SCIENTIFIC REPORT 2010 - 2011 - IOV

Major Ongoing Research Projects
The Medical Physics Unit has been recently involved in two
projects concerning the verification of the accuracy of the dose
distribution calculated by a commercial treatment planning
system.
COmmiSSiONiNG SiEmENS virTual wEDGES iN ThE
ONCENTra TrEaTmENT plaNNiNG SySTEm uSiNG
GafChrOmiC EbT film
Principal investigator: Franca Simonato
Contributors: Roberto Fabbris, Alice Ferretti, Sonia Reccanello,
Roberto Zandonà
Purpose. Virtual Wedges were introduced in Siemens
LINAC to improve the treatment workflow. The aim of the work
was the validation of dose calculation by MasterPlan-Oncentra
treatment planning system for virtual wedged beams.
Methods. The Oncor Siemens accelerator installed in the
Radiation Therapy Unit produces 6 and 15 MV photon beams.
At first, the consistency of VW LINAC production was tested and
the EBT film measuring method was verified. Then, the measured
and calculated wedge factors and beam profiles were compared.
For 15°, 30°, 45°, and 60° wedge angles, the wedge factors for
different field sizes were measured by an ionization chamber and
the dose profiles acquired by Gafchromic EBT film.
Results. The comparison between measured and calculated VW
factors shows discrepancies that increase with field size and angle.
The OTP Enhanced algorithm fits better with measurements than
the Classic one, with overall improvement visible for large angles.
The agreement between measured and planned beam profiles is
within the limits reported by the ESTRO Booklet No. 7 in terms
of confidence limits.
Conclusions. The MasterPlan-Oncentra treatment planning
system determines wedge factors and VW profiles within the
requested accuracy in the majority of treatment conditions. For
big field dimensions and wedge angle, wedge factor accordance
was worse, but it could be increased with an improvement of the
LINAC dosimetric board calibration.
COmmiSSiONiNG Of a COmmErCial TpS baSED ON ThE
vmC++ mC CODE fOr ElECTrON bEamS: valiDaTiON
aND COmpariSON wiTh EGSNrC
Principal Investigator: Andrea Martignano
Contributors: Alice Ferretti, Franca Simonato
Purpose. Some commercial TPS already use MC engines
for dose calculation. The aim of this work was to perform the
commissioning of the VMC++ Monte Carlo (MC) engine
implemented in the Oncentra Masterplan TPS for electron dose
calculation, and to verify its accuracy comparing the results to the
EGSnrc MC code.
Methods. The commissioning procedure for the TPS consists
of measurements of output factors and profiles in x,y and z
direction, in both air and water. The BEAMnrc MC code was
used as a benchmark: BEAMnrc required the geometries of the
LINAC head, which were provided by Siemens; the optimisation
was done considering PDD and profiles in water. Commissioning
results were evaluated by means of 1D Gamma Analysis (2%,
2mm), calculated with a home-made Matlab program.
Masterplan dose distribution maps were compared to the
results of BEAMnrc, in two virtual phantoms: one made of
water with an air insert, and the second with a bone insert. The
comparison was done by means of 2D Gamma Analysis (3%,
3mm), and comparing significant profiles and PDD.
Results and conclusions. The results of the commissioning
of the TPS were good. The optimisation of the BEAMnrc model
of the LINAC required the modification of some components to
match the calculated and measured profiles: the final agreement
was very good. The agreement of the dose distributions calculated
with the TPS and with EGSnrc with the air-insert phantom was
high; with the bone-insert phantom there were differences of
about 10-15% in the bone region. This is due to the fact that
the Masterplan implementation of VMC++ reports the dose as
“dose to water”, instead of “dose to medium” (therefore it is not a
dosimetric error).
THE DEPARTMENTS - DEPARTMENT OF RADIOTHERAPY AND NUCLEAR MEDICINE
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