.13 | ||
| 5 | Lower | 183.48 | 171.44 | 170.79 | −0.38 | ||
| Average (cGy ± SD) | 171.10 ± 13.58 | 174.42 ± 12.26 | 173.95 ± 12.81 | −0.27 | .4830 | ||
| TD3L | 1 | Upper | 179.12 | 181.00 | 176.08 | −2.72 | |
| 2 | Outer | 171.40 | 171.78 | 169.86 | −1.12 | ||
| 3 | Center | 179.88 | 188.07 | 190.23 | 1.15 | ||
| 4 | Inner | 145.76 | 154.67 | 157.48 | 1.82 | ||
| 5 | Lower | 183.76 | 186.07 | 183.84 | −1.19 | ||
| Average (cGy ± SD) | 171.98 ± 11.82 | 176.32 ± 11.10 | 175.50 ± 10.32 | −0.46 | .6024 | ||
Abbreviations: TD0L, TomoDirect without flash; TD1L, TomoDirect with 1L flash; TD2L, TomoDirect with 2L flash; TD3L, TomoDirect with 3L flash; OSL, optically stimulated luminescence.
Skin Dose Measurement of HT
The skin doses at each measurement point for the HT plans in the static and dynamic conditions of the phantom are shown in Table 2. The HT delivery mode demonstrated a range of skin doses from 166 to 174 cGy (83%-87% of the prescription dose) in static conditions. The average skin dose for the HT plan without a VB (NoVB) showed the largest significant dose difference for static and dynamic conditions by 9.43% (P = .0097). However, the dose difference between the two phantom conditions for a VB plan with ≥1.0 cm thickness revealed that the static and dynamic conditions were not significant the dose difference by less than 2% by 1.74% (P = .2847), 1.26% (P = .1889), and 0.26% (P = .7903) for VB1.0, VB1.5, and VB2.0, respectively. According to the HT, the VB application was superior in maintaining the skin dose following the prescription and even respiratory motion in HT treatment.
Table 2.
Skin Dose (cGy) of TomoHelical Plans for Static and Moving Phantom Conditions.
| TomoHelical | OSL | Position | Calculation | Measurement | % Dose difference (static-moving) | P value | |
|---|---|---|---|---|---|---|---|
| Static | Moving | ||||||
| Plans | (cGy) | (cGy) | (cGy) | ||||
| NoVB | 1 | Upper | 177.68 | 182.97 | 168.97 | −7.65 | |
| 2 | Outer | 190.08 | 178.72 | 166.65 | −6.75 | ||
| 3 | Center | 182.60 | 157.07 | 131.51 | −16.27 | ||
| 4 | Inner | 177.32 | 187.53 | 163.90 | −12.60 | ||
| 5 | Lower | 192.96 | 165.24 | 158.30 | −4.20 | ||
| Average (cGy ± SD) | 187.13 ± 6.38 | 174.31 ± 1.39 | 157.87 ± 13.65 | −9.43 | .0097 | ||
| VB1.0 | 1 | Upper | 200.36 | 162.28 | 161.36 | −0.57 | |
| 2 | Outer | 200.28 | 169.64 | 163.62 | −3.55 | ||
| 3 | Center | 200.88 | 173.17 | 165.68 | −4.32 | ||
| 4 | Inner | 200.56 | 172.49 | 166.88 | −3.26 | ||
| 5 | Lower | 200.44 | 160.33 | 165.78 | 3.40 | ||
| Average (cGy ± SD) | 200.50 ± 0.21 | 167.58 ± 5.30 | 164.66 ± 1.96 | −1.74 | .2847 | ||
| VB1.5 | 1 | Upper | 200.72 | 170.32 | 170.18 | −0.08 | |
| 2 | Outer | 201.44 | 164.23 | 165.66 | 0.87 | ||
| 3 | Center | 203.64 | 169.09 | 164.64 | −2.63 | ||
| 4 | Inner | 203.12 | 166.99 | 161.18 | −3.48 | ||
| 5 | Lower | 201.52 | 167.58 | 165.97 | −0.97 | ||
| Average (cGy ± SD) | 202.09 ± 1.10 | 167.64 ± 2.07 | 165.53 ± 2.88 | −1.26 | .1889 | ||
| VB2.0 | 1 | Upper | 200.96 | 165.60 | 164.17 | −0.87 | |
| 2 | Outer | 201.84 | 159.64 | 164.22 | 2.87 | ||
| 3 | Center | 203.68 | 177.20 | 173.96 | −1.83 | ||
| 4 | Inner | 203.36 | 169.44 | 165.93 | −2.08 | ||
| 5 | Lower | 201.64 | 162.54 | 163.98 | 0.88 | ||
| Average (cGy ± SD) | 202.30 ± 1.05 | 166.89 ± 6.10 | 166.45 ± 3.82 | −0.26 | 0.7903 | ||
Abbreviations: NoVB, TomoHelical without virtual bolus; VB1.0, TomoHelical with virtual bolus 1.0 cm; VB1.5, TomoHelical with virtual bolus 1.5 cm; VB2.0, TomoHelical with virtual bolus 2.0 cm.
Discussion
In this study, we investigated and compared the influence of patient respiration on skin dose during breast treatment using Helical Tomotherapy. A moving phantom was developed and controlled using the respirator of an oxygen tank, and the respiratory signal was observed using a gating device for waveform adjustment. A flash or VB application was used to compensate for phantom movement during the treatment.
Regarding the summary of skin dose in Table 3, all TD plans demonstrated a range from 87.6% to 88.4% of the prescription dose in static conditions. However, the skin dose was reduced to 84.1% to 87.8% of the prescription dose for dynamic conditions.
Table 3.
The Skin Dose (% of Prescription) of TomoDirect and TomoHelical Plans for Static and Moving Phantom Conditions.
| Delivery mode | Plans | Static phantom (% of prescription dose ± SD) | Moving phantom (% of prescription dose ± SD) | % Difference |
|---|---|---|---|---|
| TomoDirect | TD0L | 88.36 ± 5.91 | 84.09 ± 6.69 | −5.07 |
| TD1L | 87.61 ± 5.54 | 85.30 ± 6.83 | −2.70 | |
| TD2L | 87.21 ± 6.13 | 86.97 ± 6.40 | −0.27 | |
| TD3L | 88.16 ± 5.55 | 87.75 ± 5.16 | −0.47 | |
| TomoHelical | NoVB | 87.15 ± 5.70 | 78.93 ± 6.83 | −10.41 |
| VB1.0 | 83.79 ± 2.65 | 82.70 ± 0.98 | −1.77 | |
| VB1.5 | 83.82 ± 1.03 | 82.76 ± 1.44 | −1.28 | |
| VB2.0 | 83.44 ± 3.05 | 83.22 ± 1.91 | −0.26 |
Abbreviations: TD0L, TomoDirect without flash; TD1L, TomoDirect with 1L flash; TD2L, TomoDirect with 2L flash; TD3L, TomoDirect with 3L flash; NoVB, TomoHelical without virtual bolus; VB1.0, TomoHelical with virtual bolus 1.0 cm; VB1.5, TomoHelical with virtual bolus 1.5 cm; VB2.0, TomoHelical with virtual bolus 2.0 cm; OSL, optically stimulated luminescence.
Lee et al 16 measured the skin dose on the patient during TD treatment using five OSLs placed on the patient's skin. The results demonstrated that the total average skin dose for TD was 149.36 cGy (82.98% of prescription dose). The lowest measured dose (3 cm inner) was 137.6 cGy, which was consistent with our study. The inner position of TD0L showed the lowest skin dose for dynamic conditions of 147.35 cGy.
Sung et al 1 predicted skin dose for whole-breast irradiation by Helical Tomotherapy with a 10 mm VB was 79.69% of the prescription dose, which is consistent with our study. The skin dose for VB1.0 cm in the dynamic condition was 82.70%.
With respect to the VB application, the measuring skin dose from the three VB plans was lower than that from the NoVB plan because the calculated dose from treatment planning was not consistent with the actual dose due to the absence of bolus during delivery. For treatment planning, acceptable planning criteria were simply reached owing to the target being under the tissue margin. However, the delivery was not the same, and the skin dose may differ when compared with the calculation.
Regarding the actual delivery dose when applying the VB technique, our previous study demonstrated the verification of dose delivered on helical tomotherapy on the Arccheck for VB application. 17 The VB thickness 0.5, 1.0, and 1.5 cm showed acceptable value for dose distribution with more than 95% passing rate of 99.8% (VB0.5) and 100% (VB1.0), for gamma criteria by 3%/3 mm. The treatment plans with VB less than 15 mm, deliver doses that are comparable to treatment plans which were consistent in our study. The VB of at least 1.0 cm thickness (VB1.0) is able to compensate the skin dose in case of intrafraction movements by presenting a skin dose difference of less than 2% between the static and dynamic conditions. The treatment plans without dose expansion methods showed significant skin dose differences between static and dynamic conditions by 4.83% (TD) and 9.43% (HT), respectively.
However, the deviation of dose verification showed an increasing trend when VB thickness increased. The VB2.0 was not acceptable for point dose and dose distribution verification with more than 2% dose difference and less than 90% of gamma passing rate. 17
For RT centers unable to establish the DIBH technique for breast treatment, free-breathing with flash or VB application is an alternative to ensure coverage of the superficial target under respiration. Because of this study aims to investigate the skin dose in movable phantom, the real situation in the patients still needs to evaluate in further study. In vivo dosimetric study for skin dose measurement during treatment in TD and HT may confirm the application of VB for ensuring the adequacy of skin dose for breast RT due to the breathing motion.
Conclusion
All TD and HT plans demonstrated skin dose reductions under dynamic conditions. The flash two leaves plan (TD2L) showed the lowest skin dose difference between two phantom conditions. According to the HT plans, at least 1.0 cm thickness of VB application showed superiority in maintaining the skin dose following the prescription and even respiratory motion in the HT treatment. However, the deviation of dose verification showed an increasing trend when VB thickness increased. Therefore, Flash retraction in the TD2L and VB1.0 of VB application was useful for compensating the intrafraction movement and maintaining the skin dose according to the prescription.
Abbreviations
- CT
computed tomography
- CW
chest wall
- DIBH
deep inspiratory breath-hold
- HT
Helical Tomotherapy
- OSL
optically stimulated luminescence
- PTV
planning target volume
- RPM
respirations per minute
- RT
radiotherapy
- SF
skin flash
- TD
TomoDirect
- VB
virtual bolus.
Footnotes
Authors’ Contribution: (1) Wannapha Nobnop: The conception and design, acquisition of data, analysis and interpretation of data, drafting the article. (2) Nattaphol Lertananpipat: The conception and design, development the phantom. (3) Anirut Watcharawipha: Measurement, analysis and interpretation of data. (4) Anupong Kongsa: Acquisition of data, analysis and interpretation of data. (5) Damrongsak Tippanya: Acquisition of data, analysis and interpretation of data. (6) Warit Thongsuk: Acquisition of data, analysis and interpretation of data. (7) Imjai Chitapanarux: The conception and design, revising it critically for important intellectual content and final approval of the version to be published.
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding: This study was exemption approved by the Institutional Review Board for Medical Ethics of the Faculty of Medicine, ChiangMai University (study code RAD-2564-08212/Research ID: 8212).
ORCID iD: Wannapha Nobnop https://orcid.org/0000-0002-5266-2845
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