ESTRO 36 Abstract Book

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S114 ESTRO 36 2017
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Figure 2 shows the comparison between the normalized
lateral X and Y profiles of EPID images acquired with and
without the B-field
(3x3,5x5,8x8,10x10,12x12,15x15,20x20 cm2). More than
99% of the points showed local deviations smaller than 2%
for the X and Y profiles.
The 2-D γ-analysis showed that the averaged γmean was
0.42 ± 0.16 and the % γ≤1 was 96.6 ± 4.5.
Conclusion
EPID images acquired with and without B-field are
virtually identical, indicating that the presence of a small
(2.5 mT) magnetic field at the EPID level in the MR-Linac
should not become an impediment for the implementation
of EPID dosimetry in the MR-Linac.
Acknowledgements
This research was partly sponsored by Elekta AB,
Stockholm, Sweden. The authors would like to thank
Robert Spaninks (Elekta) for assistance with the
measurements.
OC-0230 Treatment log files as a tool to identify
inaccuracies in scanned proton beam delivery and
planning
M. Belosi1, R. Van der Meer 1, P. Garcia de Acilu Laa2, A.
Bolsi1, D. Weber1, A. Lomax1
1 Paul Scherrer Institute, Centre for Proton Therapy,
Villigen PSI, Switzerland
2 Hospital Universitario Puerta del Sur Hospitales de
Madrid, Radiofisica Hospitalitaria, Madrid, Spain
Purpose or Objective
Dose distributions delivered at Gantry2 (G2) at the Paul
Scherrer Institut (PSI) can be reconstructed on the patient
anatomy based on machine log files. These dose
reconstructions are a powerful tool in identifying potential
issues related to the integrity of the patients’ dose
delivery, as has already been demonstrated for a first
series of patients treated in G2 for skull base chordomas
(Scandurra et al. 2016). Here, such calculations have been
extended by investigating their dependency on planning
technique (e.g. SFUD vs IMPT, field direction etc) and on
couch position. The latter is crucial for quality assurance
of the delivery of patched fields (different sub-fields
combined to treat large areas) necessary for large H&N
and pelvic treatments.
Material and Methods
As of November 2015, 43 patients were treated on G2 for
a total of 74 plans (21 SFUD, 51 IMPT and 3 SIB) and 248
fields (average of 4 fields per plan), of which 26 fields
were patched. Parameters recorded during the treatment
delivery of these patients (spot positions, MU’s per pencil
beam, couch and gantry position) are stored into a log file
and used to reconstruct the 3D dose distribution by an in-
house developed Independent Dose Calculation software
(Meier et al. 2015). A MATLAB script calculates the dose
metrics by comparing the reconstructed to the nominal
dose distribution. These metrics include the maximum,
minimum and mean dose differences as well as the
percentage of voxels within +/- 1% of the nominal dose
(pass rate).
Results
Table 1 shows the results of the log file analysis.
Interestingly, and despite the typically higher modulation
for IMPT, the average pass rate for both SFUD and IMPT is
similar, with the 95% percentile actually being a little
better for IMPT. In addition, complex plans with steep in-
field dose gradients, such as SIB treatments, also had pass
rates >99%. Nevertheless, highly modulated plans can have
larger local dose differences as seen by the larger max
dose deviation in Table 1 and demonstrated for a specific
case in Figure 1. Hence, attention should be paid to the
location of isolated, highly weighted spots.
Finally, the results of the first patched field treatments (2
to 4 patches per field) did not show any evidence of dose
deviations at the interface between patches.
Conclusion
3D dose reconstruction using treatment log files is a
powerful tool to identify delivery problems and trends,
and to improve planning robustness. Further effort should
be invested in order to predict field robustness to delivery
fluctuations before the clinical delivery of the plan as part
of the plan’s specific QA.
OC-0231 The suitability of radiochromic film in 0.35T
magnetic field CO-60 compared with conventional 6MV
D.L.J. Barten1, L.J. Van Battum1, D. Hoffmans1, S.
Heukelom1
1VUMC, Radiotherapie, Amsterdam, The Netherlands