Journal List > Prog Med Phys > v.28(3) > 1098568

Mahmood, Ibrahim, and Aqeel: Uncertainty Assessment: Relative versus Absolute Point Dose Measurement for Patient Specific Quality Assurance in EBRT

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.

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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.
pmp-28-111f1.tif
Fig. 2.
Comparison between measured point dose (relative and absolute method) with TPS computed dose.
pmp-28-111f2.tif
Table 1.
Percentage relative standard uncertainty of the each factor associated with the field and reference ionisation chamber.
Items Percentage relative uncertainty
Reference chamber Field chamber
u (ND,W,Qo) 0.4 NA
u (kQ,Qo) 1 1
u (ktp) 0.2 0.2
u (kp) 0.1 0.1
u (ks) 0.16 0.16
Humidity u (kh) 0.17 0.17
Reproducibility of reference condition 0.4 0.4
u (M) 0.3 0.3
u (MU) 0.12 0.12
Percentage relative standard uncertainty (k=1) 1.24 1.18
Total uncertainty (k=1) 1.7 7%
Extended uncertainty (k=2) 3.4 4%
Table 2.
Percentage relative standard uncertainty of the each factor associated with the absolute and relative point dose measurement methods and total percentage relative standard uncertainty and extended uncertainty.
Items % Relative (point dose measu uncertainty urement methods)
Relative Absolute
u (ND,W,Qo)/u (NCF) 0.7 1.7
u (kQ,Qo) NA 1
u (ki) 0.4 0.4
Setup reproducibility 0.4 0.4
Long term dosimeter stability 0.3 0.3
Electrometer charge reading 0.3 0.3
u (M)    
Beam Monitor u (MU) 0.12 0.12
Percentage relative standard 1.00 2.07
uncertainty (k=1)    
Extended uncertainty (k=2) 2.0 4.2
Table 3.
Percentage dose difference between measured dose (relative and absolute method) and TPS computed dose for H&N IMRT treatment plans.
Patients Percentage Dose difference between measured and TPS computed dose
Relative method Absolute method
1 −1.1 1.19
2 −0.1 0.40
3 1.38 3.45
4 1.2 1.08
5 2.2 0.06
6 2.5 1.93
7 1.35 1.36
8 0.0 1.42
9 1.5 2.19
10 1.40 1.40
11 1.40 1.56
12 −1.3 0.21
13 2.0 1.60
SD 1.2 0.9
Table 4.
Setup information to expose the semiflex ionisation chamber in the CIRS H&N phantom to account the daily variation of LINAC output.
Parameters Values Parameters Values
Field Size (10 cm×10 cm) Gantry angle
Collimator angle Energy 6 MV
MUs 200 Dose rate 400 MU/min
TOOLS
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