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Conclusion
It was determined that the four testing materials were
MR/CT compatible, showed expected PDD curves, and
preliminary data show expected beam profiles with and
without 1.5T on the MR-Linac.
PO-0757 Variation of mean dose output from 204 UK
linacs (Jan-June 2015) and its potential clinical impact.
M. Bolt
1
, A. Nisbet
1
, C. Clark
2
, T. Chen
3
, R. Jena
4
1
St. Luke's Cancer Centre Royal Surrey County Hosp,
Radiotherapy Physics, Guildford, United Kingdom
2
National Physical Laboratory, Radiation Dosimetry,
Teddington, United Kingdom
3
University of Surrey, Chemical and Process Engineering,
Guildford, United Kingdom
4
Addenbrooke's Hospital, Department of Oncology,
Cambridge, United Kingdom
Purpose or Objective
Variation in dose delivered to patients directly impacts
the effectiveness of radiotherapy treatments. The drift
and daily fluctuations in linac beam calibration (output) is
a contributing factor to the cumulative dose received by
the patient. Knowledge of the variation in measured
outputs on a national scale provides an insight into the
uncertainties in dose delivery and its clinical impact.
Material and Methods
A request for 6MV output measurement data was sent to
all UK radiotherapy centres. In total, data was provided
for 204 linacs situated at 52 cancer centres across the UK.
The data spans 6 months from January to June 2015,
totalling almost 25,000 data points. Additional data
collected includes: linac model, year of install,
measurement equipment and recording method.
The dose response parameter, gamma, is the percentage
change in treatment response caused by a percentage
change in dose. Gamma values of 2.3 and 5.2
(representative for Head and Neck cancers) were used to
estimate the effect on TCP and NTCP respectively [1].
Results
Based on the collated data, the UK linac outputs had a
mean of -0.01% with one standard deviation of 0.88%.
There was a wide variety of recording methods, with 8
centres having no form of electronic record for daily
checks. Measurement data for both constancy devices and
ionisation chambers was provided for 29 linacs. Of these,
8 (28%) had a discrepancy between measurement devices
of greater than 0.5%, with 3 linacs (10%) having greater
than 1%.
The greatest variation in the mean output of an individual
linac was -2.1%, with 90% of linacs having a mean output
within 1%of the national mean. No significant variations
were observed based on the age of the linac.
The maximum range within a single centre for the mean
output for each linac was 2.3% (min:-1.1%, max:+1.2%) –
see figure. Assuming patients are treated on a single linac
for their treatment duration, this indicates a variation in
TCP of 5.3% and a variation in NTCP of 12% dependent on
which linac they are treated on at that centre.
Conclusion
The data collection process indicates that many
departments still rely heavily on paper QC records. The
variation in treatment outcomes caused by dose variation
alone indicates the importance of accurate QC. Output
adjustment is one of the simplest ways of maintaining
treatment consistency between individual patients, and
its significance should not be forgotten with the
introduction of more advanced techniques. This variation
in dose should be considered when participating in clinical
trials. This applies both to small scale local trials in which
the technique used may determine the treatment linac,
and therefore the dose delivered, as well as large multi-
centre trials where the dose variation should be
considered for the trial power calculations.
[1] Bentzen, S.M et al. (2000), Eur J Cancer, Mar: 36(5):
pp.615-620.
PO-0758 Development of patient-specific phantoms for
verification of SBRT planning using 3D printer
C.S. Hong
1
, D. Oh
1
, S.G. Ju
1
, M. Kim
1
, B.Y. Koo
1
, H.C.
Park
1
, D.H. Choi
1
, H. Pyo
1
1
Samsung Medical Center- Samsung Biomedical Research
Institute- Sungkyunkwan University School of Medicine,
Radiation Oncology, Seoul, Korea Republic of
Purpose or Objective
A new technique for manufacturing a patient-specific
dosimetric phantom (PSDP) using three-dimensional
printing (PSDP_3DP) was developed, and its geometrical
and dosimetric accuracy was analyzed.
Material and Methods
External body contours and structures of the spine and
metallic fixation screws (MFS) were delineated from CT
images of a patient with MFS who underwent stereotactic
body radiation therapy for recurrent thoracic spine
metastasis. Contours were converted into a
STereoLithography file format using in-house program. A
hollow, four-section PSDP was designed and manufactured
using three types of 3DP to allow filling with a muscle-
equivalent liquid and insertion of dosimetric film and grass
dosimeters in the axial and coronal planes, respectively.
To evaluate the geometrical and dosimetric accuracy of
PSDP_3DP, CT images were obtained and compared with
patient CT data for volume, mean density, and Dice
similarity coefficient (DSC) for contours. The dose
distribution in the PSDP_3DP was calculated by applying
the same beam parameters as for the patient, and the