Physica Medica: European Journal of Medical Physics - Volume 32 - Supplement 1 - February 2016

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a further optimization of linac model within the TPS, in particular the value
assigned to the underdosage effect of tongue-and-groove. Further inves-
tigations and tests are going on and will be presented.
A.158
A METHOD TO DEFINE ISODOSE-BASED STRUCTURES IN DOSE PAINTING
TREATMENT OPTIMIZATION OF GLIOBLASTOMA MULTIFORME TUMORS
(GBM)
M. Orlandi A. Botti E. Cagni R. Sghedoni L. Orsingher P. Ciammell
C. Iotti M. Iori a Arcispedale S. Maria Nuova, Fisica Medica, Reggio Emilia,
Italy; b Arcispedale S. Maria Nuova, Radioterapia, Reggio Emilia, Italy
Introduction: This work compares several strategies to fit a Dose Paint-
ing (DP) map, based on Apparent Diffusion Coefficient (ADC) map, with an
isodose structure set, to be submitted to the optimization process using a
commercial planning system.
Materials and Methods: Stereotactic Radiation Therapy of 6 GBM pa-
tients, with CTV dose escalated between 25 and 50 Gy in 5 fractions, are
considered. A non-uniform Biomarker-based Dose map (BD) is obtained from
an ADC map of each patient, registered with the planning CT scan. The ADC
voxels values, within the CTV, are converted to dose through a linear func-
tion with threshold in dose and ADC values.
In order to perform DP By Contours, 9 CTV substructures are created,
discretizing BD map in 9 subvolumes corresponding to 9 dose levels of the
BD. A Discretized Dose map (DD) is obtained assigning to each voxel, be-
longing to the dose level k, a uniform dose of value Dk.
Four different methods to create the substructures are implemented.
IsoDose Method. The dose interval prescription is divided in 9 equal intervals.
IsoVol Method. The values of the isodose levels are calculated dividing the
volume of the absolute CTV DVH in 9 intervals.
IsoVD Method. An arbitrary function, δDV(D, V), is defined and divided in
9 intervals to obtain the corresponding dose values.
minQF Method. Using a genetic algorithm, the Quality Factor is mini-
mized, generating iteratively a different DD. The Quality Factor is defined
as QF = 1 − < Qi > , where Qi = DDi/BDi for the i-th CTV voxel
The QF is used also to compare the resulting DDs of each method with the
BD.
Results: The minQF method provides QF values closer to zero for 5 pa-
tients (<QF> = 0.024); only in one case IsoVol method is better, for less than
1%, than minQF method.
Conclusions: A robust strategy in order to select the structure set that better
fit a BD map is found in minQF method, regardless of the way it used to
obtained the BD.
References
[1]
[2]
A.159
A FEASIBILITY STUDY OF TOMOTHERAPY DOSE PAINTING
HYPOFRACTIONATED TREATMENTS ON GLIOBLASTOMA MULTIFORME
(GBM) GUIDED BY MRI
M. Orlandi A. Botti E. Cagni R. Sghedoni L. Orsingher P. Ciammell
C. Iotti M. Iori a Arcispedale S. Maria Nuova, Fisica Medica, Reggio Emilia,
Italy; b Arcispedale S. Maria Nuova, Radioterapia, Reggio Emilia, Italy
Introduction: The aim of this study is to investigate the feasibility in
Tomotherapy (HT, USA) of a hypofractionated dose painting (DP) treatment
on GBM cancer patients using Apparent Diffusion Coefficient (ADC) maps.
Material and Methods: Five patients, who underwent GBM radiotherapy
treatment, are retrospectively replanned with dose to the CTV escalated
between 25 and 50 Gy in 5 fractions. The objective is that the 95% of the
CTV received at least 25 Gy. Biomarker-based Dose maps (BD) are gener-
ated from ADC maps registered with planning CT, for each patient. The ADC
voxel values, within the CTV region, are converted to dose values using a
linear function with a double threshold.
In order to perform DP By Contours 9, CTV substructures are created,
discretizing BDmap in 9 subvolumes, corresponding to 9 dose levels, bymeans
of a Discretized Dose map (DD). The DD is realized minimizing, with an it-
erative process, the difference between BD and DD, evaluated by means of
Quality Factor: QF = 1 − < Qi > , where Qi = DDi/BDi for the i-th CTV voxel
Then plans are optimized on a standard HT treatment planning system and
Planned Dose maps (PD) are obtained. For each patient these PDs are com-
pared with the prescribed BDs using Qi maps, where, in this case, Qi = PDi/
BDi. The quality of the treatment plans is evaluated in term of QF and Q0.9-
1.1, that is the CTV volume in which Qi ranges from 0.9 to 1.1.
In the end all plans are delivered, for quality assurance, on Octavius system
(PTW, Germany).
Results: All plans are clinically acceptable in terms of dose to OARs and
coverage of the CTVs. The QF mean is 0.148, while Q0.9-1.1 mean is 68%.
The mean delivery time is 49.7 min. All DQA performed are within the ac-
ceptance criteria with a mean value of γ(3 mm-3%) of 87.4%.
Conclusions: The plans obtained are deliverable and respect the OARs dose
constraints. Some concerns still exist about the HT delivery time.
References
[1]
[2]
A.160
COMMISSIONING AND IMPLEMENTATION OF DOSIMETRY CHECK WITH
TRUEBEAM FFF BEAMS AND NEW DMI-EPID
L. Orsingher *, M. Orlandi, E. Cagni, A. Botti, R. Sghedoni, M. Iori. Servizio
Di Fisica Medica, IRCCS-ASMN, Reggio Emilia, Italy
Introduction: Due to the recent findings on the weak correlation between
conventional QA gamma metric and dose errors in DVHs, pretreatment dose
QA should evolve to a patient DVH-based QA approach.
The aim of this study is to validate the commissioning of Dosimetry Check
(DC, MathResolution®) for QA plans with both flattened and FFF beams
with particular attention to stereotactic treatments.
Materials and Method: We configured DC (v.4.10) for Truebeam Stx,
equipped with the new DMI-EPID, which overcomes limitations of previ-
ous devices and permits more accurate measurements. Several dose kernels
were computed for 6MV and 6FFF using different measurements data sets.
For each energies, an all-field model and a small-field one were created
using measurements of field sizes from 1 to 28 cm and a subset of sizes
up to 15 cm, respectively.
Reference plans were calculated in the Eclipse TPS (v. 13.0) with AAA al-
gorithm against a series of homogeneous geometrical phantoms. The kernels
were validated by delivering the plans in both pre-treatment and transit
modalities.
Results: The DC model, which better agrees with measurements, was ob-
tained with a trial-and-error process changing various configuration
parameters. Dose and deconvolution kernel fits were within 1% of all com-
missioning measurements for both 6x-all-field-model and for 6FFF-small-
field model. A slightly worse agreement has been found for 6FFF-all-field-
model. The kernels were validated by delivering the plans in both pre-
treatment and transit scenarios for static, IMRT and VMAT plans.
Conclusion: The DC is able to reconstruct 3D dose on homogeneous
phantom for FFF beams with DMI-EPID. However, the commissioning process
needs a systematic check of all steps to obtain a good agreement with ref-
erence data. Our preliminary results support the ability of DC as a tool for
QA both in pre-treatment and transit dosimetry. Further investigation will
be performed with inhomogeneous phantom using different algorithms.
e47 Abstracts/Physica Medica 32 (2016) e1–e70