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irradiation measurements with the Multiplan’s DQA plan.
It generates QA reports that feature overlays of isodoses
in 2D and 3D, profiles, DVHs, voxel statistics, and pass/fail
metrics for dose difference and distance-to-agreement
according to gamma index criteria. In this study, we
performed DQA for four CK patients who received
treatment for brain metastasis, spine metastasis and
trigeminal neuralgia as recommended by AAPM TG-135.
For each patient, the DQA was done three times.
Results
Figures 1 and 2 show the CrystalBall phantom setup with
OD/cm to dose auto-calibration, 2D and 3D overlay of
isodoses for a patient, respectively.
Table 1 shows results of the study for gamma evaluation
passing averages for the DQA of the four patients. For all
patients studied, we found a passing rate of more than 96%
with gamma index criteria of 2 % dose difference and 2
mm distance-to-agreement. For 3 % and 3 mm criteria, the
passing rate is found to be above 99%.
Conclusion
Our DQA results suggest that the newly developed
CrystalBall QA phantom system for robotic radiosurgery
can be ideal tool for 3D dose verification with isotropic
sub-millimeter spatial resolution and film-equivalent
accuracy. This 3D tool can offer unique advantage over
other existing 2D tools and techniques in terms of high-
resolution DQA necessary for radiotherapy with minimal
additional physics resources.
Electronic Poster: Physics track: Radiation protection,
secondary tumour induction and low dose (incl.
imaging)
EP-1514 Planar kV imaging dose reduction study for
Varian iX and TrueBeam linacs
E. Gershkevitsh
1
, D. Zolotuhhin
1
1
North-Estonian Regional Hospital Cancer Center
Radiotherapy, Radiotherapy, Tallinn, Estonia
Purpose or Objective
IGRT has become an indispensable tool in modern
radiotherapy with kV imaging used in many departments
due to superior image quality and lower dose when
compared to MV imaging. Since, the frequency of kV
images continues to increase (intrafractional imaging,
etc.) the reduction of additional dose assumes high
priority. Many departments use manufacturer supplied
protocols for imaging which are not always optimised
between image quality and radiation dose (ALARA).
Material and Methods
Whole body phantom PBU-50 (Kyoto Kagaku ltd., Japan)
for imaging in radiology has been imaged on Varian iX OBI
1.5 and TrueBeam 2.5 accelerators (Varian Medical
Systems, USA). Manufacturer’s default protocols were
adapted by modifying kV and mAs values when imaging
different anatomical regions of the phantom (head,
thorax, abdomen, pelvis, extremities). Images with
different settings were independently reviewed by two
persons and their suitability for IGRT set-up correction
protocols were evaluated. The suitable images with the
lowest mAs were then selected. The entrance surface dose
(ESD) for manufacturer’s default protocols and modified
protocols were measured with RTI Black Piranha (RTI
Group, Sweden) and compared. Image quality was also
measured with kVQC phantom (Standard Imaging, USA) for
different protocols. The modified protocols have been
applied for clinical work.
Results
The default manufacturer’s protocols on TrueBeam linac
yielded 9.4 times lower ESD than on iX linac (range 2.5-
24.8). For most cases it was possible to reduced the ESD
on average by a factor of 3 (range 0.9-8.5) on iX linac by
optimising imaging protocols. Further ESD reduction was
also possible for TrueBeam linac.
Conclusion
The imaging doses on new TrueBeam accelerator is
substantially lower than on previous iX platform.
Manufacturer’s default IGRT protocols could be optimised
to reduce the ESD to the patient without losing the
necessary image quality for patient set-up correction. For
patient set-up with planar kV imaging the bony anatomy is
mostly used and optimization should focus on this aspect.
Therefore, the current approach with anthropomorphic
phantom is more advantageous in optimization over
standard kV quality control phantoms and SNR metrics.
EP-1515 A novel attachment system for cutouts in
kilovoltage x-ray beam therapy
M. Baumgartl
1
, G. Kohler
1
1
University Hospital Basel, Clinic for Radiooncology,
Basel, Switzerland
Purpose or Objective
Customized shielding in superficial and orthovoltage
therapy is a common procedure to spare healthy tissue and
is nowadays mainly based on in-house cutouts attached to
applicators or patients. However, the production of
customized shields could be time consuming and does not
always provide a promising result. Inaccuracies during the
replacement of cutouts may arise if the same applicator is
required to treat different patients or target volumes (TV)
immediately one after the other. An adequate, fast and
reproducible approach of shielding was developed to treat
the TV in the low and medium energy (kV) range for
standard applicators.
Material and Methods
In our department most of the cutouts have a straight-
edged shape. Our developed in-house frame-based system