Diamond Detectors for Radiotherapy X-Ray Small Beam Dosimetry

At a Glance

Metadata	Details
Publication Date	2021-04-16
Journal	Frontiers in Physics
Authors	C. Talamonti, K. Kanxheri, S. Pallotta, L. Servoli
Institutions	Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, University of Florence
Citations	43
Analysis	Full AI Review Included

Executive Summary

This review details the use and performance of synthetic Chemical Vapor Deposition (CVD) diamond detectors for high-precision X-ray small beam dosimetry in modern radiotherapy.

Dosimetry Challenge: Advanced techniques (IMRT, VMAT) use small, non-uniform radiation fields (< 3x3 cm²) characterized by high dose gradients and lack of lateral electron equilibrium, rendering traditional ionization chambers inadequate.
Diamond Value Proposition: Diamond is nearly water-equivalent (low atomic number Z ~ 6/7) and possesses high spatial resolution, making it an ideal candidate for minimizing charged particle fluence perturbations and volume averaging effects.
Fabrication Advancement: The availability of reproducible, low-cost synthetic diamonds (both single-crystal, scCVD, and polycrystalline, pcCVD) has enabled their diffusion into clinical dosimetry, overcoming the limitations of rare natural diamonds.
Performance Metrics: Both commercial (e.g., PTW 60019 microDiamond) and prototype devices demonstrate excellent signal linearity (deviation typically < 2%), high sensitivity (up to 215 nC Gy^-1 mm^-3), and minimal energy or dose-rate dependence.
Advanced Geometries: Research is focused on developing highly segmented, large-area 1D/2D detectors (e.g., DIAPIX, 3DOSE) using pcCVD matrices and 3D columnar electrodes to provide complete dose maps in a single measurement, crucial for dynamic treatments.
Operational Modes: Detectors are typically operated in photovoltaic (Schottky barrier, zero-bias) mode to eliminate polarization effects and reduce dependence on substrate crystalline quality, or in photoconduction (biased) mode for higher sensitivity.

Technical Specifications

Parameter	Value	Unit	Context
Small Field Threshold	< 3 x 3	cm²	Field size where standard dosimetry fails
scCVD Max Area	< 1	cm²	Maximum size achievable using HPHT seeds
pcCVD Max Area	25 x 25	mm²	Maximum size achievable for polycrystalline films
CVD Growth Temperature	~700	°C	Microwave Plasma Enhanced CVD (MPCVD)
HPHT Growth Temperature	> 1300	°C	High Pressure High Temperature synthesis
HPHT Growth Pressure	Several	GPa	High Pressure High Temperature synthesis
CEA-SCDD Active Volume	0.534	mm³	Single crystal CVD prototype
PTW-SCDD Active Volume	0.004	mm³	Commercial microDiamond (PTW 60019)
CEA-SCDD Sensitivity	215	nC Gy^-1 mm^-3	At -50 V bias
3DOSE Cell Size	70 x 114	µm²	Area of 3D columnar electrode cells
3DOSE Sensitive Volume	n x 4 10	mm^-3	Smallest sensitive volume pixels
DIAPIX Matrix Size	12 x 12	pixels	Polycrystalline 2D array
DIAPIX Pixel Pitch	2	mm	Spacing between sensitive volumes
DIAPIX Dose Rate Range	50-500	cGy/min	Range tested for rise/fall times
3DOSE Rise Time	0.5	s	Signal response time
3DOSE Fall Time	0.3	s	Signal response time
Raman Peak (Pure Crystal)	1,332.8	cm^-1	Used for crystal structure analysis
Dose Linearity (Max Deviation)	< 2	%	Observed maximum deviation for 3DOSE prototype
Dose Rate Dependence (A value)	0.975 ± 0.003	N/A	Measured for SCDDo (close to ideal A=1)

Key Methodologies

Substrate Growth: Synthetic diamond substrates are primarily grown using Chemical Vapor Deposition (CVD).
- Single Crystal (scCVD): Grown homo-epitaxially on HPHT seeds, resulting in high-quality material suitable for high-performance point detectors.
- Polycrystalline (pcCVD): Grown hetero-epitaxially on non-diamond matrices or nano-crystal seeds, enabling larger area detectors (e.g., 2D arrays) despite having structural defects (grain boundaries).
Electrode Fabrication and Geometry:
- Planar (Sandwich) Configuration: Electrodes (often Al, Cr/Au, or Ag) are deposited on the front and back surfaces. This configuration supports both biased (photoconduction) and zero-bias (photovoltaic/Schottky) operation.
- 3D Columnar Electrodes: Electrodes are fabricated directly inside the diamond bulk using pulsed laser techniques to create highly segmented, small-volume cells perpendicular to the surface (e.g., 3DOSE).
- Ohmic Contact Development: Focus on stable contacts, often using H-terminated or oxygenated diamond surfaces, or incorporating Diamond-Like Carbon (DLC) layers (1-3 nm thick) to facilitate charge carrier tunneling and improve adhesion.
Detector Operation:
- Photovoltaic Mode (Schottky): Utilizes the built-in electric field at the metal-diamond interface, requiring no external bias. This minimizes bulk trapping effects and polarization, leading to faster response dynamics.
- Photoconduction Mode (Ohmic): Requires a constant external bias (up to 1 V/µm) to drift charged carriers, often used for higher sensitivity, but susceptible to polarization effects from deep defects.
Readout Systems:
- Single Channel: Standard electrometers (e.g., Keithley) used for point detectors (like PTW microDiamond).
- Multichannel/Integrated ASICs: Required for 2D pixelated arrays (DIAPIX, 3DOSE). Advanced Application-Specific Integrated Circuits (ASICs) are integrated close to the detector to handle simultaneous readout of multiple pixels, converting charge into counts for real-time dose mapping.

Commercial Applications

The primary application focus is high-precision dosimetry in clinical radiotherapy, driven by the need for accurate measurements in complex, highly conformal treatments.

Clinical Radiotherapy:
- Small Field Dosimetry: Used as a reference detector for measuring output factors and dose profiles in fields < 3x3 cm² (e.g., CyberKnife, GammaKnife, modern Linacs).
- Intensity Modulated Radiation Therapy (IMRT/VMAT): Used for dose verification and quality assurance (QA) due to fast response times capable of tracking rapid beam intensity changes.
- Stereotactic Radiotherapy (SRT): Used for high-dose, high-gradient measurements requiring extremely high spatial resolution.
Commercial Products:
- PTW 60019 microDiamond: Currently the only commercially available single-crystal diamond dosimeter routinely used in hospitals for small field relative dosimetry.
Advanced Research Prototypes (Future Commercialization):
- DIAPIX (2D pcCVD Array): Developed for large-area dose mapping and patient pre-treatment verification, reducing the uncertainty associated with sequential point measurements.
- 3DOSE (3D Columnar pcCVD): Designed for ultra-high spatial resolution in very small fields, aiming to overcome volume averaging effects.

View Original Abstract

Many new X-Ray treatment machines using small and/or non-standard radiation fields, e.g., Tomotherapy, Cyber-knife, and linear accelerators equipped with high-resolution multi-leaf collimators and on-board imaging system, have been introduced in the radiotherapy clinical routine within the last few years. The introduction of these new treatment modalities has led to the development of high conformal radiotherapy treatment techniques like Intensity Modulated photon Radiation Therapy, Volumetric Modulated Arc Therapy, and stereotactic radiotherapy. When using these treatment techniques, patients are exposed to non-uniform radiation fields, high dose gradients, time and space variation of dose rates, and beam energy spectrum. This makes reaching the required degree of accuracy in clinical dosimetry even more demanding. Continuing to use standard field procedures and detectors in fields smaller than 3 × 3 cm 2 , will generate a reduced accuracy of clinical dosimetry, running the risk to overshadowing the progress made so far in radiotherapy applications. These dosimetric issues represent a new challenge for medical physicists. To choose the most appropriate detector for small field dosimetry, different features must be considered. Short- and long-term stability, linear response to the absorbed dose and dose rate, no energy and angular dependence, are all needed but not sufficient. The two most sought-after attributes for small field dosimetry are water equivalence and small highly sensitive (high sensitivity) volumes. Both these requirements aim at minimizing perturbations of charged particle fluence approaching the Charged Particle Equilibrium condition as much as possible, while maintaining high spatial resolution by reducing the averaging effect for non-uniform radiation fields. A compromise between different features is necessary because no dosimeter currently fulfills all requirements, but diamond properties seem promising and could lead to a marked improvement. Diamonds have long been used as materials for dosimeters, but natural diamonds were only first used for medical applications in the 80 s. The availability of reproducible synthetic diamonds at a lower cost compared to natural ones made the diffusion of diamonds in dosimetry possible. This paper aims to review the use of synthetic poly and single-crystal diamond dosimeters in radiotherapy, focusing on their performance under MegaVoltage photon beams. Both commercial and prototype diamond dosimeters behaviour are described and analyzed. Moreover, this paper will report the main related results in literature, considering diamond development issues like growth modalities, electrical contacts, packaging, readout electronics, and how do they affect all the dosimetric parameters of interest such as signal linearity, energy dependence, dose-rate dependence, reproducibility, rise and decay times.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2021.632299

References

2012 - Cancer and radiation therapy: Current advances and future directions [Crossref]
2007 - Small fields: Nonequilibrium radiation dosimetry [Crossref]
2017 - The energy dependence of the lateral dose response functions of detectors with various densities in photon-beam dosimetry [Crossref]
2015 - On the monte carlo simulation of small-field micro-diamond detectors for megavoltage photon dosimetry [Crossref]
2019 - A monte carlo study on the ptw 60019 microdiamond detector [Crossref]
2017 - Medical and health physics
2017 - Dosimetry of small static fields used in external beam radiotherapy
2008 - A new formalism for reference dosimetry of small and non-standard fields [Crossref]
2018 - Different dosimeters/detectors used in small-field dosimetry: Pros and cons [Crossref]
2004 - The behavior of several microionization chambers in small intensity modulated radiotherapy fields [Crossref]