ESTRO 36 Abstract Book

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S456 ESTRO 36
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beam-on
time.
Results
The mean equivalent doses in 2-Gy fractions to 40%
(EQD40%) of the volumes of the bilateral hippocampus
were for 9.90/5.31 Gy for coVMAT/VMDWAT, respectively.
The EQD40% for VMWAT were < 7.3 Gy, which is the
threshold predicting cognitive impairment, as defined by
Gondi et al., and were significantly lower than those for
coVMAT. The mean equivalent doses in 2-Gy fractions to
2, 10, 20, 30, 50, 80, 98 % (EQD2-98%) of the volumes of
the bilateral hippocampus was also significantly lower
than those of coVMAT. VMDWAT also significantly reduced
the EQD40% and EQD2-98% of the left hippocampus. While
the normal brain volume receiving 5 Gy (V5) was
significantly larger in VMDWAT, as compared to coVMAT,
the normal brain volume receiving 10, 15, 20, 25, 30, 35,
40, 45, and 50 Gy (V10–50) was significantly smaller in
VMDWAT. The conformity and homogeneity indices were
significantly better in VMDWAT. The mean treatment time
of VMDWAT was significantly longer than that of to
VMDWAT (67.1/70.1 seconds in coVMAT/VMDWAT,
respectively).
Conclusion
VMDWAT significantly reduced the doses to the bilateral
and left hippocampus compared to coVMAT. The target
conformity and homogeneity were significantly better in
VMDWAT. Although the treatment time and V5 of the
normal brain was increased in VMDWAT, V10–50 of the
normal brain was significantly decreased in VMDWAT.
VMDWAT could be a promising treatment technique for
pituitary adenomas and craniopharyngiomas.
PO-0844 Dosimetric Evaluation of MLC and Fixed Cone
for Patients in the Prone Position with CyberKnife
S.K. Ahn1, J.H. Cho1, K.C. Keum1
1Yonsei Cancer Center- Yonsei University, Department of
Radiation Oncology- Yonsei Cancer Center- Yonsei
University Health System- Seoul- Korea, Seoul, Korea
Republic of
Purpose or Objective
The constraints of systems using fixed cones have been
improved with the recent introduction of the multileaf
collimator (MLC) to the CyberKnife® system. This study
evaluated the dosimetric impact of the MLC in stereotactic
body radiation therapy for spine lesions, with the patient
in the prone position.
Material and Methods
Sixteen patients with spinal tumors, who were treated
with CyberKnife® M6 TM, were placed in a body fixer and
scanned with four-dimensional computed tomography
(4DCT).
A total of 32 treatment plans were set up with two fixed
cones (ray tracing algorithm) and MLC (finite-sized pencil-
beam algorithm), using the MultiPlan® System. A total of
24 Gy in four fractions was prescribed to the 78%–83%
isodose line, encompassing at least 95% of the planning
target volume (PTV).
The XSight® prone tracking method was used for target
tracking, and the Synchrony® Respiratory Tracking System
was used for motion tracking. For the PTV, the maximum
dose, homogeneity index (HI), and conformity index (CI)
were analyzed. For the spinal cord and bowel, the
maximum dose (D 0.03 cc) was analyzed. The other
analyzed parameters included monitor unit, treatment
time, beam number, and node number.
Results
Regardless of the type of collimator, the difference among
the maximum dose, HI, and CI values of the PTV was
3.1±2%, while the maximum dose of the spinal cord and
bowel was 9.7±4.5%, indicating clinically insignificant
differences. For the other parameters, the values of the
treatment plan using MLC were lower by 53.8±8.4% for MU,
by 39.5±7.5% for treatment time, by 49.3±7.3% for beam
number, and by 49.7±7.1% for node number, compared to
the use of fixed cones. The differences were larger when
the tumors were greater in size.
Conclusion
There are dosimetric advantages to evaluating patients in
the prone position for lesions that are anatomically
located in the back, such as spinal tumors. However, MLC,
which has fewer treatment nodes and a shorter treatment
time, is also useful in the prone position because the
maintenance of positional reproducibility is critical.
PO-0845 Automatic treatment planning of FFF VMAT
for breast cancer: fast planning and fast treatment
E.L. Lorenzen1, K.L. Gottlieb1, C.R. Hansen1, H.R.
Jensen1, J.D. Jensen2, M.H. Nielsen2, M. Ewertz2
1Odense University Hospital, Laboratory of Radiation
Physics, Odense, Denmark
2Odense University Hospital, Department of Oncology,
Odense, Denmark
Purpose or Objective
Forward planned tangential radiotherapy with wedges or
few segments is the standard technique in many centres
for radiotherapy after breast conserving surgery. Helical
techniques such as Thomotherapy and VMAT can be used
to increase conformity but may increase the volume
receiving low doses and the treatment planning can be
time-consuming. In the present study we evaluate FFF
VMAT using automated planning by comparison with
manually planned tangential radiotherapy on its plan
quality as well as its efficiency in both treatment planning
and delivery.