923 / 1096
S907
ESTRO 36
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the rectum constraints. There was an average weight gain
of 668 gr between the pCT day and the first day of
treatment, but no significant relation with not fulfilling
the prescription goals or organ at risk constraints was
observed.
Conclusion
The significant differences between CTV 57 prescribed
doses and those actually delivered do not have a clinical
impact because the average D98 CTV 57 is higher than the
prescribed dose. The V 36.5 delivered to the rectum in
37.5% of the patients exceed the planned constraints,
although this difference is not significant. The subgroup
analysis has shown significant anatomical variations. The
fraction five adaptation point for the accumulated doses
in the rectum (1194 cGy) allows to significantly predict
when the risk of not fulfilling the rectum V36.5 constraint
is high and a plan adaptation is needed. The significant
weight gain between the pCT day and the first day of
treatment has no significant relation with not fulfilling the
prescription goals or organ at risk constraints
.
EP-1667 MR-Guided Radiotherapy of Head and Neck
Cancers: Adaptive Planning Strategies
N. Dogan
1
, K. Padgett
1
, M. Duffy
1
, M. Samuels
1
1
University of Miami- Sylvester Comprehensive Cancer
Center, Department of Radiation Oncology, Miami-
Florida, USA
Purpose or Objective
Adaptive Radiotherapy (ART) with frequent imaging has
been used to improve dosimetric accuracy by accounting
for anatomical variations, such as primary tumor shrinkage
and/or body weight loss, in head-and-neck (H&N) cancer
patients. MR-guided radiotherapy technology provides
daily real time MR images in the treatment room, hence
has a great potential for online adaptive
radiotherapy. The purpose of this study is to provide an
assessment of different adaptive planning strategies using
three-source Co
60
and Magnetic Resonance Imaging (MRI)
Guided Radiation Therapy (MR-IGRT) System for treatment
of H&N cancer patients.
Material and Methods
Patients with locally advanced H&N cancers were
selected for this study. For each patient, six weekly MR
imaging were acquired on the ViewRay MR-IGRT system
during the course of radiotherapy. PTVs, parotids, cord,
brainstem, mandible, oral cavity and larynx were
contoured on planning MR image and all structures were
deformably-mapped on the weekly MR images. Three ART
planning strategies were explored: 1) A new optimized
IMRT plan on each weekly MR image (WeeklyAdapt) 2) A
new optimized IMRT plan on week four MR image only
(OneAdapt) and 3) Calculating the plan from the planning
MR on each weekly MR image (NoAdapt). The PTV coverage
for all ViewRay MR-IGRT plans were such that 95% of the
PTV received 100% the prescription dose. The differences
between accumulated doses on planning MR for all three
ART strategies were evaluated using the dose-volume
constraints for targets and critical structures.
Results
For PTV70, PTV60 and PTV54, as compared to the D95
coverage on planning MR, the differences in D95
accumulated doses between three ART strategies were
<2%. The maximum dose to both cord and brainstem
between WeeklyAdapt and only OneAdapt were very
similar and <1% as compared to the planning MR
values. However, NoAdapt Dmax for brainstem and cord
were >27% and 25% than the original planning MR Dmax
values respectively. The mean left and right parotid dose
remained very similar both WeeklyAdapt and OneAdapt
strategies although there were up to 10.7% increase in the
mean dose to the parotids with NoAdapt strategy.
Conclusion
This study demonstrated that no significant differences in
accumulated doses were observed between weekly ART
and only one ART at week four during MR IGRT of H&N
cancer patients. Further studies are needed to evaluate
benefits of daily online ART during MR IGRT.
EP-1668 Dose calculation accuracy using CBCT images
for head and neck VMAT
M.A. Carrasco Herrera
1
, B. Quintana
2
, J.M. Nieto
2
, F.J.
Luis Simon
1
, C. Santa Marta Pastrana
3
1
Hospital Universitario Virgen del Rocio, Medical Physics
department, Sevilla, Spain
2
Hospital Universitario Virgen del Rocio, Radiation
Oncology department, Sevilla, Spain
3
Universidad Nacional de Educación a Distancia UNED,