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considering all the techniques, only 4.4% of mean g
mean
tests resulted out of tolerance. In addition, removing
class-2 errors, this percentage decreases to approximately
3%. Actually the workload of IVD procedures on 9 patients
is 1 hour per day.
Conclusion
IVD performed using SOFTDISO assures: (i) a rapid response
of dose delivery alert with a reduced workload; (ii) a large
number of patients tested daily and (iii) for out- of-
tolerance tests repeating IVD in the subsequent day, the
possibility to verify the efficacy of the adopted
corrections.
EP-1477 Evaluating gamma-index quality assurance
methods for Nasopharynx Volumetric Arc Therapy
(VMAT)
E.M. Pogson
1,2,3
, S. Arumugam
2
, S. Blake
1
, N. Roberts
4
, C.
Hansen
5,6
, M. Currie
7
, M. Carolan
7
, P. Vial
2
, J. Juresic
2
, C.
Ochoa
2
, J. Yakobi
2
, A. Haman
2
, A. Trtovac
2
, L.
Holloway
1,2,3,4,8
, D.I. Thwaites
1
1
University of Sydney, Institute of Medical Physics-
School of Physics- Faculty of Science, Sydney NSW,
Australia
2
South Western Sydney Local Health District, Liverpool
and Macarthur Cancer Therapy Centres, Liverpool,
Australia
3
Ingham Institute, Medical Physics, Liverpool, Australia
4
University of Wollongong, Centre for Medical Radiation
Physics- School of Physics, Wollongong, Australia
5
Odense University Hospital, Laboratory of Radiation
Physics, Odense, Denmark
6
University of Southern Denmark, Faculty of Health
Sciences- University of Southern Denmark- Denmark,
Odense, Denmark
7
Illawarra and Shoalhaven Local Health District,
Illawarra Cancer Care Centre, Wollongong, Australia
8
University of New South Wales, South Western Sydney
Clinical School, Sydney, Australia
Purpose or Objective
Pre-treatment dose verification is often performed on
dose measuring phantoms with some form of gamma
evaluation. However it has been shown that the clinical
relevance of a 3% and 3mm pass rate tolerance is
questionable. The purpose of this study is to simulate
machine errors of clinical significance for nasopharynx
patients and test if these errors can be detected on a
standard commercial phantom. In this study systematic
errors including collimator rotation, gantry rotation, MLC
shifts, and MLC field sizes are investigated.
Material and Methods
Ten retrospective VMAT patients were planned with a
department protocol. Machine errors were deliberately
introduced to all plans. Plans were modified by increments
using Python to create simulated error plans; -5 to 5° for
gantry and collimator angles and -5 to +5mm for MLC shift
and MLC field size, considering each parameter
separately. Simulated error plans (Dose
error
) were
compared to the original non-error plan (Dose
Baseline
)
utilising
equation
(1).
(1)
All error plans doses were then recalculated in Pinnacle
3
.
Plans were reviewed against acceptable tolerance limits.
Plans were above tolerance and considered unacceptable
if PTV D95%, Brainstem D1cc or spinal cord D1cc were
beyond a ±5% deviation in dose. Additionally if either of
the left or right parotid mean doses were beyond ±10%,
this was also considered an unacceptable plan.
The smallest unacceptable error plan for each error type
(including the Gantry (G), Collimator (C), MLC Shift (S),
and MLC Field Size (F) error was delivered on an Elekta
Linac and dose was measured using an ArcCheck. Gamma
analysis was performed in SNCpatient version 6.6 utilising
a global 3%/3 mm (10% threshold with correction off)
gamma pass rate. Before measurement, the Linacs were
tested for MLC, gantry and dose accuracy. Only one
patient’s -5° gantry error was deemed unacceptable and
subsequently measured, patient 2 with this error detected
(gamma pass rate of 68.8%).
Results
The results for 10 patients are shown in Figure 1.
Figure 1. Gamma pass rate (%) for non-error (NE) plans
and for deliberately introduced errors (including the
Collimator (C), MLC shift (S), and MLC Field Size (F)
error), where the latter are selected as exceeding dose
tolerances by the smallest magnitude. *Patient 1 F error
of -5 was not deliverable due to machine tolerances,
hence an F error of -2 is utilised here.
The global 3%/3mm gamma pass is able to detect the
majority of unacceptable plans, however some MLC field
size plans still pass. Decreasing the MLC field size by 1mm
can result in significantly reduced dose to the PTV, which
affects tumour control e.g. patient 3 MLC FS-1 passed,
however this plan under-doses the PTV63Gy by -5.4%
relative to the original non-error plan.
Conclusion
Not all deliberately introduced clinically significant errors
were discovered for VMAT plans using a typical 3%/3mm
(10% threshold with correction off) gamma pass rate.
EP-1478 A split field beam model of Beam Modulator
linear accelerator in Pinnacle treatment planning
system
M. Chandrasekaran
1
, S. Worrall
1
, M.K.H. Chan
1
, N.
Khater
1
, C. Birch
1
1
University Hospital Southampton NHS Foundation Trust,
Radiotherapy Physics, Southampton, United Kingdom