Experimental determination of the lateral dose response functions of detectors to be applied in the measurement of narrow photon-beam dose profiles

At a Glance

Metadata	Details
Publication Date	2015-11-19
Journal	Physics in Medicine and Biology
Authors	Daniela Poppinga, Jutta Meyners, B. Delfs, A Muru, D. Harder
Institutions	Pius Hospital Oldenburg
Citations	23
Analysis	Full AI Review Included

6CCVD Technical Analysis: Lateral Dose Response in Narrow Photon Beams

Paper Analyzed: Experimental determination of the lateral dose response functions of detectors to be applied in the measurement of narrow photon-beam dose profiles. (Poppinga et al., 2015 Phys. Med. Biol. 60 9421)

This technical documentation provides an analysis of the research concerning the characterization of synthetic diamond detectors for high-resolution dosimetry, aligning the material requirements with the specialized manufacturing capabilities of 6CCVD.

Executive Summary

Core Finding Confirmed: The study experimentally confirmed the Monte-Carlo prediction that high-density detectors, specifically synthetic MPCVD diamond (PTW 60019) and silicon, exhibit significant negative curve portions in their lateral dose response functions $K(x)$.
Physical Mechanism: These negative portions are crucial indicators of the perturbation of the secondary electron field caused by the detector’s enhanced electron density (3.2 relative to water for diamond) compared to the surrounding water phantom.
Application Impact: The demonstrated negative $K(x)$ values are directly linked to the overresponse observed in diamond detectors during small-field output measurements in advanced radiotherapy techniques (e.g., VMAT, IMRT).
Measurement Technique: The response functions $K(x)$ were accurately determined using a 0.5 mm $6$ MV photon slit beam setup combined with high-spatial-resolution radiochromic film (EBT3) dosimetry and analytical deconvolution using Gaussian fits.
Solution Pathway: Knowing the exact $K(x)$ kernel allows engineers to accurately deconvolve measured signal profiles $M(x)$ to obtain the true dose profiles $D(x)$, thereby providing essential correction factors for clinical small-field applications.
Material Resolution Requirement: The microDiamond detector achieved high resolution due to its ultra-thin sensitive volume (approximately $1$ µm thickness), validating the need for ultra-high precision SCD manufacturing.

Technical Specifications

The following parameters pertain to the synthetic diamond detector used in the study and the experimental setup utilized for determining the dose response function $K(x)$.

Parameter	Value	Unit	Context
Detector Material Type	Synthetic Diamond	N/A	PTW 60019 MicroDiamond
Sensitive Volume Diameter	2.2	mm	Sensitive area, rotationally symmetric
Sensitive Volume Thickness (Approx.)	1	µm	Implied thickness, contrasting the Si diode ($30$ µm)
Mass Density (Diamond)	3.5	g cm^-3	High density compared to water (1.0 g cm^-3)
Relative Electron Density	3.2	N/A	Relative to water (Key factor causing secondary electron perturbation)
Radiation Source	$6$	MV	Photon beam energy
Slit Beam Width	0.5	mm	Narrow beam used for high-resolution profiling
Phantom Material	Water	N/A	Large water phantom
Measurement Depth	3	cm	Depth for cross-beam profile acquisition
Deconvolution Fit Method	Weighted Sum of 5	Gaussian Functions	Used to obtain $K(x)$ in the frequency domain

Key Methodologies

The experimental determination of the detector-specific 1D lateral dose response function $K(x)$ relied on a specialized high-resolution setup and advanced deconvolution techniques:

Narrow Beam Setup: A $6$ MV photon beam was collimated into a highly defined $0.5$ mm wide slit using a tertiary collimator (lead blocks) positioned $8.5$ cm above the water phantom surface (SSD = $80$ cm).
True Dose Profile Measurement ($D(x)$): The undisturbed true cross-beam dose profile $D(x)$ was measured at $3$ cm depth using high-spatial-resolution radiochromic film (EBT3).
Detector Signal Profile Measurement ($M(x)$): The perturbed signal profiles $M(x)$ were recorded by scanning the synthetic diamond (PTW 60019) and other detectors across the slit beam in fine steps ($0.1$ mm step width).
Gaussian Fitting of Profiles: Both $D(x)$ and $M(x)$ profiles were fitted using weighted sums of three centered 1D Gaussian functions to reduce noise and discretization effects inherent in experimental data.
Analytical Deconvolution: The fitted profiles were analytically Fourier transformed. The Fourier transform of the response kernel $FT[K(x)]$ was obtained by dividing $FT[M(x)]$ by $FT[D(x)]$.
Response Function Synthesis: $FT[K(x)]$ was subsequently fitted by a weighted sum of five centered Gaussian functions (Equation 10). The final 1D lateral dose response function $K(x)$ was obtained by analytical inverse Fourier transformation of this sum (Equation 11).
Rotational Kernel Calculation: The rotationally symmetric 2D kernel $K(r)$ was derived from the measured 1D projection $K(x)$ using the Hankel transformation (mathematically demonstrated by relating Equation 11 and Equation 12).

6CCVD Solutions & Capabilities

The findings underscore the critical role of material density and precise dimensional control, especially sensitive volume thickness, in minimizing the volume effect and accurately characterizing the secondary electron field perturbation ($K(x)$).

Applicable Materials

To replicate or advance high-resolution photon dosimetry research, 6CCVD recommends materials with the tightest tolerances:

Single Crystal Diamond (SCD) Substrates: Essential for fabricating high-purity, ultra-thin sensitive volumes ($0.1$ µm to $500$ µm thickness range). The high crystallographic perfection ensures consistent electronic properties crucial for precise charge collection, surpassing the performance of commercial polycrystalline detectors.
- Recommendation for Sensitive Volume: Optical or Electronic Grade SCD, polished to Ra < 1 nm, ensuring ultra-flat surfaces necessary for uniform contacts and thin layer deposition (e.g., $1$ µm sensitive layer thickness, as utilized in the PTW 60019).
Polycrystalline Diamond (PCD): Available in larger custom sizes (up to $125$ mm) for developing large-area arrays or specialized detectors where the highest purity SCD is not required. 6CCVD offers high-quality polishing for inch-size PCD to Ra < 5 nm.
Boron-Doped Diamond (BDD): Ideal for robust electrodes or highly conductive contact layers due to its metallic properties.

Customization Potential

The experimental setup necessitates precision manufacturing beyond standard off-the-shelf components. 6CCVD directly addresses these needs:

Requirement from Paper	6CCVD Engineering Solution	Impact / Sales Driver
Ultra-Thin Sensitive Volume (approx. 1 µm)	Custom Thickness SCD: Fabrication down to $0.1$ µm with tight uniformity control.	Enables next-generation ultra-high-resolution detectors that minimize volume averaging effects.
Complex Electrical Contacting	Precision Metalization: In-house deposition of Au, Pt, Pd, Ti, W, and Cu layers.	Facilitates robust electrode design necessary for reliable signal collection and minimizing noise.
Non-Standard Detector Shapes	Laser Cutting and Shaping: High-precision laser modification for custom geometric designs (e.g., small circular or slit-like sensitive areas).	Allows researchers to optimize detector geometry for specific narrow-beam applications and Monte Carlo verification studies.
Global Logistics	Global Shipping Expertise: DDU default, DDP available.	Ensures rapid and reliable delivery of specialized diamond materials to research facilities worldwide.

Engineering Support

Understanding and correcting for the observed negative response function portions in diamond detectors requires expert knowledge in both material physics and radiation transport modeling. 6CCVD’s in-house PhD team can assist researchers and engineers with material selection, defect engineering, and modeling parameters necessary for small field dosimetry projects, particularly those focused on:

Minimizing secondary electron field perturbation effects.
Designing SCD detectors with optimized thickness for superior spatial resolution.
Integrating custom metal contacts for low-noise readout in clinical environments.

Call to Action: For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

This study aims at the experimental determination of the detector-specific 1D lateral dose response function K(x) and of its associated rotational symmetric counterpart K(r) for a set of high-resolution detectors presently used in narrow-beam photon dosimetry. A combination of slit-beam, radiochromic film, and deconvolution techniques served to accomplish this task for four detectors with diameters of their sensitive volumes ranging from 1 to 2.2 mm. The particular aim of the experiment was to examine the existence of significant negative portions of some of these response functions predicted by a recent Monte-Carlo-simulation (Looe et al 2015 Phys. Med. Biol. 60 6585-607). In a 6 MV photon slit beam formed by the Siemens Artiste collimation system and a 0.5 mm wide slit between 10 cm thick lead blocks serving as the tertiary collimator, the true cross-beam dose profile D(x) at 3 cm depth in a large water phantom was measured with radiochromic film EBT3, and the detector-affected cross-beam signal profiles M(x) were recorded with a silicon diode, a synthetic diamond detector, a miniaturized scintillation detector, and a small ionization chamber. For each detector, the deconvolution of the convolution integral M(x) = K(x) ∗ D(x) served to obtain its specific 1D lateral dose response function K(x), and K(r) was calculated from it. Fourier transformations and back transformations were performed using function approximations by weighted sums of Gaussian functions and their analytical transformation. The 1D lateral dose response functions K(x) of the four types of detectors and their associated rotational symmetric counterparts K(r) were obtained. Significant negative curve portions of K(x) and K(r) were observed in the case of the silicon diode and the diamond detector, confirming the Monte-Carlo-based prediction (Looe et al 2015 Phys. Med. Biol. 60 6585-607). They are typical for the perturbation of the secondary electron field by a detector with enhanced electron density compared with the surrounding water. In the cases of the scintillation detector and the small ionization chamber, the negative curve portions of K(x) practically vanish. It is planned to use the measured functions K(x) and K(r) to deconvolve clinical narrow-beam signal profiles and to correct the output factor values obtained with various high-resolution detectors.

Tech Support

Original Source

DOI: https://doi.org/10.1088/0031-9155/60/24/9421