ESTRO 36 Abstract Book

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S811 ESTRO 36
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The new IBA Dolphin (IBA Dosimetry, Germany) is a dose
transmission detector (DTD) mounted onto the gantry for
online treatment verification as well. Aim of this study is
to compare the results of the Dolphin/Compass with the
traditional 3D dosimetry phantom Delta 4 (Scandidos,
Sweden) for lung stereotactic body radiation therapy
treatmentsand to mesure the detector attenuation for
online dose verification.
Material and Methods
At first the two systems were compared in terms of ability
of error detection of leaf position. A box treatment was
measured three times with introduction of a shift of one
leaf bank in steps from 0 to 2 mm and the analysis of
gamma index or DVHs was carry out. Afterward ten
patients with lung cancer, treated by sbrt, were included
in the study. All treatment plans were
simultaneuslyverified with the Dolphin and the Delta
4.The treatment plans were generated by Monaco system
(ver.5.0, Elekta AB, Sweden). Dolphin with the Compass
software (v. 4.0) permits the 3D dose distribution
reconstruction on a patient CT and the Compass itself is a
model-based dose computation, with a collapsed cone
dose engine; the beam model of the Compass was
validated and accepted. For the quantitative analysis
parameters of dose–volume based indices for PTV (V80%,
D98%, mean dose, D2% and Gamma index 2%-2mm ) and
OARs doses (Dmax and dose at the threshold volume
according to AAPM TG101) were evaluated for Compass
calculation and DTD reconstruction. At the same time
gamma index (2%-2mm) was calculated based on Delta 4
measurements.
The detector attenuation was estimated in a clinical
context comparing the median dose inside the Delta 4
detector with and without the Dolphin mounted.
Results
Error detection ability : the fig. 1 shows the variation
between difference % of mean dose in a Roi limited to the
irradiation beams for DTD versus leaf position shift and the
% of points with gamma index > 1 for Delta 4.
Quantitative analysis: table 1 shows the results of the
comparison between Dolphin/Compass and Delta 4
phantom. The PTV average gamma was 0.64±0.12; the
mean percentage differences of V80%, D98%, mean dose
and D2% were inferior to 3%. The difference in Gy for OARs
were under or equal to 1 Gy, except for D(4cc) of trachea
(1.15 Gy). The maximum difference was found for rib D max
(4.4 Gy). The mean % of point with gamma < 1 for Delta 4
was 83.2±0.06; one patient was considered failed with 72%
of points with g<1 in Delta 4.
Detector attenuation : a value of 10.5±0.5 % was found.
Table 1. Comparison between Compass computed and
reconstructed doses
*Dmax defined at 0.035 cc
fig 1 shift leaf detectability
Conclusion
The DTD system seems to be more sensitive than 3D
detector for error detection ability. The Dolphin/Compass
system is a useful tool to perform QA patients in a SBRT
context offering more clinical evaluable informations than
3D phantoms only. For the online dosimetry, the
methodology proposed led to an attenuation correction
factor not negligible but constant
.
EP-1513 CyberKnife robotic radiotherapy delivery
quality assurance using CrystalBall 3D Dosimetry System
M.A. Al Kafi1, A. Al Moussa1, M.J. Maryanski2, B. Moftah1
1King Faisal Specialist Hospital and Research Centre,
Biomedical Physics, Riyadh 11211, Saudi Arabia
2MGS Research- Inc., d.b.a. 3D Dosimetry, Madison- CT,
USA
Purpose or Objective
Stereotactic radiosurgery/radiotherapy (SRS) and
stereotactic body radiotherapy (SBRT) deliver high dose to
the tumor accurately and precisely. With hypo-