Fast Beam Scanning and Accurate Output Factor Measurements for Small-Field Dosimetry Using a Novel Scintillation Detector

At a Glance

Metadata	Details
Publication Date	2025-08-26
Journal	bioRxiv (Cold Spring Harbor Laboratory)
Authors	Yiding Han, Jingzhu Xu, Yao Hao, Baozhou Sun
Institutions	St. Luke’s Medical Center, Washington University in St. Louis
Analysis	Full AI Review Included

Executive Summary

The study validates the Blue Physics Plastic Scintillation Detector (BP-PSD Model 11) as a superior tool for small-field dosimetry, emphasizing speed and accuracy in clinical quality assurance (QA).

Correction-Free Dosimetry: The BP-PSD is water-equivalent, energy-independent, and dose-rate independent, eliminating the need for field output correction factors (k-value = 1) required by micro-diamond and micro-silicon detectors (TRS 483).
Significant Time Savings: The fast-scan mode allows complete profile and Percentage Depth Dose (PDD) datasets across multiple small fields (down to 0.5x0.5 cm²) to be acquired in less than 8 minutes, compared to approximately 40 minutes required by conventional detectors.
Accurate Output Factors (FOF): FOF measurements showed excellent agreement with reference detectors (Exradin W2, micro-diamond, micro-silicon diode). Maximum variation was 1.6% at the smallest field size (0.5x0.5 cm²), significantly improving upon previous literature reports (12%).
Novel Indirect PDD Method: A profile-based technique was introduced to derive PDD curves from 62 fast lateral scans, achieving 100% gamma passing rate (3%/1mm) for extremely small fields (1x1 cm²) against TPS simulation, while inherently minimizing errors from detector misalignment and beam inclination.
High Consistency: Direct PDD measurements achieved a 98% gamma passing rate (3%/1mm) against an Ion Chamber reference for 3x3 cm² and 10x10 cm² fields. Penumbra measurements (80%-20%) agreed with micro-diamond and micro-silicon detectors with less than 1% variation.

Technical Specifications

Parameter	Value	Unit	Context
Detector Type	BP-PSD Model 11	N/A	Plastic Scintillation Detector
Scintillator Core Material	Polystyrene	N/A	Doped with fluorescent compounds
Sensitivity Volume (Cylinder)	1 x 1	mm	Diameter x Length (0.785 mm³ volume)
Response Time	Nanoseconds	N/A	Short fluorescence decay time
Acquisition Mode	Pulse-to-pulse	N/A	Raw data reflects individual linac pulses
Integration Window	750	µs	Dual-channel interleaved integrator circuits
ACR (Cerenkov Correction Factor)	0.963	N/A	Adjacent Channel Ratio determined experimentally
Scanning Speed (Small Fields)	10	mm/s	Used for fields ≤ 3x3 cm²
Scanning Speed (Large Fields)	20	mm/s	Used for fields 10x10 cm²
FOF Variation (1x1 cm²)	< 1	%	Variation between BP-PSD and other detectors
FOF Variation (0.5x0.5 cm²)	1.6	%	Variation between BP-PSD and micro-diamond
Indirect PDD Scan Time	12	minutes	Total time for 62 profiles (0 to 280 mm depth)
Indirect PDD Gamma Pass Rate	100	%	3%/1mm criteria, referenced to TPS simulation (1x1 cm²)
Direct PDD Gamma Pass Rate	98	%	3%/1mm criteria, referenced to Ion Chamber (3x3 cm²)

Key Methodologies

The dosimetric evaluation was performed on a Varian TrueBeam 6XFFF photon beam using a PTW BeamScan water tank system.

Detector Comparison: BP-PSD Model 11 was benchmarked against four reference instruments: Exradin W2 (PSD), PTW micro-diamond (TN60019), PTW micro-silicon diode (TN60023), and a large-volume Ion Chamber (TN31013).
Cerenkov Light Correction: The system utilized two channels (Sensor Rs and Cerenkov Rc). The Cerenkov contribution was removed using the Adjacent Channel Ratio (ACR) formula, determined experimentally to be 0.963.
Data Acquisition and Smoothing: Raw pulse-by-pulse data from the BP-PSD was processed using in-house software. A rolling smoothing technique with a 40 ms window was applied to reduce noise and facilitate comparison with integrating detectors.
Scanning Speeds:
- BP-PSD: 10 mm/s (for fields ≤ 3x3 cm²) and 20 mm/s (for 10x10 cm²).
- Reference Detectors (PDD/Profiles): 2 mm/s or 1 mm/s (significantly slower).
Field Output Factor (FOF) Measurement: FOFs (Ω) were measured at two depths (5 cm and 10 cm) for field sizes ranging from 0.5x0.5 cm² to 4x4 cm², referenced to a 10x10 cm² jaw-defined field. A k-value of 1 was applied for all PSD measurements.
Indirect PDD Measurement (Novel Method):
- 62 lateral beam profiles were scanned (10 mm/s speed) from the water surface down to 280 mm depth (total time: 12 minutes).
- The PDD value at each depth was derived by identifying the peak dose of the corresponding lateral profile, thereby bypassing errors associated with vertical detector drift or beam inclination.
Evaluation Criteria: PDD curves were analyzed using 1D gamma passing rates (3%/1mm and 2%/2mm). Beam profiles were evaluated by comparing penumbra lengths (lateral distance between 80% and 20% isodose lines).

Commercial Applications

The BP-PSD Model 11 is specifically designed to enhance efficiency and accuracy in high-precision radiation therapy environments.

Stereotactic Radiosurgery (SRS) and Therapy (SRT/SABR): Provides accurate dosimetry for extremely small fields (down to 0.5x0.5 cm²) where conventional detectors struggle with volume averaging and correction factors.
Clinical Quality Assurance (QA): Enables significantly faster routine QA processes (5 to 10 times quicker), reducing linac downtime required for commissioning and daily checks.
Dosimetry Commissioning: Facilitates rapid and accurate measurement of PDDs and beam profiles, especially for small fields, satisfying stringent accuracy and stability requirements.
Water Tank Scanning Systems: The fast response time and indirect PDD method are uniquely practical for integration into existing scanning water tank systems (like PTW BeamScan), offering a high-throughput solution for large volumes of tasks.

View Original Abstract

Abstract Background The most used instruments for small-field dosimetry have notable limitations, including the need for correction factors, limited scanning speeds, and challenges in alignment for percentage depth dose (PDD) measurements, particularly for extremely small-fields. However, plastic scintillation detectors (PSDs) are an attractive alternative for small-field dosimetry due to their correction-free nature, linear dose response, and fast response time. Purpose This study evaluates the robustness and accuracy of the dosimetric measurements using a new water-equivalent PSD in small-field dosimetry. The study also aims to report an indirect method for measuring PDD in small-fields, with a scanning time that is 5 to 10 times faster than traditional methods. Method PDDs, profiles and output factors were measured on a Varian TrueBeam 6XFFF photon beam for the field size of 0.5×0.5 cm 2 , 1×1 cm 2 , 2×2 cm 2 , 3×3 cm 2 , 4×4 cm 2 using a new PSD from Blue Physics (BP-PSD). These measurements were compared with those obtained using a well-established PSD (Standard Imaging W2), micro-diamond (TN60019, PTW-Freiburg, Germany), and micro-silicon detectors (TN60023, PTW-Freiburg, Germany). Owing to its fast response, the BP-PSD enabled the collection of beam profiles at 31 depths, which were used to derive the PDD while avoiding detector misalignment along the beam path. Data was collected in a water tank controlled by the PTW BeamScan software. The pulse-by-pulse raw data from BP-PSD were converted to respective dosimetry data using in-house software. Result The BP-PSD demonstrated excellent agreement with other detectors for small-field output factors (FOFs), with a maximum variation of 1.6%. The BP-PSD also showed strong agreements in PDD measurements with an ion chamber (TN31013) for both 3×3 cm 2 and 10×10 cm 2 field sizes, achieving a 98% gamma passing rate (gamma criteria: 1mm,3%). For the profile measurements, the BP-PSD showed consistency with both the micro-diamond and micro-silicon diode detectors, with less than 1% variation in measured penumbra length. At a 3×3 cm 2 field size, the measured penumbra length (4 mm) agreed with previously published data (3.86-4.2 mm). Additionally, for field size less than 3×3 cm 2 the indirect PDD measurements derived from profiles showed significant improvement compared to the direct measurements using various detectors, using TPS-calculated PDD as a reference. Conclusion The BP-PSD has proven to be a robust and reliable detector for small-field dosimetry. It exhibits excellent agreement with other detectors in measuring small FOFs and provides accurate measurements with significantly faster scanning speeds in a water tank. The fast response feature enables the indirect PDD measurement method

Tech Support

Original Source

DOI: https://doi.org/10.1101/2025.08.22.25334184

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