Static small radiation fields and detectors for relative small field dosimetry in external beam radiotherapy
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-08-26 |
| Journal | Doklady BGUIR |
| Authors | V. S. Piskunou, І. Г. Тарутин |
| Institutions | N.N. Alexandrov National Cancer Centre, State Healthcare Institution “Regional Clinical Oncological Dispensary” |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This analysis focuses on the challenges and suitable detectors for relative dosimetry in small static radiation fields used in modern external beam radiotherapy (EBRT).
- Core Problem: The increasing use of advanced techniques (SRS, SBRT, IMRT) relies on small fields, violating the standard reference conditions (10 x 10 cm) and increasing clinical dosimetry uncertainty.
- Small Field Definition: A photon beam is defined as “small” if it exhibits Lateral Charged Particle Equilibrium (LCPE) loss, partial primary source occlusion by collimators, or if the detector size is comparable to the field size (volume averaging effect).
- Detector Requirements: Ideal detectors must offer high spatial resolution, water equivalence, linear response independent of energy and dose rate, and minimal angular dependence.
- Inadequate Standards: Conventional ionization chambers (0.3-0.6 cm³) are generally unsuitable due to significant volume averaging and perturbation effects in high-gradient small fields.
- Recommended Detectors: Liquid ionization chambers, silicon diodes, diamond detectors, organic scintillators, radiochromic film, TLDs, and OSLDs are recommended for relative small field dosimetry, provided appropriate correction factors are applied.
- Measurement Focus: Relative dosimetry primarily involves determining percentage depth dose (PDD), lateral profiles, and output factors (OF) as a function of field size and shape.
- Conclusion: No single ideal detector exists; clinical practice requires using multiple detector types and applying specific corrections to ensure accuracy.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Reference Field Size | 10 x 10 | cm2 | Standard dosimetry protocol (IAEA TRS 398, AAPM TG-51) |
| Small Field Definition 1 | < 2 x 2 | cm2 | Fields where LCPE loss and source occlusion are pronounced |
| Small Field Definition 2 | 2 x 2 - 4 x 4 | cm2 | Intermediate small fields |
| Classic IC Volume | 0.3-0.6 | cm³ | Generally too large for small field dosimetry |
| Mini IC Volume | 0.01-0.3 | cm³ | Suitable for fields > 2 x 2 cm2 |
| Micro IC Volume | 0.002-0.01 | cm³ | Suitable for fields < 2 x 2 cm2 |
| Silicon Diode Volume | Typically < 0.2 | mm³ | High resolution, but high angular and energy dependence |
| Organic Scintillator Volume | Approx. 1 | mm³ or less | Excellent water equivalence and dose rate linearity |
| Liquid IC Operating Voltage | 800-1000 | V | Required high voltage for operation |
| Scintillator Dose Rate Range | 10 mGy/min to 10 Gy/min | Dose Rate | Range suitable for distant beam therapy |
| Radiochromic Film Resolution | 0.1 to 0.2 | mm | Limited by densitometer scanning resolution |
Key Methodologies
Section titled “Key Methodologies”The primary methodologies involve measuring dose distributions under non-standard conditions, focusing on mitigating three major physical effects:
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Addressing Lateral Charged Particle Equilibrium (LCPE) Loss:
- LCPE is lost when the beam radius is less than the maximum range of secondary electrons, leading to a net outflow of charge and reduced absorbed dose relative to collision kerma.
- Measurements must account for this imbalance, which is exacerbated by higher photon energies and lower medium density.
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Mitigating Source Occlusion Effect:
- In very small fields, the collimator partially shields the extended primary photon source, causing a reduction in the output factor (dose on the central axis) compared to fully open fields.
- Accurate output factor measurements require detectors with minimal perturbation and volume averaging effects to correctly capture the reduced intensity.
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Compensating for Volume Averaging Effect:
- The detector signal represents the average dose absorbed across its sensitive volume. In fields with steep dose gradients (like the penumbra), this averaging distorts the measured profile.
- Detectors with extremely small sensitive volumes (e.g., <1 mm³) are preferred to minimize this effect, especially when measuring lateral beam profiles.
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Selecting and Correcting Detectors:
- Measurements (PDD, profiles, OFs) are performed using solid-state detectors (diodes, diamonds, scintillators) or micro-ionization chambers.
- Correction factors must be applied for non-water equivalence, dose rate dependence (especially for unflattened beams), angular dependence (critical for diodes), and polarization effects (for ICs and diamond detectors).
Commercial Applications
Section titled “Commercial Applications”The research on small field dosimetry detectors is critical for the clinical implementation and quality assurance of advanced radiotherapy techniques:
- Stereotactic Radiosurgery (SRS): Requires highly accurate dose delivery to very small targets, necessitating detectors capable of measuring fields < 1 cm.
- Stereotactic Body Radiotherapy (SBRT): Uses small, high-dose fractions delivered to extracranial tumors, demanding precise dose verification.
- Intensity Modulated Radiotherapy (IMRT): Relies on complex, highly modulated fields often composed of small beamlets, requiring high spatial resolution for quality assurance.
- Linear Accelerator (Linac) Commissioning: Essential for initial setup and calibration of new treatment machines, particularly those equipped with high-definition multi-leaf collimators (MLCs).
- In Vivo Dosimetry: Use of TLDs and OSLDs for verifying the absorbed dose delivered to the patient during treatment.
- Quality Assurance (QA): Routine checks of beam profiles and output factors to maintain the accuracy and safety of clinical radiation delivery.
View Original Abstract
The aim of this work is to analyze existing detectors for the relative dosimetry of small radiation fields in external beam radiation therapy and the requirements for them, consider the problems in carrying out dosimetry of small radiation fields, determine the physical conditions under which an external photon beam can be designated as a small field. In modern radiation therapy, there is an increase in the use of small static fields, which is facilitated by the general availability of standard and optional multileaf collimators and new generation treatment machines of various designs. There is growing interest in the use of such radiation techniques as stereotactic radiosurgery, stereotactic body radiotherapy, intensity modulated radiotherapy, which are widely used small fields. This has increased the uncertainties in clinical dosimetry, especially for small fields. Accurate dosimetry of small fields is important when commissioning linear accelerators and is a difficult task, especially for very small fields used in stereotactic radiotherapy. In the course of the work, a study of topical problems in the dosimetry of small radiation fields in external beam radiation therapy has been carried out. The physical conditions under which the external photon beam can be designated as a small field are considered. A review and analysis of existing detectors for the relative dosimetry of small radiation fields, as well as an analysis of the requirements for the character. The analysis revealed that liquid ionization chambers, silicon diodes, diamond detectors, organic scintillators, radiochromic films, thermoluminescent dosimeters and optically stimulated luminescence detectors are considered suitable for relative dosimetry of small photon fields and are recommended for use in clinics where radiotherapy is performed.