ESTRO 36 Abstract Book

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S843 ESTRO 36 2017
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For Total Body Irradiation (TBI) very few experiences of
dose calculation using Treatment Planning Systems (TPS)
have been reported. The estimation of local dose
inhomogeneity and sparing of critical organs is usually
performed based spreadsheet calculation fine-tuned by in-
vivo measurements. No reports exist for treatment
planning with a TPS and dose accumulation based on
deformable registration. We report the implementation of
these techniques using RayStation ® commissioned to
perform any type of treatment plan and to estimate actual
dose distribution as delivered with a Volumetric
Modulated Arc Therapy (VMAT)-based TBI technique.
Material and Methods
For TPS commissioning, the PTW® water tank was
positioned at 170cm from the LINAC (Elekta Synergy ®).
PDD, profile and output factors were acquired (8.5x8.5,
10x10, 17x17, 34x34, 51x51, 68x68, 68x8.5, 68x17, 68x34,
68x51 cm2). The TBI plan was calculated using a total body
CT in supine and prone position and the inverse planning
and 3DCRT module available in the TPS. A dose of 600cGy
in 6 fractions was prescribed to the midline of the patient
outline as contoured in supine and prone position. A
longitudinal VMAT-technique with 8 arcs [330°÷30° with
step of 10°] was simulated using 6MV photons. The
ANACONDA algorithm was applied to perform the elastic
image registration (0.15cm of grid size) between the two
CTs. The deformed vectors fields obtained were used to
warp the dose grid allowing, the dose summation between
the supine and the prone treatment.
Results
The relative weighting factors of the beams were
experimentally obtained and confirmed by the calculation
of the TSP; a specific MU/fx was set for each gantry angle
into the range of [100÷191]MU/fx, based on the previously
published VMAT-technique. For the supine and prone
treatments, 607cGy and 544cGy were recorded at the
respective prescription points in the separate treatment,
respectively. The summation of both dose distributions
witch dose warping using the SumDoses tool developed as
described above resulted in an average dose of 1154cGy
with quite uniform irradiation.
Conclusion
This study assessed the possibility to apply a dedicated
TPS with hybrid algorithms to the novel VMAT-TBI
technique. A real-time dosimetry was obtained simulating
the patient treatment in both supine and prone setup and
a cumulative dose was analyzed using deformation and a
summation of the dose grids. With this planning approach,
lung sparing can be performed as before with
individualized lung blocks but this approach would also
provide the possibility to directly modulate dose
reductions over the lungs (provided appropriate patient
positioning can be assured). After full commissioning of
the TPS for extended SSD-conditions within the
requirements of the TBI-delivery technique, an optimized
comprehensive treatment planning approach would be
available for this treatment paradigm.
EP-1585 A practical method to reduce monitor units in
prostate cancer RapidArc plans
D. Sánchez-Artuñedo1 , S. Jiménez-Puertas1, M. Sancho-
Navarro1, M. Hermida-López 1
1 Hospital Universitari Vall d'Hebron, Servei de Física i
Protecció Radiològica, Barcelona, Spain
Purpose or Objective
While optimizing a RapidArc (RA) plan with Eclipse
Progressive Resolution Optimizer (PRO) algorithm (Varian
Medical Systems), it is possible to select a monitor units
(MU) objective value with the MaxMU dimensionless
parameter. We aimed to minimize MU for prostate RA
plans as a function of the MaxMU value.
Material and Methods
We retrospectively evaluated 40 prostate RA plans
optimized with the PRO algorithm of Eclipse v.11.
Prescribed doses were 57 Gy/ 59.5 Gy for PTV1 (including
prostate and seminal vesicles), and 15 Gy/17.5 Gy for a
simultaneous boost to the prostate only, for 30/35
fractions. For each patient, the original plan optimized
without a MaxMU constraint (base plan ) was reoptimized
(reoptimized plan) for six values of MaxMU: 1000, 800,
700, 600, 500 and 400. Differences in dosimetric
parameters of PTV and OAR between the base and
reoptimized plans were analyzed with a paired samples t-
test. For PTVs the mean dose, D2%, D98%, V95% and (D5%-
D95%) were evaluated. For all OAR (rectum, bladder and
femoral heads) the mean dose was considered.
Furthermore, in rectum, V50, V60, V70; in bladder, D67%,
V30 and in femoral heads and bladder, D2%.
Results
For MaxMU ≤ 0.75×MUbase_plan , the mean reduction of MU
was of -2.3% (p<0.001). For MaxMU between 0.75×MU and
MU, the mean reduction of MU was -1.3% (p=0.01). For
MaxMU ≥ MU, the mean MU increased by 1.2% (p<0.001).
Base plans with MU > 650 showed a MU mean reduction of
-2.2% (p<0.001). The maximum mean decrease was
obtained for MaxMU=500 (-3.7%; p<0.001). Base plans with
MU < 650 showed a MU mean increase of 1.7% (p<0.001).
However, for some plans the MU decreased (see graph).
No clinically relevant differences were found for the
analyzed dosimetric parameters.’
Conclusion
Prostate cancer Rapid Arc plans may be divided in two
groups based on the MU of the base plan. For plans with
MU < 650, MU tend to increase if MaxMU parameter is used,
but not in all cases (see graph). Due to this variability, we
recommend to use this parameter and to compare the
obtained plan with the base plan. For plans with MU > 650,
a mean MU decrease was obtained being maximum for
MaxMU=500. We recommend in this case to reoptimize
using this value. The MU reduction was achieved without
compromising plan quality.
EP-1586 ART and VMAT–the benefits in bone marrow
sparing for patients with bladder cancer
M. Poncyljusz1, D. Blatkiewicz1, J. Chorazy1, B. Czyzew1,
P.F. Kukolowicz 1, M. Piziorska1
1The Maria Skłodowska-Curie Memorial Cancer Centre
and Institute of Oncology, Department of Medical
Physics, Warsaw, Poland
Purpose or Objective
The combination of Adaptive Radiotherapy (ART) and
VMAT allows to create more conformal plans for patients
with bladder cancer without risk of missing target during
changes of bladder volume and shape. Relative position of
small bowel is influenced by the volume of treated
bladder. Recent reviews demonstrated the advantages of
using ART in bowel sparing during bladder irradiation. The
aim of this study was to assess feasibility of simultaneous