910 / 1082
S894
ESTRO 36 2017
_______________________________________________________________________________________________
Purpose or Objective
To evaluate if planned doses for prostate and rectum are
equal to the doses which are actually delivered and to
determine adaptation points for the accumulated dose.
Material and Methods
Twenty four patients with intermediate and high-risk
prostate cancer who were going to be treated with image
guided radiotherapy were enrolled. A plan-CT (pCT) and
nine treatment kilovoltage conebeam-CT (kvCBCT) scans
were acquired prospectively during the first three weeks
of a prostate lGRT treatment (a total of 240 CTs). A rectal
emptying preparation and a full bladder protocol were
used. For each patient, a deformable image registration
(DIR) from the pCT to each of the nine kvCBCT was
performed with RayStation treatment planning system. All
registers were revised and recontoured by a Radiation
Oncologist, establishing regions of interest (ROIs) for a
second DIR with control of such ROIs. For every patient, a
hypofractionated VMAT schedule (15 x 3.82 Gy) was
planned and correlated with their kvCBCT images, being
able to determine the accumulated and total doses that
would have been actually delivered. Since the pCT day, a
nutritional evaluation control with anthropometric and
biochemical parameters was performed for each of the 24
patients.
Results
A significant difference between planned and delivered
D98 CTV 57 (p=0.026) and D2 CTV 57 (p=0.005) was
observed; however, the average D98 CTV 57 delivered was
higher than the prescription dose. Despite not having
observed a significant difference in V36.5 of the planned
and delivered to the rectum, the delivered doses to 50%
of the rectum exceeded the planned constraints in 37.5%
of the patients. A significant rectum volume variation was
observed during the first week of treatment. An
accumulated delivered dose to 50% of rectum > 1194 cGy
in fraction five was a significant predictor for exceeding
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