Can IQM catch errors before they become clinically relevant?
Lake Constance Radiation Oncology Center Friedrichshafen
3 clinical VMAT plans
Prostate / Breast / Head&Neck
IQM signal stability (inter fraction repeatability):
10 measurements for each plan
Detection of geometric errors:
systematic MLC leafbank shift (0.5-8mm) and random leafbank errors (2/5/8 mm @ 4 control points)
Detection of dosimetric errors:
Total #MU changed and Random errors @ single control points
IQM Signal stability
Cumulative signal reproducibility (10 measurements)
Prostate: 0.32%STDEV
Breast: 0.23%STDEV
H&N: 0.43%STDEV
Geometric errors
Systematic MLC bank shift from 0.5mm to 8mm in a VMAT prostate plan
Systematic MLC bank shift
Y2+0.5mm: 1.2% rel. disagreement
Y2+1mm: 2.6% rel. disagreement
Y2+2mm: 6% rel. disagreement
Y2+5mm: 15.5% rel. disagreement
Y2+8mm: 24.2% rel. disagreement
Random leafbank errors of 8mm at 3 consecutive control points
Dosimetric errors
Total #MU changed
IQM Monitor Immediately issues an alert
Random MU errors at single control points
IQM immediately detects the deviation
Summary
Easy & robust workflow
No software crashes
User friendly interface
High sensitivity of cumulative signal
Leaf bank shift of 1mm is detectable
Systematic errors detectable
Random errors detectable
Detecting VMAT delivery errorsAzienda Provinciale per i Servizi Sanitari, Trento
How well does IQM detect changes in MU’s or shifts in MLC position on clinical H&N VMAT plans?
Deviations were introduced into 4 clinical H&N VMAT plans:
3, 5 and 10 % errors on total delivered MUs (dosimetric error)
and
3, 5 and 10 mm MLCs shift (geometric error)
Results:
MLC Shift errors were well detected and
IQM shows a linear response with dose.
Conclusion:
IQM shows appreciable features in detecting real-time errors
and for saving time during QA procedures.
Error detection capability compared to Scandidos Delta 4(Radboud UMC, Nijmengen, The Netherlands)
Various IMRT and VMAT clinical beams with induced errors: One segment/control point retraction of leaves by 10, 5 and 2mm and one segment increase of 10, 5 or 2MU
Sensitivity and specificity can be expected to be sufficient for clinical practice, and at least equal to the Scandidos Delta 4
Sensitivity and specificity can be expected to be sufficient for clinical practice, and at least equal to the Scandidos Delta 4
Simulated errors(University of Florence, Italy)
Errors were introduced to an H&N IMRT treatment composed by seven beams (gantry angles = 0°, 40°, 80°, 140°, 220°, 280°, 320°). Errors were introduced by modifying the number of delivered MU (between 1 and 3 ) and by introducing small deviations in leaf positions for each segment of each beam (mimicking an MLC bank error).
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Error detection capability compared to PTW Octavius(Azienda Provinciale per i Servizi Sanitari, Trento, Italy)
The IQM capability in recognizing errors was performed introducing deviations in 4 clinical H&N VMAT plans: 3, 5 and 10% errors on total delivered MUs and 3, 5 and 10mm MLCs shift by means of an homemade Matlab(MathWorks, Natick, MA) script. The cumulative IQM checksum value was measured and the percentage difference was calculated with respect to the non-modified plan. At the same time they obtained dose distribution maps through the PTW 2D array inserted in a rotating QA phantom (RT-smartIMRT, dose.point GmbH). The local gamma pass rates (2%/2mm) were compared to the original plan values.
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Both methods detect specifically MLC shift errors, while MUs variations were better identified by IQM. IQM shows a linear response with dose (R2=0.9995), while gamma analysis seems to have difficulty in identifying 3% and 5% MUs variations.