ESTRO 36 Abstract Book

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S823 ESTRO 36 2017
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Conclusion
Non-coplanar technique can reduce MLD, lung V20 and
spinal cord dose in both FB and DIBH. While these
reductions were relatively small in our patient group as a
whole - and compared to reductions possible by DIBH alone
- they were substantial in some patients. Therefore, the
NC approach should be exploited in patients not
compatible with DIBH for OAR dose reduction.
EP-1549 Cyberknife Iris based versus InCise based
plans for 20 cases of prostate oligonodal metastases
C.L. Chaw 1, N.J. VanAs1 , V.S. Khoo1
1 Royal Marsden Hospital Trust & Institute of Cancer
Research, Academic Uro-oncology, London, United
Kingdom
Purpose or Objective
To compare dosimetry and delivery efficiency between Iris
collimator and InCise multileaf collimator (MLC) (Accuray
Inc.Sunnyvale, CA) for patients with oligometastatic nodal
disease from prostate cancer.
Material and Methods
Treatment plans for 20 patients were performed on
MultiplanTM5.1.3 treatment planning system utilizing MLC
and Iris for 30Gy in 3 fractions. To minimize variation
between cases, nodal metastases located in pelvis and
abdomen with a distance of 0-10mm to organs at risk
(OAR; rectum, small bowel and duodenum) were chosen.
The clinical target volume (CTV) to planning target volume
(PTV) margin is 3mm. Dosimetric evaluation included PTV
coverage, CTV coverage, conformity index (CI), Paddick’s
new CI (nCI), homogeneity index, and gradient index.
Treatment delivery efficiency is measured by beam
delivery time (start of first beam to end of final beam,
including beam-on-time, robot motion, and intra-fraction
imaging), number of monitor units and number of beams
used. OAR dose sparing were analysed by D maxsmall bowel
dose constrained at D0.5cc: 25.2Gy, D5cc: 17.7Gy,
Dmax rectal dose constrained at D0.5cc:28.2Gy and
Dmax duodenum constrained at D0.5cc: 22.2Gy;
D5cc:16.5Gy and D10cc:11.4Gy. Statistical significance
was tested using Wilcoxon signed rank test.
Results
There were no statistically significant differences in
conformity indices or target coverage, but MLC plans were
more homogenous with small but significantly lower mean
target dose than Iris (2% difference in PTV mean dose;
4.8% difference in CTV mean dose; all P < 0.001). Gradient
index was also improved by 13% using MLC plans (P <
0.001). All OAR constraints were satisfied by both devices.
The small bowel mean dose was significantly lower by 52%
using MLC (p < 0.001). There was a significant reduction
in delivery time by 47% (mean 19.7 mins [range:13-30
mins] vs 37.0 mins [24-56], total monitor units used by
46%, and 74% reduction in number of beams by 74% with
the MLC-based plans (all p < 0.001).
Conclusion
Compared to the Iris, the InCise MLC produced comparable
target coverage but was significantly better in dosimety
with significant improved delivery efficiency.
EP-1550 Investigating the advantages of CyberKnife
M6 MLC over Iris collimator for Liver SBRT plans
R. Doro1, L. Masi1, V. Di Cataldo2, S. Cipressi2, I.
Bonucci 2, M. Loi3, L. Livi4
1IFCA, Medical Physics, Firenze, Italy
2IFCA, Radiation Oncology, Firenze, Italy
3University of Florence, Department of Clinical and
Experimental Biomedical Sciences "Mario Serio", Firenze,
Italy
4Azienda Ospedaliera Universitaria Careggi,
Radiotherapy Unit, Firenze, Italy
Purpose or Objective
The purpose of this study is to evaluate the performance
of the CyberKnife M6 systems equipped with MLC for Liver
SBRT plans. To this aim, MLC plans were compared to
clinical plans generated using circular apertures.
Material and Methods
21 clinical treatment plans for Liver SBRT created with IRIS
variable aperture collimator were optimized again on
Multiplan 5.3 TPS using MLC. Plans were created both for
first and second treatment cases and were prescribed
either in 3 or 5 fractions with prescription doses ranging
from 30 Gy to 45 Gy. PTV dimensions ranged from 25.7 cm3
to 233 cm3. The same OAR constraints were applied both
for IRIS and MLC plans. Evaluation parameters of each plan
included PTV coverage, Paddick's new CI (nCI),
homogeneity index (HI), gradient index (GI) and
prescription isodose. OAR (duodenum, stomach, bowel,
hearth) dose sparing was analyzed using the maximum and
mean doses (Dmean). Liver dose sparing was analyzed
using mean dose and the volume either inside 15 Gy (3
fractions) or 21 Gy (5 fractions) isodose. The dose delivery
efficiency was evaluated on the basis of planned monitor
units (MUs) and the reported treatment time per fraction.
The dose to the PTV was also summarized by the
generalized equivalent uniform dose (gEUD), using a=-20.
The mean values, standard deviation and p-values (two
tailed Student's t test) were computed between the two
comparison groups and statistical significance set at p<
0.05.
Results
The evaluation parameters for the MLC and IRIS plans are
shown in table 1. MLC plans achieved equivalent PTV
coverage and conformity when compared to IRIS plans and
minimized the low dose extension improving significantly
(p<0.001) the dose fall-off gradient with GI
increasing from 2.65 (MLC) to 3.13 (IRIS). Plans created
using MLC were generally prescribed to higher isodose
levels (73% vs 70.5%), which resulted in significantly more
homogeneous dose inside the PTV (HI=1.37 vs HI=1.42,
p=0.02). This, however, did not affect significantly the
PTV gEUD which was equivalent between IRIS and MLC. No
significant difference was observed for OAR dose sparing
between the two groups of plans, with the exception of
Bowel mean dose which was significantly lower for MLC.
Average treatment time was significantly (p=0.01)
reduced from 34.7 min. to 29.2 min when using MLC. MLC
MU mean value was lower than IRIS MU, but statistical