Dosimetric effects of brass mesh bolus on skin dose and dose at depth for postmastectomy chest wall irradiation

Innovative applications of visualized thermosensitive color-changing personalized boluses in post-mastectomy radiotherapy: a dosimetric analysis

BACKGROUND AND PURPOSE

To explore the feasibility and advantages of the visualized thermosensitive color-changing personalized bolus in post-mastectomy radiotherapy (PMRT).

MATERIALS AND METHODS

Forty PMRT patients (June 2023-June 2024) were randomized into two groups. Group A (experimental group, 20 patients) underwent two CT scans: A1 (without compensator) and A2 (with the visualized thermosensitive color-changing personalized bolus), followed by treatment with the thermosensitive color-changing personalized bolus. Group B (control group, 20 patients) also underwent two CT scans: B1 (without bolus) and B2 (with a conventional commercial bolus), followed by treatment with the commercial bolus. Treatment plans were generated for virtual bolus (A1-Plan, B1-Plan) and real bolus (A2-Plan, B2-Plan). A3-Plan (A1-Plan applied to thermosensitive bolus treatment) and B2-Plan (B1-Plan applied to commercial bolus treatment) were compared to evaluate dosimetric differences in target volumes, organs at risk (OARs), and skin toxicity.

RESULTS

In Group A, A1-Plan and A2-Plan showed no significant differences in OAR doses (e.g., ipsilateral lung, heart, contralateral breast, skin Dmax/Dmean) or target metrics (V50Gy, Dmax, homogeneity index (HI), conformity index (CI), monitor units (MU)). A3-Plan compared to A1-Plan had minor differences in target coverage (94.05% vs. 95.14%), HI (0.148 vs. 0.147), and CI (0.83 vs. 0.84). In Group B, B2-Plan had significantly reduced target coverage (89.9% vs. 95%), homogeneity (0.153 vs. 0.136), and conformity (0.817 vs. 0.810) compared to B1-Plan, attributed to air gaps from the commercial bolus. The thermosensitive color-changing personalized bolus had better skin adherence, significantly reduced air cavity volumes (3833 mm³ vs. 21498 mm³), and maintained equivalent dosimetric performance to virtual boluses. Skin toxicity was Grade I in all patients without differences between groups.

CONCLUSIONS

The visualized thermosensitive color-changing personalized bolus demonstrated superior skin adherence, smaller air gaps, and better positional reproducibility compared to commercial boluses. Its dosimetric performance was consistent with virtual bolus plans, ensuring target coverage and OAR protection without increased skin toxicity. These findings support its clinical application in PMRT.

An investigation of high-Z material for bolus in electron beam therapy

Highlight. Electron beam treatment often requires bolus to augment surface dose to nearly 100%. There are no optimum bolus materials and hence a high-Z based clothlike material is investigated to reduce air column in treatment that provides optimum surface dose. This material is well suited as it can be used multiple times and can be sanitized. Characteristics of W-Si material is provided.Purpose /Objective(s). Electron beams are frequently used for superficial tumors. However, due to electron beam characteristics the surface dose is 75-95% of the prescribed dose depending on beam energy thus requiring placement of bolus to augment surface dose. Various types of boluses are commonly used in clinics, each having it's own unique limitation. Most bolus devices do not conform to the skin contour and create airgaps that are known to produce dose perturbations creating hot and cold spots. A cloth-like high-Z materials; Tungsten, (Z = 74) and Bismuth, (Z = 83) impregnated in silicone gel is investigated for electron bolus.Materials/Methods. Super soft silicone-gel based submillimeter thin tungsten and bismuth sheets were investigated for bolus for 6-12 MeV. Parallel plate ion chamber measurements were performed in a solid water phantom on a Varian machine. Depth dose characteristics were measured to optimize the thickness for surface dose to be 100% for selected electron therapy and validated with Monte Carlo simulations.Results. Silicone-gel tungsten and bismuth sheets produce significant electrons thus increasing surface dose. Based on measured depth dose, our data showed that tungsten sheets of 0.14 mm, 0.18 mm and 0.2 mm and Bismuth sheets of 0.42 mm, 0.18 mm and 0.2 mm provide 100% surface dose for 6, 9 and 12 MeV beams, respectively without any significant changes in depth dose except increasing surface dose.Conclusions. The new high-Z clothlike sheets are extremely soft but high tensile metallic bolus materials that can fit flawlessly on any skin contour. Only 0.2 mm thick sheets are needed for 100% surface dose without degradation of the depth dose characteristics. These materials are reusable and ideal for bolus in electron beam treatment. This investigation opens a new frontier in designing new bolus materials optimum for patient treatment.

Post-mastectomy radiotherapy: Impact of bolus thickness and irradiation technique on skin dose

PURPOSE

To determine 10 MV IMRT and VMAT based protocols with a daily bolus targeting a skin dose of 45 Gy in order to replace the 6 MV tangential fields with a 5 mm thick bolus on alternate days method for post-mastectomy radiotherapy.

METHOD

We measured the mean surface dose along the chest wall PTV as a function of different bolus thicknesses for sliding window IMRT and VMAT plans. We analyzed surface dose profiles and dose homogeneities and compared them to our standard 6 MV strategy. All measurements were performed on a thorax phantom with Gafchromic films while dosimetric plans were computed using the Acuros XB algorithm (Varian).

RESULTS

We obtained the best compromise between measured surface dose (mean dose and homogeneity) and skin toxicity threshold obtained from the literature using a daily 3 mm thick bolus. Mean surface doses were 91.4 ± 2.8% [85.7% - 95.4%] and 92.2 ± 2.3% [85.6% - 95.2%] of the prescribed dose with IMRT and VMAT techniques, respectively. Our standard 6 MV alternate days 5 mm thick bolus leads to 89.0 ± 3.7% [83.6% - 95.5%]. Mean dose differences between measured and TPS results were < 3.2% for depths as low as 2 mm depth.

CONCLUSION

10 MV IMRT-based protocols with a daily 3 mm thick bolus produce a surface dose comparable to the standard 6 MV 5 mm thick bolus on alternate days method but with an improved surface dose homogeneity. This allows for a better control of skin toxicity and target volume coverage.

Characterization of brass mesh bolus for electron beam therapy

Purpose. Bolus is often required for targets close to or on skin surface, however, standard bolus on complex surfaces can result in air gaps that compromise dosimetry. Brass mesh boluses (RPD, Inc., Albertville, MN) are designed to conform to the patient's surface and reduce air gaps. While they have been well characterized for their use with photons, minimal characterization exists in literature for their use with electrons.Methods and materials.Dosimetric characteristics of brass mesh bolus was investigated for use with 6, 9 and 12 MeV electrons using a 10 × 10 cm2applicator on standard multi-energy LINAC. Measurements for bolus equivalence and percentage depth doses (PDDs) under brass mesh, as well as surface dose measurements were performed on solid water and a 3D printed resin breast phantom (Anycubic Photon MonoX, Shenzhen, China) using Markus®parallel-plate ionization chamber (Model 34045, PTW Freiburg, Germany), thermoluminescent detectors (TLD) and EBRT film. After obtaining surface dose measurements, these were compared to dose calculated on the Pinnacle3 treatment planning system (TPS, 16.2, Koninklijke Philips N.V.).Results. Measurements of surface dose under brass mesh showed consistently higher dose than without bolus, confirming that brass mesh can increase the PDD at surface up to ∼ 94% of dose at dmax, depending on incident electron energy. This increase is equivalent to using ∼ 7.2 mm water equivalent bolus for 6 MeV, ∼ 3.6 mm for 9 MeV and ∼ 2.2 mm bolus for 12 MeV electrons. TPS results showed close agreement within-vivomeasurements, confirming the potential for brass mesh as bolus for electron irradiation, provided blousing effect is correctly modelled.Conclusions. To increase electron surface dose, a brass mesh can be used with equivalent effect of water-density bolus varying with electron energy. Proper implementation could allow for ease of treatment, as well as increase bolus conformality in electron-only plans.

Analysis of Individualized Silicone Rubber Bolus Using Fan Beam Computed Tomography in Postmastectomy Radiotherapy: A Dosimetric Evaluation and Skin Acute Radiation Dermatitis Survey

Objective: To investigate the dosimetric effects of using individualized silicone rubber (SR) bolus on the target area and organs at risk (OARs) during postmastectomy radiotherapy (PMRT), as well as evaluate skin acute radiation dermatitis (ARD). Methods: A retrospective study was performed on 30 patients with breast cancer. Each patient was prepared with an individualized SR bolus of 3 mm thickness. Fan-beam computed tomography (FBCT) was performed at the first and second fractions, and then once a week for a total of 5 times. Dosimetric metrics such as homogeneity index (HI), conformity index (CI), skin dose (SD), and OARs including the heart, lungs, and spinal cord were compared between the original plan and the FBCTs. The acute side effects were recorded. Results: In targets' dosimetric metrics, there were no significant differences in Dmean and V105% between planning computed tomography (CT) and actual treatments (P > .05), while the differences in D95%, V95%, HI, and CI were statistically significant (P < .05). In OARs, there were no significant differences between the Dmean, V5, and V20 of the affected lung, V5 of the heart and Dmax of the spinal cord (P > .05) except the V30 of affected lung, which was slightly lower than the planning CT (P < .05). In SD, both Dmax and Dmean in actual treatments were increased than plan A, and the difference was statistically significant (P < .05), while the skin-V20 and skin-V30 has no difference. Among the 30 patients, only one patient had no skin ARD, and 5 patients developed ARD of grade 2, while the remaining 24 patients were grade 1. Conclusion: The OR bolus showed good anastomoses and high interfraction reproducibility with the chest wall, and did not cause deformation during irradiation. It ensured accurate dose delivery of the target and OARs during the treatment, which may increase SD by over 101%. In this study, no cases of grade 3 skin ARD were observed. However, the potential of using OR bolus to reduce grade 1 and 2 skin ARD warrants further investigation with a larger sample size.

Comparing brass mesh to tissue equivalent bolus materials for volumetric modulated arc therapy chest wall irradiation

PURPOSE

To compare the superficial dose when using brass mesh bolus (BMB), no bolus, or 3 mm tissue-equivalent bolus with a pseudo-flash volumetric modulated arc therapy (VMAT) breast treatment planning technique.

METHODS

Two different beam arrangements for right-sided irradiation and one beam arrangement for bilateral irradiation were planned on an inhomogeneous thorax phantom in accordance with our clinical practice for VMAT postmastectomy radiotherapy (PMRT). Plans were optimized using pseudo-flash and representative critical organ optimization structures were used to shape the dose. Plans were delivered without bolus, with 3 mm tissue-equivalent bolus (TEB), or with one-layer BMB. Optically stimulated luminescence dosimeter (OSLD) and radiochromic film measurements were taken and analyzed to determine the superficial dose in each case and the relative enhancement from the no bolus delivery.

RESULTS

Superficial dose measured with OSLDs was found to be 76.4 ± 4.5%, 103.0 ± 6.1%, and 98.1 ± 5.8% of prescription for no physical bolus (NB), TEB, and BMB, respectively. Superficial dose was observed to increase from lateral to medial points when measured with film. However, the relative increase in superficial dose from NB was consistent across the profile with an increase of 43 ± 2.1% and 34 ± 3.3% of prescription for TEB and BMB, respectively. The results are in good agreement with expectations from the literature and the experience with tangential radiotherapy.

CONCLUSION

Three millimeter TEB and one-layer BMB were shown to provide similar enhancement to the superficial dose compared to delivery without bolus. BMB, which does not significantly affect dose at depth and is more conformal to the patient surface, is an acceptable alternative to 3 mm TEB for chest wall PMRT patients treated with pseudo-flash PMRT.

Double enhancement effect of a surface dose with tungsten rubber bolus in photon radiotherapy for keloids and superficial tumors

To clarify the dosimetric characteristics of a real-time variable shape rubber-containing tungsten (STR) bolus in a clinical plan and investigate the efficacy of the STR bolus in photon radiotherapy for keloids and other superficial tumors. A 5 mm gel bolus or 1 mm STR bolus was placed on a solid water phantom. Tangential irradiation was performed using a TomoTherapy Radixact-X9 and 6 MV X-ray flattening-filter-free beam, and the surface dose was measured with radiochromic film. Clinical-like plans (TomoDirect; TD and TomoHelical; TH) were applied with the same geometry and the dose distributions were measured. The increase in surface dose by the build-up effect and backscatter was 37.7% and 8.0% for the gel bolus, and 40.5% and 26.4% for the STR bolus, respectively. In the TD and TH plans, the increase in surface dose was 27.4% and 48.3% for the gel bolus, and 39.0% and 63.2% for the STR bolus. Similary, changes in the sagittal plane dose were - 3.9% and 6.1% for the gel bolus, and - 6.3% and 6.9% for the STR bolus. The STR bolus effectively increased the surface dose by the build-up effect and backscatter in photon radiotherapy for keloids and other superficial tumors.

Surface Dose Measurements in Chest Wall Postmastectomy Radiotherapy to Achieve Optimal Dose Delivery with 6 MV Photon Beam

Aim

A tissue-equivalent bolus of sufficient thickness is used to overcome build up effect to the chest wall region of postmastectomy radiotherapy (PMRT) patients with tangential technique till Radiation Therapy Oncology Group (RTOG) Grade 2 (dry desquamation) skin reaction is observed. The aim of this study is to optimize surface dose delivered to chest wall in three-dimensional radiotherapy using EBT3 film.

Materials and Methods

Measurements were conducted with calibrated EBT3 films with thorax phantom under "open beam, Superflab gel (0.5 cm) and brass bolus conditions to check correlation against TPS planned doses. Eighty-two patients who received 50 Gy in 25# were randomly assigned to Group A (Superflab 0.5 cm gel bolus for first 15 fractions followed by no bolus in remaining 10 fractions), Group B or Group C (Superflab 0.5 cm gel or single layer brass bolus, respectively, till reaching RTOG Grade 2 skin toxicity).

Results

Phantom measured and TPS calculated surface doses were within - 5.5%, 4.7%, and 8.6% under open beam, 0.5 cm gel, and single layer of brass bolus applications, respectively. The overall surface doses (OSD) were 80.1% ±2.9% (n = 28), 92.6% ±4.6% (n = 28), and 87.4% ±4.7% (n = 26) in Group A, B, and C, respectively. At the end of treatment, 7 out of 28; 13 out of 28; and 9 out of 26 patients developed Grade 2 skin toxicity having the OSD value of 83.0% ±1.6% (n = 7); 93.7% ±3.2% (n = 13); and 89.9% ±5.6% (n = 9) in Groups A, B, and C, respectively. At the 20^th-23^rd fraction, 2 out of 7; 6 out of 13; and 4 out of 9 patients in Groups A, B, and C developed a Grade 2 skin toxicity, while the remaining patients in each group developed at the end of treatment.

Conclusions

Our objective to estimate the occurrence of optimal dose limit for bolus applications in PMRT could be achieved using clinical EBT3 film dosimetry. This study ensured correct dose to scar area to protect cosmetic effects. This may also serve as quality assurance on optimal dose delivery for expected local control in these patients.

Dosimetric characteristics of a thin bolus made of variable shape tungsten rubber for photon radiotherapy

In this study, we aim to clarify the dosimetric characteristics of a real time variable shape rubber containing tungsten (STR) as a thin bolus in 6-MV photon radiotherapy. The percentage depth doses (PDDs) and lateral dose profiles (irradiation field = 10 × 10  cm²) in the water-equivalent phantom were measured and compared between no bolus, a commercial 5-mm gel bolus, and 0.5-, 1-, 2-, and 3-mm STR boluses. The characteristics of the PDDs were evaluated according to relative doses at 1 mm depth (D_1mm) and depth of maximum dose (d_max). To determine the distance of the shift caused by the STR bolus, the PDD value at a depth of 100 mm without a bolus was obtained. For each STR thickness, the difference between the depth corresponding to this PDD value and 100 mm was calculated. The penumbra size and width of the 50% dose were evaluated using lateral dose profiles. The D_1mm with no bolus, 5-mm gel bolus, and 0.5-, 1-, 2-, and 3-mm STR boluses were 47.6%, 91.5%, 78.2%, 86.6%, 89.3%, and 89.4%, respectively, and the respective d_max values were 15, 10, 13, 12, 11, and 10 mm. The shifting distance of the 0.5-, 1-, 2-, and 3-mm STR boluses were 2.7, 4.4, 4.8, and 4.9 mm, respectively. There were no differences for those in lateral dose profiles. The 1-mm-thick STR thin bolus shifted the depth dose profile by 4.4 mm and could be used as a customized bolus for photon radiotherapy.

Dosimetric assessment of brass mesh bolus and transparent polymer-gel type bolus for commonly used breast treatment delivery techniques

We investigated skin dose enhancements of brass mesh bolus (BMB) and a recently developed transparent polymer-gel bolus (PGB) for clinically relevant breast treatment delivery techniques. The dose enhancement of the breast surface with BMB and PGB were compared to that of tissue-equivalent bolus. Three breast treatment plans were generated on CT scans of an anthropomorphic chest phantom: tangential step-and-shoot 3D conformal (3DCRT) planned using Field-in-Field (FiF), tangential sliding-window 3DCRT using Electronic Compensator (EC), and volumetric modulated arc therapy (VMAT). All plans were created using 6 MV photons and a prescription dose (Rx) of 180 cGy per fraction. Skin doses of all 3 plans were measured with radiochromic films, separately delivered in triplicate. Each plan was delivered to the phantom without bolus, and then with BMB (1 or 2 layers; 3 or 10 mm tissue-equivalent), PGB, and Superflab (3, 5, and 10 mm tissue-equivalent). Doses were determined by reading the radiochromic films with a flatbed scanner, and analyzing the images using a calibration curve for each specific batch. For all bolus types and plans, surface doses averaged over the 3 measurements were between 88.4% and 107.4% of Rx. Without bolus, average measured skin doses were between 51.2% and 64.2% of Rx. Skin doses with BMB and PGB were comparable to that with tissue-equivalent bolus. Over all 3 treatment delivery techniques, using BMB resulted in average skin doses of 92.8% and 102.1% for 1- and 2 layers, respectively, and using PGB results in average skin doses of 94.8%, 98.2%, and 99.7% for 3, 5, and 10-mm tissue-equivalent, respectively. The average measured skin doses with BMB and PGB agreed within ± 3% compared to the tissue-equivalent thickness bolus. We concluded that BMB and PGB are clinically equivalent in skin dose enhancement for breast treatment as the 3, 5, and 10 mm tissue-equivalent bolus.

eXaSkin: A novel high-density bolus for 6MV X-rays radiotherapy

PURPOSE

To evaluate eXaSkin, a novel high-density bolus alternative to commercial tissue-equivalent Superflab, for 6MV photon-beam radiotherapy.

MATERIALS AND METHODS

We delivered a 10 × 10 cm² open field at 90° and head-and-neck clinical plan, generated with the volumetric modulated arc therapy (VMAT) technique, to an anthropomorphic phantom in three scenarios: with no bolus on the phantom's surface, with Superflab, and with eXaSkin. In each scenario, we measured dose to a central planning target volume (PTV) in the nasopharynx region with an ionization chamber, and we measured dose to the skin, at three different positions within the vicinity of a neck lymph node PTV, with MOSkin™, a semiconductor dosimeter. Measurements were compared against calculations with the treatment planning system (TPS).

RESULTS

For the static field, MOSkin results underneath the eXaSkin were in agreement with calculations to within 1.22%; for VMAT, to within 5.68%. Underneath Superflab, those values were 3.36% and 11.66%. The inferior agreement can be explained by suboptimal adherence of Superflab to the phantom's surface as well as difficulties in accurately reproducing its placement between imaging and treatment session. In all scenarios, dose measured at the central target agreed to within 1% with calculations.

CONCLUSIONS

eXaSkin was shown to have superior adaptation to the phantom's surface, producing minimal air gaps between the skin surface and bolus, allowing for accurate positioning and reproducibility of set-up conditions. eXaSkin with its high density material provides sufficient build-up to achieve full skin dose with less material thickness than Superflab.

Verification using in vivo optically stimulated luminescent dosimetry of the predicted skin surface dose in patients receiving postmastectomy radiotherapy

The purpose of this study was to evaluate whether dose to the skin surface underneath bolus, was accurately predicted by a 3D treatment planning system (TPS) in patients receiving 50 Gy/25# postmastectomy radiotherapy (PMRT) using optically stimulated luminescent dosimetry (OSLD) for verification. In vivo dosimetry using OSLDs was performed in 20 consecutive patients receiving PMRT. An array of 9 OSLDs were applied to the chest wall or neobreast in a grid arrangement. Dosimetry data were recorded on 3 separate treatment fractions, averaged, and extrapolated to 25 fractions. On the 3D TPS, the predicted dose was calculated using the departmental planning algorithm at points corresponding to the OSLDs. The mean within patient difference between the planned and measured dose at each of the 9 points was calculated and Bland-Altman limits of agreement used to quantify the extent of agreement. Paired t-tests were used to test for evidence of systematic bias at each point. The coefficient of variation of the 3 OSLD readings per patient at each of the 9 points was low for 8 points (≤4.4%) demonstrating comparable dose received per fraction at these points. The mean ratio between the in vivo measured extrapolated OSLD (IVME OSLD) dose and the planned TPS dose ranged between 0.97 and 0.99 across all points (standard deviation range 0.05 to 0.08). The mean within patient difference between the IVME OSLD and planned TPS was <1 Gy at 7 of the 9 points and the t-test for evidence of systematic bias was significant (p = 0.03) at only 1 of the 9 points. Our commercially available 3D TPS closely predicted PMRT skin surface dose underneath bolus as verified by OSLDs. At all sites, the average ratio of delivered to predicted dose was >0.97 but <1. This practical and feasible OSLD assessment of only 3 of 25 fractions facilitates quality assurance of a TPS in predicting skin surface dose under bolus.

In vivo dosimetry using MOSkin detector during Cobalt-60 high-dose-rate (HDR) brachytherapy of skin cancer

The MOSkin, a metal-oxide semiconductor field-effect transistor based detector, is suitable for evaluating skin dose due to its water equivalent depth (WED) of 0.07 mm. This study evaluates doses received by target area and unavoidable normal skin during a the case of skin brachytherapy. The MOSkin was evaluated for its feasibility as detector of choice for in vivo dosimetry during skin brachytherapy. A high-dose rate Cobalt-60 brachytherapy source was administered to the tumour located at the medial aspect of the right arm, complicated with huge lymphedema thus limiting the arm motion. The source was positioned in the middle of patients' right arm with supine, hands down position. A 5 mm lead and 5 mm bolus were sandwiched between the medial aspect of the arm and lateral chest to reduce skin dose to the chest. Two calibrated MOSkin detectors were placed on the target and normal skin area for five treatment sessions for in vivo dose monitoring. The mean dose to the target area ranged between 19.9 and 21.1 Gy and was higher in comparison with the calculated dose due to contribution of backscattered dose from lead. The mean measured dose at normal skin chest area was 1.6 Gy (1.3-1.9 Gy), less than 2 Gy per fraction. Total dose in EQD₂ received by chest skin was much lower than the recommended skin tolerance. The MOSkin detector presents a reliable real-time dose measurement. This study has confirmed the applicability of the MOSkin detector in monitoring skin dose during brachytherapy treatment due to its small sensitive volume and WED 0.07 mm.