Clinical in‐vivo dosimetry for total skin electron therapy using gafchromic™ EBT4 film

Dosimetric and clinical validation of a high‐density bolus for superficial skin tumors treated with radiotherapy

Abstract Background Accurate dose deposition in superficial radiotherapy remains challenging due to the inherent skin‐sparing effect of megavoltage photon beams and difficulties in ensuring bolus conformity. Purpose This study aims to comprehensively evaluate the dosimetric characteristics of the room‐temperature‐malleable high‐density bolus, to evaluate the accuracy of dose calculation and to assess clinical outcomes in a clinical cohort of 55 patients. Materials and Methods The dosimetric validation was performed through the use of dose measurements in a solid water phantom on top of which a 1.0 cm thick high‐densitybolus was positioned. Absolute depth dose measurements beyond the bolus were obtained by combining an absolute point dose (ion chamber) measurement at 1.0 cm depth beyond the bolus with relative depth dose measurements by using a microDiamond detector. Measurements were performed using 6 megavoltage (MV) flattening‐filter‐free (FFF) photon beams on an Halcyon and on a TrueBeam treatment unit. Obtained data were compared to dose calculations with both photon dose calculations available in the Eclipse treatment planning system (TPS): Analytical Anisotropic Algorithm (AAA) and Acuros XB (AXB). The impact of non‐conforming interfaces was assessed by introducing 0.5 to 2.0 cm thick air cavities between bolus and skin. Skin dose loss due to the air cavities was measured for static open fields and for dynamic sweeping gap fields. Clinical outcomes of 55 patients treated between October 2023 and March 2025 were retrospectively analyzed. Results Depth dose measurements confirm that a 1.0 cm high‐density bolus was sufficient to effectively overcome the build‐up region of the 6FFF photon beam. The AXB dose calculation algorithm provided the best agreement between measurements and calculations. Air gaps between the bolus and the skin resulted in clinically relevant skin dose reductions, ranging from approximately 1% to 20%, depending on field size and gap thickness, especially for clinical target volumes < 5.0 cm, large (> 1.0 cm) air gaps, or the use of large static fields. Among the 55 included patients, acute dermatitis toxicity was predominantly Grade 1 (49%) or Grade 2 (47.0%), with a low incidence of severe adverse events (Grade III: 4 %, Grade IV: 0%). Conclusion The high‐density bolus demonstrated satisfying dosimetric reliability, particularly when modeled with the Acuros XB algorithm. The bolus addressed the build‐up challenge inherent to MV photons, even in the presence of small air gaps, without increased toxicity in a large cohort of patients. This strategy represents a viable and accessible solution for ensuring accurate and conformal dose delivery in superficial tumors.

In vivo rectal dosimeter with MRI marker

Background Hypofractionated external beam radiotherapy for prostate cancer necessitates precise rectal dose evaluation. We fabricated a radiochromic polyurethane-based in vivo rectal dosimeter with a custom MRI marker and a patient-specific applicator for in vivo dose verification (IDV) during gated MR-image guided radiotherapy (MR-IGRT). Methods The dosimeter featured a radiochromic polyurethane active layer, incorporating leucomalachite green (LMG) and tartrazine. To accommodate anatomical variations, a detachable PMMA applicator was designed in five sizes. For localization, four elastomeric materials, including two polyurethane-based and two silicone-based materials, were evaluated as candidate MRI markers. Post-irradiation fading was assessed over time to evaluate measurement stability. A dose-response calibration was performed to establish a linear relationship between net optical density (OD) and absorbed dose. Furthermore, dose uncertainty was analyzed based on the law of error propagation. For verification, in vivo measurements were conducted for two patients and compared with TPS-calculated doses. Results Vyta Flex 20 was selected as the optimal MRI marker due to its high signal intensity and ease of fabrication. Post-irradiation net OD showed a dose-dependent temporal response, and the readout time was standardized to 2 h. The dosimeter’s sensitivity was 0.00253 cGy-1. Dose uncertainties were determined to be 2.1%, 2.0%, and 1.6% at 100, 200, and 300 cGy, respectively. In vivo verification showed mean dose differences of 3.7 ± 1.4% (95% CI, 0.1–7.3%) for patient #1 and 5.9 ± 2.1% (95% CI, 0.8–11.1%) for patient #2. Measured doses were consistently higher than TPS-calculated doses, suggesting possible contributions from localization uncertainty in high-dose-gradient regions and the material-dependent response of the radiochromic active layer. Conclusion The fabricated radiochromic dosimeter with a custom MRI marker and adjustable applicator demonstrated preliminary feasibility as a proof-of-concept system for in vivo rectal dose verification during MR-IGRT. Further studies with larger patient cohorts and more treatment fractions are required to validate its reproducibility, statistical robustness, and clinical utility. Further refinement in positioning is also needed, particularly in dose-gradient regions.

Evaluation of skin dose robustness in breast radiotherapy using VMAT planning technique: a phantom-based surface dosimetry study

Objective. Minor anatomical changes from evolving edema during breast radiotherapy can compromise skin dose, particularly for volumetric modulated arc therapy (VMAT). This study experimentally evaluated the robustness of conventional, skin-flash, and robust-optimization-based VMAT techniques against simulated contour variations using phantom-based surface dosimetry. Approach. Two configurations of an anthropomorphic phantom simulated clinical scenarios: with a breast attachment (large/reconstructed) and without (small/postmastectomy flat chest wall). Four plans were compared: 3D-CRT, conventional VMAT, VMAT with skin-flash, and VMAT with robust optimization (RO). Plan robustness was assessed via treatment planning system (TPS) simulations and physical optically stimulated luminescence dosimeter (OSLD) measurements at five surface points. To simulate controlled rigid surface contour change, the couch was shifted anterior-posteriorly by ±2, ±4, and ±6 mm as a simplified surrogate for edema-induced variations. Shifted OSLD measurements were normalized to the 0 mm baseline, and absolute dose deviations were evaluated against the 3D-CRT reference. Main Results. TPS simulations suggested VMAT with skin-flash offered the greatest stability; however, physical measurements showed the opposite. Conventional VMAT was more variable than 3D-CRT (SD 9.47% vs 3.62% at ±6 mm), and the evaluated VMAT with skin-flash implementation was the least robust (SD 12.60%; worst-case minimum dose 62.49% with 75% of points outside ±5%), despite its favorable TPS prediction. In contrast, VMAT with RO approached the stability of 3D-CRT (SD 5.48%) and showed even smaller deviations than 3D-CRT in the phantom with a breast attachment. Significant. These physical measurements demonstrate that, contrary to TPS predictions, the evaluated VMAT with skin-flash implementation does not ensure skin-dose robustness, whereas VMAT with RO substantially improved robustness to a level comparable to that of 3D-CRT, with the greatest benefit observed in the phantom with a breast attachment.

Are Water-Based Algorithms Still Adequate for Dose Calculation in Modern HDR Interventional Radiotherapy? A Review with Special Focus on 3D-Printed Applicators and Heterogeneous Materials

Background: High-dose-rate interventional radiotherapy (HDR IRT) relies on accurate dose calculations to ensure safe and effective treatment. The AAPM TG-43 formalism, based on homogeneous water assumptions, has long been the clinical standard, but the growing use of patient-specific applicators and heterogeneous materials challenges its accuracy. Materials and Methods: A literature narrative review was performed using PubMed and Scopus to identify studies comparing TG-43 and model-based dose calculation algorithms (MBDCAs), including deterministic methods (ACE, Acuros BV) and Monte Carlo simulations. Thirty-five studies were selected and qualitatively analyzed. Results: TG-43 yielded higher dose compared with MBDCAs, particularly in the presence of tissue heterogeneities, air gaps, shielding materials, and limited scatter conditions. Differences ranged from 2 to 5% in relatively homogeneous settings to more than 10–20% in complex geometries such as superficial mould treatments and head-and-neck IRT. Model-based approaches showed better agreement with Monte Carlo simulations and experimental measurements, especially in contact HDR IRT and applications involving 3D-printed applicators. Conclusion: While TG-43 remains clinically established and widely used, its limitations are increasingly evident in modern personalized HDR IRT. Model-based dose calculation algorithms provide greater dosimetric accuracy and should be progressively integrated into clinical practice, particularly in treatments involving significant heterogeneities.

An end-to-end quality assurance procedure for pelvic VMAT using optically stimulated luminescence dosimeters and an anthropomorphic phantom.

Skin dosimetry in megavoltage radiotherapy using hydrogenated amorphous silicon on a tissue-equivalent flexible substrate

Acute skin toxicity after radiation treatments highlights the need for improved dose monitoring and measurement, as the treatment planning system does not accurately estimate skin dose. Wearable radiation detectors may be the solution for most in vivo dosimetry needs, but they require innovative materials. The purpose of this work is to overcome the challenges that limit accurate skin dosimetry by exploring the use of hydrogenated amorphous silicon (a-Si:H) fabricated on a flexible polyimide substrate for direct radiation detection. The a-Si:H detector was fabricated on a 125 μm polyimide substrate with a thickness of 3.6–10 μm. The detector’s response to the build-up region of the percentage depth dose (PDD) of the MV photon beam was compared to Geant4 radiation transport simulations and benchmarked to Attix ionization chamber measurements. Angular dependency and surface field factors at the phantom surface were compared to Attix IC. The PDD measurement is within 2% of Geant4’s simulation and the Attix chamber’s response from 150 μ m to 25 mm in a plastic water phantom. All samples showed linear dose responses with 0.37% reproducibility. The a-Si:H device matches the Attix chamber for surface field factor measurements (6–10 MV photon beams, for 5 to 25 cm field sizes). The angular dependence (−60° to +60°) compared to the Attix IC confirms the sensor’s WED of 150 ± 5 µm. This study demonstrates that a-Si:H sensors on flexible plastic substrates have a potential for accurate surface dose measurements and agree with reference detectors. This thin, flexible detector provides real time measurements and is stable under high radiation doses. The possibility to assemble with ease an array of a-Si:H pixels over large areas with different sizes and shapes, makes this technology attractive for in vivo dosimetry.