ESTRO 36 Abstract Book

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S675 ESTRO 36 2017
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The MRIdian MRI 60 Co radiotherapy system (ViewRay,
Oakwood, Ohio) combines an open split-solenoid MRI
scanner equipped for parallel imaging and three 60Co
gamma-ray sources.
The quantification of dose distribution perturbations due
to the presence of 0.35 T magnetic field represents an
issue.
The MRIdian TPS is equipped by two Montecarlo based
algorithms to calculate the dose distribution: the first one
has faster calculation time; and does not account for the
presence of magnetic field (B off). The second one has
slower calculation time; and takes into account for the
presence of magnetic field (B on). Hardware and software
implementation on MRIdian will be completed at our
institution within February 2017. Experimental
measurements are planned when the system will become
clinically operative.
Aim of this study was to compare the two algorithms in
order to evaluate which could be more accurate in an in
silico treatment planning study designed for locally
advanced rectal cancer (LARC). Material and Methods
This study includes 7 cases of patients affected by LARC.
For each patient of the study two plans were developed.
Same priority values for the optimization were applied.
The calculation of dose distribution was performed using
the two different algorithms object of this study (B off and
Bon).
Plans were performed in IMRT modality, adopting same
beams geometry consisting in one pseudo-arc composed
by three beam triplets.
The treatment plans were optimized according to usual
Quality Assurance protocols adopted in our Institution for
Linac IMRT treatments: the PTV1 was represented by
tumor and corresponding mesorectum; the PTV2 by
mesorectum in toto and pelvic nodes. Isotropic 0.7 cm
margins were added to PTVs. The total prescribed dose for
PTV1 was 55 Gy and 45 Gy for PTV2 through Simultaneous
Integrated Boost.
All plans were optimized for PTV coverage and sparing of
bowel bag and bladder. For PTVs coverage V95 and V105
were considered. For bowel bag V45 and for bladder the
mean dose were considered, respectively. Plans were
normalized at target median. Results
Table 1 summarizes the median values for PTV coverage
and organs at risk sparing obtained in the two cases. No
significant differences have been reported between the
two algorithms.
Conclusion
The system appears to be able to compensate the
disomogenities due to the presence of magnetic field
through the use of optimizer.
EP-1268 Tumor response according to NK cell change
during preoperative chemoradiotherapy in rectal
cancer
J. Heo 1, Y.T. Oh 1, O.K. Noh1, M. Chun1, J.E. Park2, S.R.
Cho3
1Ajou University School of Medicine, Radiation Oncology,
Suwon, Korea Republic of
2Ajou University School of Medicine, Pediatrics, Suwon,
Korea Republic of
3Ajou University School of Medicine, Laboratory
Medicine, Suwon, Korea Republic of
Purpose or Objective
The objective of this prospective study was to evaluate
the relationship between the circulating lymphocyte
subpopulation counts during preoperative
chemoradiotherapy (CRT) and tumor response in locally
advanced rectal cancer.
Material and Methods
In this prospective study, from August 2015 to June 2016,
10 patients treated with preoperative CRT followed by
surgery were enrolled. Patients received conventional
fractionated radiotherapy (50.4 Gy) with fluorouracil-
based chemotherapy. Surgical resection was performed at
4 to 8 weeks after the completion of preoperative CRT.
The absolute blood lymphocyte subpopulation was
obtained prior to and after 4 weeks of CRT. We analyzed
the association between a tumor response and change in
the lymphocyte subpopulation during CRT.
Results
Among 10 patients, 2 (20%) had evidence of pathologic
complete response. In 8 patients with clinically node
positive, 4 (50%) had nodal tumor response. All
lymphocyte subpopulation counts at 4 weeks after CRT
were significantly lower than those observed during
pretreatment (p < 0.01). A high decrease in NK cell count
during CRT (baseline cell count − cell count at 4 weeks)
was associated with node down staging (p = 0.034).
Conclusion
Our results suggest that the change of lymphocyte subset
to preoperative CRT may be a predictive factor for tumor
response in rectal cancer.
EP-1269 Comparison of 2 and 3 arc VMAT versus fixed
field IMRT and proton beam therapy in anal cancer
C. Kronborg1, E.E. Wilken2, J. Hansen1, L. Nyvang1, J.B.
Petersen1, E. Serup-Hansen2, K.L.G. Spindler1
1Aarhus University Hospital, Oncology, Aarhus C,
Denmark
2Herlev and Gentofte Hospital, Oncology, Herlev,
Denmark
Purpose or Objective
Chemoradiotherapy is the standard treatment for
squamous cell carcinoma of the anus (SCCA) and is the
source of both acute and late toxicity. Advanced
radiotherapy treatment techniques aim at reducing dose
to organs at risk (OAR) while maintaining target coverage
and dose homogeneity. Further, VMAT techniques shorten
delivery time considerably. We compared dosimetric
advantages of fixed field IMRT, 2 and 3 arc VMAT and
additional 3- and 4-field pencil beam scanning proton
therapy.
Material and Methods
Twenty patients with SCCA treated at two different
centres were included. Standard treatment was 64-51,2
Gy/32 F or 60-49,5/30 Gy/F delivered with 2 or 3 arc VMAT
technique and concurrent chemotherapy according to
local practice. Alternative treatment plans were
generated for all patients using 5- or 6- fixed field IMRT
and 3 arc VMAT (All Varian Eclipse planning system). Four
patients with doses above normal constraints (ex high V40
Gy to the bowel) were selected for additional proton
therapy planning; both 3- and 4- field plans were
generated (Eclipse ver. 10 Multi Field Optimization
(IMPT)). Bowel was delineated as potential bowel cavity
and bladder as total circumference.
Results