Basic Metrology: High-Dose Dosimetry Uncertainty Tables

Summary

The high-dose dosimetry program supports radiation-processing applications by assuring that the absorbed dose to the product, often prescribed or limited by regulatory agencies, is traceable to NIST standards. In addition, our most accurate measurements using small alanine-pellet dosimeters for these high-dose processes show promise to provide traceability to national measurement standards in clinical applications for the small-field radiation beams increasingly being used in radiation therapy.

High-dose dosimetry service descriptions can be found in Procedures 11 and 12 of the RPD Quality System.

The tables below are the uncertainty budgets for the high-dose dosimetry services. Irradiation geometries have been established for a variety of dosimeters including: films (alanine and radiochromic), alanine pellets (unpackaged or TapeTab), and optichromic. Alanine transfer dosimetry uncertainties are for pellet dosimeters only. While these uncertainty budgets support the majority of the high-dose dosimetry services other uncertainties have been characterized and may be reported for source-specific irradiation geometries.

Description

Gammacell™ Calibration Geometry Dose Rate

Uncertainty Source	Type A (%)	Type B (%)
Water Calorimetry in Vertical Beam	0.16	0.51
Gammacell/Pool Source Ratio Data	0.10
Pool/B036 Source Ratio Data	0.17
Geometry Correction Factor	0.11
Field Uniformity		0.01
Source Timer (irrad time > 8min)		0.20
Co-60 Decay Correction		0.02
sqrt(sum)	0.28	0.55
combined in quadrature		0.60
coverage factor at 95.45%		2.0
Expanded Uncertainty at 95.45% conf.		1.2

 

High-Dose Alanine Response

Uncertainty Source	Type A (%)	Type B (%)
Repeatability and Reproducibility	0.30
Mass Determination	0.20
Interspecimen Contamination		0.10
Ruby Correction		0.05
System Drift		0.10
sqrt(sum)	0.36	0.15

 

Alanine Transfer - Photons > 100 Gy

Uncertainty Source	Type A (%)	Type B (%)
Alanine Dose Rate	0.25	0.55
Alanine Response	0.36	0.15
Dose Rate Effect		0.10
Temperature Correction		0.10
Calibration Curve	0.50	0.10
sqrt(sum)	0.66	0.59
combined in quadrature		0.89
coverage at 95.45 %		2.0
Expanded Uncertainty at 95.45% conf.		1.8

 

Alanine Transfer - Photons <= 100 Gy

Uncertainty Source	Type A (%)	Type B (%)
Alanine Dose Rate	0.25	0.55
Alanine Response	0.36	0.15
Dose Rate Effect		0.10
Temperature Correction		0.10
Calibration Curve	0.50	0.10
Check Dose c.f.	0.44	0.10
sqrt(sum)	0.80	0.60
combined in quadrature		1.00
coverage factor at 95.45%		2.0
Expanded Uncertainty at 95.45% conf.		2.0

 

Alanine Transfer - Dose(Silicon)

Uncertainty Source	Type A (%)	Type B (%)
Alanine Dose Rate	0.25	0.55
Alanine Response	0.36	0.15
Dose Rate Effect		0.10
Temperature Correction		0.10
Calibration Curve	0.50	0.10
Uen Ratio Water to Silicon		1.80
sqrt(sum)	0.66	1.90
combined in quadrature		2.01
coverage factor at 95.45%		2.0
Expanded Uncertainty at 95.45% conf.		4.0

 

Alanine Transfer - E-beam

Uncertainty Source	Type A (%)	Type B (%)
Alanine Dose Rate	0.25	0.55
Alanine Response	0.36	0.15
Dose Rate Effect		0.10
Temperature Correction		0.10
Calibration Curve	0.50	0.10
Electron Energy Dependence		0.10
sqrt(sum)	0.66	0.60
combined in quadrature		0.90
coverage factor at 95.45%		2.0
Expanded Uncertainty at 95.45% conf.		1.8

 

Alanine Transfer - Cs-137

Uncertainty Source	Type A (%)	Type B (%)
Alanine Dose Rate	0.25	0.55
Alanine Response	0.36	0.15
Dose Rate Effect		0.10
Temperature Correction		0.10
Calibration Curve	0.50	0.10
Cs-Co Correction	1.00	0.10
sqrt(sum)	1.20	0.60
combined in quadrature		1.34
coverage factor at 95.45%		2.0
Expanded Uncertainty at 95.45% conf.		2.7

 

Alanine Transfer - Cs-137 <=50 Gy

Uncertainty Source	Type A (%)	Type B (%)
Alanine Dose Rate	0.25	0.55
Alanine Response	0.36	0.15
Temperature Correction		0.10
Dose Rate Dependance		0.10
Calibration Curve	0.44	0.10
Measurement c.f.	0.44	0.10
Energy Dependence	1.00	0.10
sqrt(sum)	1.26	0.61
combined in quadrature		1.40
coverage factor k=2		2.0
Expanded Uncertainty at 95.45% conf.		2.8

 

Revisions History	Date
Calibration Geometry Dose Rate changed: 1) GC207 to Gammacell; 2) GC/Pool Ratio from 0.08 to 0.10; 3) Added Geometry Correction Factor term of 0.11 to budget; 4) df/t-factor changed from 16/2.17 to 19/2.14. Changes allow budget to represent the largest uncertainty expected for the majority of geometries available.	Dec 13,2010
Coverage factor changed from varied to a uniform k=2.0 for all tables.	March 15,2013