Prog Med Phys.  2017 Sep;28(3):111-121. 10.14316/pmp.2017.28.3.111.

Uncertainty Assessment: Relative versus Absolute Point Dose Measurement for Patient Specific Quality Assurance in EBRT

• Mahmood T
• Ibrahim M
• Aqeel M

• KMID: 2393372
• DOI: http://doi.org/10.14316/pmp.2017.28.3.111

Abstract

Verification of dose distribution is an essential part of ensuring the treatment planning system's (TPS) calculated dose will achieve the desired outcome in radiation therapy. Each measurement have uncertainty associated with it. It is desirable to reduce the measurement uncertainty. A best approach is to reduce the uncertainty associated with each step of the process to keep the total uncertainty under acceptable limits. Point dose patient specific quality assurance (QA) is recommended by American Association of Medical Physicists (AAPM) and European Society for Radiotherapy and Oncology (ESTRO) for all the complex radiation therapy treatment techniques. Relative and absolute point dose measurement methods are used to verify the TPS computed dose. Relative and absolute point dose measurement techniques have a number of steps to measure the point dose which includes chamber cross calibration, electrometer reading, chamber calibration coefficient, beam quality correction factor, reference conditions, influences quantities, machine stability, nominal calibration factor (for relative method) and absolute dose calibration of machine. Keeping these parameters in mind, the estimated relative percentage uncertainty associated with the absolute point dose measurement is 2.1% (k=1). On the other hand, the relative percentage uncertainty associated with the relative point dose verification method is estimated to 1.0% (k=1). To compare both point dose measurement methods, 13 head and neck (H&N) IMRT patients were selected. A point dose for each patient was measured with both methods. The average percentage difference between TPS computed dose and measured absolute relative point dose was 1.4% and 1% respectively. The results of this comparative study show that while choosing the relative or absolute point dose measurement technique, both techniques can produce similar results for H&N IMRT treatment plans. There is no statistically significant difference between both point dose verification methods based upon the t-test for comparing two means.

Keyword

Uncertainty; EBRT; IMRT; Point dose; QA

MeSH Terms

Calibration
Hand
Head
Humans
Methods
Neck
Radiotherapy
Uncertainty*

Figure

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  Fig. 1. Experimental setup to measure the nominal calibration factor. (a) LINAC calibration setup (solid water phantom), (b) The CIRS H&N phantom measurement setup.

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  Fig. 2. Comparison between measured point dose (relative and absolute method) with TPS computed dose.

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