Microdosimetry for hadron therapy - A state of the art of detection technology

At a Glance

Metadata	Details
Publication Date	2022-11-24
Journal	Frontiers in Physics
Authors	Gabriele Parisi, F. Romanò, Giuseppe Schettino
Institutions	Istituto Nazionale di Fisica Nucleare, Sezione di Catania, National Physical Laboratory
Citations	20
Analysis	Full AI Review Included

Executive Summary

This review analyzes the state-of-the-art detection technologies for microdosimetry, focusing on their suitability for clinical Hadron Therapy (HT) applications, which require high spatial resolution and operation at high fluence rates.

Core Value Proposition: Microdosimetry provides a unique characterization of beam quality and a direct link to radiobiology, enabling more accurate Relative Biological Effectiveness (RBE) estimation than conventional dosimetry methods.
Technology Gap: A standard device and accredited experimental methodology for clinical microdosimetry have not yet been established, hindering widespread clinical adoption.
Solid-State Detector (SSD) Performance: Silicon and Diamond SSDs are highly promising alternatives to conventional Tissue Equivalent Proportional Counters (TEPCs), offering micrometric size, fast response, and operation at clinical fluence rates (up to 10⁷ cm^-2 s^-1).
Material Selection: Diamond SSDs are particularly attractive due to their superior radiation hardness (allowing for hundreds of treatments before degradation) and fair tissue-equivalence, minimizing material correction uncertainties compared to silicon.
Advanced Techniques: ΔE-E telescope detectors enable simultaneous particle discrimination and energy measurement, significantly improving radiation quality characterization in mixed fields (e.g., distal Bragg peak).
Future Standard: The SQUID-based microbolometer, while currently complex and requiring cryogenic operation (~6 K), shows potential as a future microdosimetry primary standard due to its extreme accuracy and low energy resolution (~0.2 eV).
Clinical Requirement: The development of hybrid systems (e.g., mini-TEPC for entrance QA, thin SSD for Bragg peak characterization) and standardized protocols is necessary to achieve comprehensive, reliable characterization across the entire depth-dose profile.

Technical Specifications

Parameter	Value	Unit	Context
Simulated Tissue Volume (TEPC)	1	µm	Simulated diameter of tissue sphere
TEPC Gas Pressure Range	0.9 to 9.5	kPa	Required to simulate 1 µm tissue volume
Conventional TEPC Diameter	10 to 150	mm	Typical size range
Mini-TEPC Operable Fluence Rate	10⁶	cm^-2 s^-1	Rate without major pile-up issues
Avalanche Confinement TEPC Site Size	25 to 300	nm	Simulated range
GEM Foil Thickness	~50	µm	Typical insulating foil
Silicon SSD Depletion Thickness	1 to 10	µm	Typical range
Silicon Bridge Microdosimeter SV Volume	30 x 30 x 10	µm³	Sensitive Volume (SV) dimensions
Silicon SSD Low Energy Cut-off (Best)	0.2	keV µm^-1	Achieved by Silicon Bridge detector
Diamond Detector LET Linearity Range	100 to 3,000	keV µm^-1	Demonstrated linearity range
Diamond Detector Radiation Hardness	>500	treatments	Expected CCE drop of only 1%
SQUID Microbolometer Operational Temp	~6	K	Requires liquid helium cryogenic system
SQUID Microbolometer Energy Resolution	~0.2	eV	Preliminary test result
TEPC Low Energy Cut-off (Typical)	0.1 to 1	keV µm^-1	Lowest achieved threshold

Key Methodologies

The following methodologies detail the fabrication and operation of the most relevant microdosimetry detectors:

Tissue Equivalent Proportional Counter (TEPC) Operation:
- The detector uses a macroscopic sensitive volume (SV) defined by A-150 plastic walls (cathode) and a central wire (anode).
- The SV is filled with low-pressure TE gas (e.g., methane-based or propane-based) to simulate micrometric tissue volumes (1 µm).
- A high voltage (typically 600-800 V) is applied to induce gas multiplication, converting ionization events into measurable current pulses.
Silicon-on-Insulator (SOI) SSD Fabrication (CMRP Mushroom Microdosimeter):
- The device is built on a SOI wafer, utilizing a thin silicon oxide (SiO₂) layer for electrical insulation, which prevents the field funneling effect common in bulk silicon.
- Well-defined cylindrical SVs (diameter and thickness ~10 µm) are fabricated using 3D detector technology.
- Cylindrical n⁺ columnar electrodes and surrounding p⁺ ring electrodes are created via etching and gas implantation to ensure precise SV definition and efficient Charge Collection Efficiency (CCE).
CVD Diamond SSD Fabrication (Layered p-i-n Structure):
- A conductive p-type boron-doped diamond layer is deposited on a High Pressure High Temperature (HPHT) diamond substrate using Microwave Plasma Enhanced Chemical Vapor Deposition (MWPECVD).
- The intrinsic diamond layer (acting as the SV, thickness 1 µm to tens of µm) is homoepitaxially grown on the p-type layer.
- A thin metal electrode is deposited on the intrinsic layer surface to complete the p-i-n junction structure, defining the sensitive area.
ΔE-E Telescope Detector Operation:
- Consists of two cascaded stages: a thin front stage (ΔE, ~2 µm thick) acting as the microdosimeter, followed by a thicker stage (E, ~500 µm thick) measuring residual energy.
- The detector is strongly direction-dependent, requiring the particle to cross ΔE first, then E.
- The ΔE-E scatter plot is used to identify the particle type (discrimination) and determine the total incident particle energy, allowing for accurate stopping power corrections for material conversion.

Commercial Applications

The technologies reviewed are primarily focused on high-precision radiation measurement in specialized fields:

Hadron Therapy (HT):
- Quality Assurance (QA) and beam characterization for clinical proton, helium, and carbon ion beams.
- Accurate estimation of Relative Biological Effectiveness (RBE) for treatment planning, especially around the steep dose gradients of the Bragg peak.
Radiation Protection and Monitoring:
- Measurement of mixed neutron-gamma fields.
- Monitoring radiation environments in aviation and space (low dose rate environments).
Neutron Capture Therapy:
- Specialized microdosimeters (e.g., twin miniaturized TEPC, boron-coated silicon detectors) for Boron Neutron Capture Therapy (BNCT).
Fundamental Metrology:
- Development of microdosimetry primary standards (SQUID-based microbolometers).
- Particle tracking and 3D energy deposition measurements (GEMpix).

View Original Abstract

The interest in hadron therapy is growing fast thanks to the latest technological advances in accelerators and delivery technologies, to the development of more and more efficient and comprehensive treatment planning tools, and due to its increasing clinical adoption proving its efficacy. A precise and reliable beam quality assessment and an accurate and effective inclusion of the biological effectiveness of different radiation qualities are fundamental to exploit at best its advantages with respect to conventional radiotherapy. Currently, in clinical practice, the quality assurance (QA) is carried out by means of conventional dosimetry, while the biological effectiveness of the radiation is taken into account considering the Relative Biological Effectiveness (RBE). The RBE is considered a constant value for protons and it is estimated as a function of the absorbed dose in case of carbon ions. In this framework, microdosimetry could bring a significant improvement to both QA and RBE estimation. By measuring the energy deposited by the radiation into cellular or sub-cellular volumes, microdosimetry could provide a unique characterisation of the beam quality on one hand, and a direct link to radiobiology on the other. Different detectors have been developed for microdosimetry, from the more conventional tissue equivalent proportional counter (TEPC), silicon-based and diamond-based solid-state detectors, to Δ E -E telescope detectors, gas electrons multiplier (GEM), hybrid microdosimeters and a micro-bolometer based on Superconducting QUantum Interference Device (SQUID) technology. However, because of their different advantages and drawbacks, a standard device and an accredited experimental methodology have not been unequivocally identified yet. The establishment of accepted microdosimetry standard protocols and code of practice is needed before the technique could be employed in clinical practice. Hoping to help creating a solid ground on which future research, development and collaborations could be planned and inspired, a comprehensive state of the art of the detector technologies developed for microdosimetry is presented in this review, discussing their use in clinical hadron therapy conditions and considering their advantages and drawbacks.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2022.1035956

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