| Περίληψη: | The purpose of the current study was to investigate the patient specific quality assurance
performed by the ArcCHECK phantom and the 3DVH software) and to assess the suitability of the
latter for volumetric modulated arc therapy (VMAT) verification of treatment plans and finally to
define the tolerance and action limits within whichsuch procedure should be performed in clinical
practice.Patient Specific Quality Assurance (PSQA) was performed in 23 cases of
Head & Neck (H&N) tumor for a total of 67 treatment plans and for 30 cases of Prostate tumor for a
total 69 of treatment plans. The measurements for dosimetric comparison were performed using a
cylindrical phantom. The comparison between calculated and measured dose distribution was carried
out using the γ index method. The acceptance criterion used for quality assurance acceptance was set
at 3%/2mm for both the 2D and 3D analysis. 3D analysis was carried out using the 3DVH software,
through which the dose difference DD to the structures of interest were detected and the corresponding
3D %GPs were collected. 2D %GP and 3D %GP were correlated with the individual 3D %GP of the
structures of interest, the dose differences DD for each dosimetric parameter of the structures of interest
were collected and correlated with both total 2D %GP and 3D %GP. Institutional tolerance and action
limits were established using thirty prostate and thirty H&N treatment plans. It is observed that the correlations are higher between the 3D %GP rates than those of 2D %GP with respect to PTV 3D %GP and OAR 3D %GP. There is a moderate correlation between 2D
%GP and 3D %GP. In the case of H&N plans, it was observed that 3D %GP becomes systematically
greater except for phase one. In the prostate plans, no systematic increase or decrease in 3D %GP was
observed. The larger PTV volume has a higher probability of a larger dose difference. 2D %GP was
moderately correlated with the DD of DVH values for PTV structures, whereas in most cases 3D %GP
was highly correlated with the corresponding DD using the 3%/2mm acceptance criterion. 2D %GP
and 3D %GP were weakly to moderately correlated with the DD for the organs at risk (OAR).
Concerning 2D %GP, the tolerance limits were tightened by 1.6% and 1.9% for H&N and prostate
cases respectively, compared to the limits defined by the TG-218 protocol. Regarding total 3D %GP,
the tolerance limits are 97.5% and 98.8% for H&N and prostate incidents, respectively, while the
action limits are 87.7% and 94.8%, respectively. The tolerance limits of 3D %GP of each structure
appear to be tighter than the ones defined in the TG-218 protocol for 2D %GP, except for PTV in H&N
and rectum in prostate cases. The action limits of 3D %GP of each structure in the H&N area have a
wider range compared to 90% except in the parotid cases. The 3D %GP action limits of each structure
in the prostate area have a tighter range compared to 90% except in cases of PTV, rectum and bladder.
Correlations between 3D %GP rates are higher than those between 2D %GP rates with
PTV 3D %GP and OAR 3D %GP. Correlation between total 3D and 2D %GP was moderate. Weak to
moderate correlation of %GP with dose differences to the critical organs (Organs At Risk, OAR)
confirms that the 2D %GP should not be used as a stand-alone quality assurance metric of treatment
plans but should combined with both dosimetric and volumetric parameters. 3D %GP appear to
correlate more strongly with DD than 2D %GP. The present study suggests that for PSQA in the
VMAT technique, institution specific TL and AL for PTV and OAR should be considered and the
PSQA not be relied only on the total 2D %GP or 3D %GP.
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