817 / 1082
S801
ESTRO 36 2017
_______________________________________________________________________________________________
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
*D
max
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 Kafi
1
, A. Al Moussa
1
, M.J. Maryanski
2
, B. Moftah
1
1
King Faisal Specialist Hospital and Research Centre,
Biomedical Physics, Riyadh 11211, Saudi Arabia
2
MGS 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-
fractionation, even small relative errors can lead to
serious complications to the normal tissue or recurrences
of the tumor. So delivery quality assurance (DQA) in
SRS/SBRT is very critical and poses unique challenges due
to extremely high dose gradients and lack of electronic
equilibrium. For this reason, dose rate independent
dosimeters with precise, high spatial resolution and 3D
capabilities are essential as reported by the Council on
Ionizing Radiation Measurements and Standards (CIRMS).
Material and Methods
The new CrystalBall system (3D Dosimetry, Madison, CT,
USA) is designed for DQA with sub-millimeter spatial
resolution in 3D. The system is composed of a fast laser CT
scanner (OCTOPUS, MGS Research, Inc, Madison, CT) and
reusable tissue-equivalent radiochromic polymer gel
sphere-mounted on a special QA phantom. Gold fiducial
markers are affixed in different locations of the phantom
for image guidance with fiducial tracking for CyberKnife
(CK) robotic SRS/SBRT system (Accuray, Sunnyvale, CA).
The CT images of the CrystalBall gel phantom were
transferred to the CK Multiplan treatment planning
system. A DQA plan was generated by superimposing a
patient plan onto the gel phantom CT data set. The DQA
plan was then sent for CK irradiation. The CrystalBall’s
VOLQA software registers the plan DICOM CT dataset with
the laser CT of the irradiated gel, creates OD/cm to dose
calibration curve and then compares the CrystalBall