Comparing the HighDose Region Isodose FallOff for Protons versus IMRT in Prostate Cancer: Is IMRT Su

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Comparing the High-Dose Region Isodose Fall-Off
for Protons versus IMRT in Prostate Cancer Is
IMRT Superior?S. Keole, G. Zhao, W. Hsi, C.
Vargas, C. Morris, W. Mendenhall, R.
Henderson, N. Mendenhall
University of Florida Proton Therapy Institute,
Jacksonville, Florida

Department of Radiation Oncology, Boca Raton
Community Hospital, Florida
Purpose
Results
There is no significant objection that overall
patient dose with proton therapy (PT) is lower
than that seen with intensity-modulated radiation
therapy (IMRT). There is concern that in the
high dose (HD) regions, the PT dose fall-off is
not as rapid as seen with IMRT, leading to the
volume of normal tissue in this area to receive a
higher dose. Simply stated, many believe that
the HD normal tissue sparing may be superior for
IMRT versus PT. This study focuses on the HD
regions by comparing prospective plans created
for both PT and IMRT with an emphasis on HD
fall-off in the rectum. This project gives
further explanation to the previously published
work from our institution (Vargas et al, IJROBP
2008) with particular focus on the HD region.
Conclusions
Materials and Methods
In discussing CTV-80 IDL, PT was overall
superior to IMRT in towards the rectum and
equivalent to IMRT towards the bladder. IMRT was
superior to PT laterally. This was due to large
smearing, the use of lateral PTV expansions and
larger than required proximal and distal margins
used in this 1st generation of treatments at
UFPTI. Many of these planning parameters have
been modified. In discussing 95-80 IDL
fall-off, PT is superior to IMRT in the axial
vectors but is 1 mm more in the cranial-caudal
vectors, which makes sense, as the IMRT planning
was co-planer. The small advantage of photons
versus protons in regards to LP is seen here. It
is negated in the other vectors since multiple
fields are required for IMRT. To answer the study
question, IMRT is not superior to PT in either
the HD region or in regards to HD falloff.
Nine patients treated at our institution in 2006
were prospectively planned for both PT and IMRT.
Measurements were taken and recorded in the
isocentric plane in all 6 vectors from the
isocenter to the prostate edge (CTV), PTV edge,
95 isodose and 80 isodose lines (IDL) in the
axial (Ax) and sagittal (Sa) views. The HD
region was defined as the distance between the
CTV and the 80 IDL. HD falloff was also
evaluated by the 95 IDL to 80 IDL distance. In
sum, 8 sets of measurements in 6 vectors were
recorded for each patient (the anterior and
posterior vectors were replicated in Ax and Sa).
Additional posterior points were taken on the Sa
view 1.5 cm superior and 1.5 cm inferior to the
isocenter plane. In sum, 10 sets of HD region
measurements were taken for each patient
representing 8 different positions.
Vargas et al, IJROBP 2008
Vargas et al, IJROBP 2008