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S789
ESTRO 36 2017
_______________________________________________________________________________________________
3
Liverpool School of Medicine, Department of
Biostatistics, Liverpool, United Kingdom
Purpose or Objective
Using flattenning filter free (FFF) beams shortens the
treatment time especially for stereotactic treatment
techniques when the high dose rate is used. It is not
possible to do quality assurance (QA) with all types of
amorphous silicon (aS) detectors because of their
saturation limit. In this study, verifications of volumetric
modulated arc therapy (VMAT) stereotactic treatment
plans were evaluated with PDIP and GLAaS algorithms by
using a new unsaturated aS detector and the results were
compared.
Material and Methods
aS1200 image detection unit (IDU) which has aS detector
integrated on a Varian TrueBeam STx linac was used
(Table 1). Portal Dosimetry (PD) (v.13.0.26) with PDIP
algorithm (Varian Medical Systems, Palo Alto) and Epiqa
(v.4.0.11) with GLAaS algorithm (Epidos s.r.o., Bratislava)
QA tools were configured at SDD: 100 cm with 2400
MU/min dose rate through the detector without saturation
issue to use for pre-treatment verification.10 MV FFF
VMAT treatment plans (2400 Dose Rate) of 35 patients with
in total 72 arcs were calculated by Anisotropic Analytical
Algorithm (AAA, ver.13.0.26) in Eclipse Treatment
Planning System. The verification plans were irradiated on
the aS1200 imager. The evaluations for both QA tools were
done with the technique of gamma analysis (GA). The GA
criterias for Distance to Agreement (DTA) and Dose
Difference (DD) were defined as 3%/3mm, 2%/2mm and
1%/1mm and applied for "field" (defined with jaws) and
"field+1cm" areas. The results were analysed with 2
sample T-test.
Table 1. Specifications of aS1200 IDU
Maximum active field size 43cm x 43cm
Maximum readout speed 20 frame/second
Matrix
1280 x1280
Resolution
0.0336 cm
Results
Epiqa had dramatically low GA (gamma<1.0) results; less
than 95% with the criterias DD:%1, DTA:1mm for field
(Average: 85,03, SD:±7,8) and field+1cm (Average: 93,30,
SD: ±2,4) comparison areas. There is no significant
difference (p>0,05) between PD and Epiqa for GA results
of DD:%3, DTA:3mm
criterias.
Table 2. Average and standard deviation (SD) results of
each method
Evaluated
field
Method,
DD, DTA
Average
value
Standard
deviation
p
value
Field+1cm PDIP, %3,
3mm
99.95
0.14
>0.06
GLAaS, %3,
3mm
99.98
0.05
PDIP, %2,
2mm
99.87
0.18
<0.05
GLAaS, %2,
2mm
99.72
0.30
PDIP, %1,
1mm
99.14
0.75
<0.05
GLAaS, %1,
1mm
93.30
2.44
Field
PDIP, %3,
3mm
99.92
0.16
>0.707
GLAaS, %3,
3mm
99.90
0.31
PDIP, %2,
2mm
99.59
0.50
<0.05
GLAaS, %2,
2mm
98.95
1.68
PDIP, %1,
1mm
97.03
2.08
<0.05
GLAaS, %1,
1mm
85.03
7.78
Conclusion
We could able to detect more errors with the hardest
criterias (DD:%1, DTA:1mm) as expected. Epiqa had better
performance for detecting the errors. It could be the
result of the differences in workflow. Epiqa compares the
dose calculated with clinical algorithm and irradiated
image, but PD compares the calculated dose with PDIP and
irradiated image. PD and Epiqa can be used for
stereotactic VMAT plans with aS1200 detector without
saturation problem at SSD: 100cm reliably. If the speed is
important for the clinics have high workload, PD could be
prefered through being an internal software.
EP-1492 Influence of induced accelerator’ errors on
dosimetric verification result and DVH of treatment
plan
M. Kruszyna
1
, K. Matuszewski
1
1
Greater Poland Cancer Centre, Medical Physics
Department, Poznan, Poland
Purpose or Objective
The commonly used gamma criteria of 3% dose difference
(global method) and 3 mm distance to agreement could
mask clinically relevant errors. The aim of this work was
to evaluate the influence of induced accelerator’s errors
on 3D gamma method results with the varies criteria and
on the patient’ dose distribution (DVHs).
Material and Methods
In the treatment prostate plan with VMAT high-
fractionated (2x7.5Gy), FFF technique the errors of dose
(differences ±1%; 2%; 3%; 5% 7%, 10%), collimator angle
(rotations in both directions: 0.5; 1.0; 1.5; 2.0; 2.5; 3.0)
and MLC shifts were introduced. For each modified plan,
the pre-treatment verification plan was created and
measured with 2D-arrays: 729 and SRS 1000 with rotational
phantom Octavius® 4D and Verisoft 6.1 software with DVH
option (PTW, Freiburg, Germany). Measured (with errors)
and calculated (reference plan) dose distributions were
analyzed with 3D gamma evaluation method for various
tolerance parameters DTA [mm] and DD [%] 1.0; 1.5; 2.0;
2.5; 3.0, by global and local dose methods with a 5%
threshold. To detect errors, the achieved score should be
less than the assumed tolerance of 95%. Additional the
DVHs from error-induced and reference plan were
analyzed for CTV D
50
, D
98,
D
2
, and D
25
, D
50
for OARs.
Results
For 12 error-induced plan with dose discrepancies, proper
detection for 729 and SRS 1000 were obtained as follows:
3/12 and 6/12 (G3%/3mm); 8/12 and 6/12 (L3%/3mm);
8/12 and 7/12 (G2%/2mm); 8/12 and 8/12 (L2%/2mm).
The rotations of collimator were detected >3° for 729 and
>2° for SRS 1000. The MLC errors were discovered for plans
with 1 leaf (MLC1) and 1 pair of leaves (MLC2) blocked, for
all leaves shifted about 0.05cm (MLC3) misalignment
weren’t indicated so obvious. The clinical relevance of
plan with MLC errors and chosen discrepancies for
collimator rotation (3°) and dose differences (+5%) were
presented in the table 1.