856 / 1082
S840
ESTRO 36 2017
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accuracy (always <1mm, further details have been
discussed in another our work).
Results
The mean D
98%
, D
mean
and D
2%
of the PTV were (90.0±3.5)%,
(99.4±1.8)%, and (106.9±3.1)% respectively. The minimal
dose to the target was slightly lower than the objective,
however it was sacrificed in order to have low doses to the
healthy brain. The mean values of conformity,
homogeneity and gradient score indexes were 1.4±0.2,
1.08±0.03 and 57±10 respectively and all indexes were
comparable with published results (Table 1). The V
12Gy
of
the healthy brain was strictly respected except in two
cases, where multiple lesions were treated and the
prescription was 20Gy and 21Gy. The median V
12Gy
was
8.8cc. No acute toxicity of any kind was recorded. The
mean treatment time was 15±4minutes with a maximum
of 46minutes for a patient treated with two consecutive
plans irradiating 4 lesions.
Conclusion
HT radiosurgery for single and multiple brain metastases
appears feasible with satisfying dosimetric results.
Moreover, it appears to have encouraging clinical
outcomes and the use of non-invasive set-up improves the
treatment cosmetic and patient comfort.
EP-1579 Practical dosimetrical issues in Intraoperative
electron radiation therapy
S. Wadi-Ramahi
1
, F. Alzorkany
1
, B. Moftah
1
, A.
Alsuhaibani
2
1
King Faisal Specialist Hospital and Research Center,
Biomedical Physics, Riyadh, Saudi Arabia
2
King Faisal Specialist Hospital and Research Center,
Radiation Oncology, Riyadh, Saudi Arabia
Purpose or Objective
Our intraoperative radiation therapy (IORT) service is an
active one treating around 50 patients/year. Sarcomas
and gastric tumors are the most we treat with IORT.
Certain cases pose clinical challenges, such as the use of
cone blocking, abutting treatment fields, retreats, and
bone partially covering the treatment area. The purpose
of this work is to discuss some of these situations and our
proposed solutions used clinically at the time.
Material and Methods
We have the Mobetron® linac (IntraOp, Ca) which can
deliver 6, 9 and 12MeV high dose rate electron beams, and
an array of cones ranging in diameter from 3 to 10 cm for
zero, 15 and 30 deg bevel angles. Only three clinically-
faced dosimetrical issues will be discussed here: a)Use of
lead shielding to conform to an irregularly shaped tumor,
b)The presence of bone in the treatment field as in
intraarticular sarcoma and pelvic tumors, c)Abutting two
electron cones to treat a long tumor bed.
Results
a) Following a case requiring ~30% of the area to be
blocked, we measured the output for 30% blocked cones
using a 0.125cc chamber in water. Each output was
normalized to that of the 10cm open cone to calculate
OF
blocked
. For cones with diameter > 6 cm the OF
blocked
was
within -5% of the OF
open
, for those with diameter <6 cm
had a decreased OF
blocked
reaching almost to 80% of the
OF
open
. This behavior was noted for all energies, Fig.1. The
use of the OF
blocked
for treatment is a physics decision,
taking into account the amount of blocked area of the
cone, cone size and beam energy. Noting that the OF
blocked
were measured for 30% blocked area. b) For a tumor that
is partially shielded by bone and treated with a single en
face electron beam requires balancing penetration
through the bone with exposing normal tissue. The
thickness of the bone in a plane parallel to the direction
of the beam is measured and then converted to effective
depth by using the coefficient of equivalent thickness
(CET) using the electron density of bone,
1.65electron/mL. This distorts the shape of the tumor by
introducing deep arms, Fig2a. One such case was a
sarcoma extending to the intraarticular space and the
humeral head covering part of the field. The energy in this
case is chosen such that 90% isodose is at the deepest part.
Shielding of the normal tissue underneath the shallower
parts of tumor is discussed and applied according to
practicality of site. C) For long tumors, abutting circular
cones at the surface will leave lateral parts under-dosed,
while overlapping the cones will introduce high dose,
Fig2b. The clinical decision for these cases is to abut at
the surface to avoid hot spots in the overlapped area.
Overlap of low doses from each cone at depth is deemed
acceptable.