Detector response and dose calculation lateral to material interfaces for 6 MV photon beam
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-10-21 |
| Journal | Journal of Applied Clinical Medical Physics |
| Authors | Jonas Ringholz, Otto A. Sauer, Sonja Wegener |
| Institutions | UniversitĂ€tsklinikum WĂŒrzburg |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Core Challenge Addressed: Systematic study and quantification of detector response and dose calculation errors near high-density material interfaces (simulating bone/tissue boundaries) using a 6 MV photon beam.
- Detector Performance: Detector signals showed significant variability (up to 2.5% difference) near the aluminum cylinder interface due to non-equilibrium effects (electron scatter, volume averaging).
- Algorithm Discrepancies: Commercial treatment planning system (TPS) algorithms exhibited large differences (up to 3% maximum deviation) compared to measured detector values, particularly for the older Collapsed Cone algorithms, which failed to model backscattering.
- Recommended Detectors: The PinPoint Ion Chamber and the microDiamond detector provided the most consistent and reliable relative dose measurements in the vicinity of the inhomogeneity.
- Calculation Recommendation: Verification of dose calculation near inhomogeneities requires using advanced algorithms (Monte Carlo or Acuros XB) and corroborating results with measurements from at least two different detector types.
- Small Field Findings (2x2 cm2): In the small field setup, dose ratios suggested higher measured dose (up to 0.005 higher at x = 20 mm) than calculated dose, indicating complex interactions outside the primary beam.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Photon Beam Energy | 6 | MV | Elekta Synergy linac |
| Source-to-Surface Distance (SSD) | 92.5 | cm | Phantom setup |
| Measurement Depth | 7.5 | cm | In water phantom |
| Inhomogeneity Material | Aluminum Alloy | N/A | Simulating bone |
| Aluminum Density (Measured) | 2.8 | g/cm3 | Used in physical setup |
| Aluminum Density (TPS/MC Model) | 2.7 | g/cm3 | Used in calculation |
| Aluminum Cylinder Diameter | 3 | cm | Inhomogeneity size |
| Field Size (Large) | 10 x 10 | cm2 | Standard field |
| Field Size (Small) | 2 x 2 | cm2 | Small field |
| Maximum Signal Increase (Ion Chamber) | 2.5 | % | 3 mm from cylinder edge (10x10 cm2 field) |
| Minimum Signal Increase (Shielded Diode) | 1.0 | % | 3 mm from cylinder edge (10x10 cm2 field) |
| Maximum TPS Dose Difference (10x10 cm2) | 3 | % | Collapsed Cone vs. Detector values |
| Ion Chamber Active Diameter (PinPoint 3D) | 2.9 | mm | Monte Carlo model |
| Film Active Layer Thickness (EBT3 Model) | 28 | ”m | Used in EGSnrc simulation |
| TPS Dose Grid Resolution | 1 or 2 | mm | Used in Pinnacle and Eclipse calculations |
Key Methodologies
Section titled âKey Methodologiesâ-
Phantom and Irradiation Setup:
- An aluminum cylinder (3 cm diameter) was placed in a PTW MP3-XS water phantom at a depth of 7.5 cm.
- Irradiation was performed using a 6 MV photon beam at 92.5 cm SSD with nominal field sizes of 10x10 cm2 and 2x2 cm2.
- Measurements were taken radially outwards from the cylinder surface.
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Detector Measurement:
- Five detector types were used consecutively: PinPoint 3D Ion Chamber, microDiamond, Shielded Diode (60016), Unshielded Diode (60012), and EBT3 radiochromic film.
- The PinPoint chamber and the unshielded diode were tested in radial orientation to minimize the distance to the interface.
- All detectors were cross-calibrated in a homogeneous water phantom setup at the same measurement positions.
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Treatment Planning System (TPS) Calculation:
- CT images of the setup were imported into Pinnacle (v16.2.1) and Eclipse (v18.0).
- Dose calculations utilized various algorithms: Acuros XB (Varian), Photon Monte Carlo (Raystation), and Collapsed Cone Convolution (Philips/Raystation).
- Dose was calculated with a spatial resolution of 1 mm or 2 mm, depending on computational capacity.
-
Monte Carlo (MC) Simulation (EGSnrc):
- The DOSRZnrc package was used for radial symmetric geometry simulations.
- The 6 MV spectrum was modeled using 17 monoenergetic photon energies (0.1 MeV to 8 MeV).
- Detailed MC models were created for the PinPoint chamber, the unshielded diode, and the EBT3 film (including a 28 ”m active layer model) to accurately simulate dose-to-medium.
- Inter-leaf leakage was factored into the small field simulations.
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Data Processing:
- Measured signals and calculated doses were normalized to the central axis dose in the 10x10 cm2 homogeneous water phantom.
- Ratios of profiles (with cylinder / without cylinder) were calculated to isolate the effect of the inhomogeneity.
- Volume averaging effects were estimated for the PTW-60012 diode data by fitting and integrating an exponential function over the detection dimensions.
Commercial Applications
Section titled âCommercial Applicationsâ- Clinical Radiotherapy Quality Assurance (QA): Essential for validating dose delivery in complex patient geometries, particularly in the head-and-neck or lung regions where bone or metal implants create significant inhomogeneities.
- Advanced TPS Commissioning: Provides critical benchmark data for commissioning and validating Type C (Acuros XB, Monte Carlo) dose calculation engines, ensuring they accurately model electron transport and scatter near high-Z materials.
- Detector Selection and Protocol: The findings guide clinical physicists in selecting appropriate detectors (PinPoint Ion Chamber and microDiamond are preferred) and developing robust protocols for measuring dose in steep dose gradient and non-equilibrium regions.
- Stereotactic Body Radiotherapy (SBRT): Directly applicable to SBRT treatments targeting tumors adjacent to or within bone (e.g., spine SBRT), where dose accuracy within a few millimeters of the interface is paramount.
- Dosimetry Tool Development: The Monte Carlo models developed for the detectors (PinPoint, Diode) provide a foundation for future studies aiming to derive specific correction factors for detector response in highly inhomogeneous fields.
View Original Abstract
Abstract Background Dose calculation around inhomogeneities is challenging for many algorithms. A validation of dose distributions in these conditions is not straightforward, as detector response close to material interfaces is affected by the nonâequilibrium situation, the changing energy spectrum and the volume effect in a steep dose gradient. Purpose Detector response lateral to an inhomogeneity of high density was studied, mimicking the situation of bone surrounded by soft tissue. Methods Profiles obtained with different detectors (diodes, ion chamber, synthetic diamond) in water in the vicinity of an aluminum cylinder were compared. Dose deposition was also calculated with different commercial treatment planning systems and algorithms as well as using Monte Carlo simulations within and lateral to the cylinder and compared to measurements. Results Dose deposition in the vicinity of an inserted aluminum cylinder changes and is registered by the detectors to a different degree. The ion chamber shows the largest change irradiated with a 10Ă10 cm 2 at 3 mm distance from the cylinder surface (2.5%), followed by the synthetic diamond (1.7%), then the unshielded (1.4%) and finally the shielded diode (1.0%). Dose calculated by different commercial dose engines differed up to 3% at that point from the detector values (Collapsed Cone). Conclusions Dose calculation near inhomogeneities depends on the used algorithm, dose measurements in the same region differ depending on the detector type used. We recommend verification of dose calculation with second type of algorithm and measurements with at least two detector types.