ESTRO 36 Abstract Book

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S984 ESTRO 36 2017
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risks (OARs: spinal cord, brainstem, optic nerves, parotids
and oral cavity) were outlined in Eclipse TPS. The
posterior aspect of patient skin was also contoured.
Two different approaches were proposed to define the
"body structure": 1) the patient outer contour plus the
immobilization device (Fig 1); and 2) as previous but also
including the air gap between the immobilization device
and the patient outer contour (Fig 2). Dose distributions
were calculated using identical IMRT plans for each
approach. The differences in the minimum (D98%),
maximum (D2%) and mean (Dmean) doses to the PTVs and
OARs as well as the skin mean doses were compared.
Results
Differences within ±1% were found in the dosimetric
parameters analyzed for PTVs and OARs. Mean skin was up
to 2% greater when the gap air between patient surface
was considered.
Conclusion
Little dose differences were observed between the
approaches of including or not the air gap existing
between the immobilization device and the patient
surface.
EP-1822 Monitoring of parotid gland changes in
radiotherapy of NPC with parapharyngeal space
involvement
W.C.V. Wu1, Y. Zhang2, C. Lin2, J. Wu2
1 Hong Kong Polytechnic University, HTI, Kowloon- Hong
Kong- SAR, China
2 Sun Yat-sen University Cancer, Cancer Hospital,
Guangzhou, China
Purpose or Objective
Parapharyngeal space (PPS) involvement is present in over
70% of nasopharyngeal carcinoma (NPC) patients. Since
PPS is close to parotid gland, a radical course of
radiotherapy for this group of patients may deliver high
dose to this organ. The purpose of this study was to
evaluate the parotid gland changes of NPC patients with
PPS involvement during radiotherapy and up to 3 months
after treatment.
Material and Methods
Kilovoltage computed tomography (CT) scans of head and
neck region of 39 NPC patients with PPS involvement
recruited from Sun Yat-sen University Cancer Center
between January 2011 and April 2013 were performed at
pre-radiotherapy, 10th, 20th and 30th fractions, and 3
months after treatment. All patients were treated with
intensity modulated radiotherapy using 6 MV photons with
prescribed doses of 66-70 Gy to the target volume. The
parotid glands were contoured in pre-radiotherapy
planning CT scan and in subsequent scans. At each time
interval, DICE similarity coefficient (DSC), percentage
volume change and centroid movement between the
planning CT and the subsequent CTs were obtained from
the contouring software. In addition, the distance
between medial and lateral borders of parotid glands from
the midline at various time intervals were also measured.
Results
The ipsilateral parotid gland received a mean dose of
about 5 Gy higher than the contralateral side (56.3±6.2 Gy
vs 51.7±9.2 Gy). The mean DSC for ipsilateral parotid
gland decreased to 0.63 at 30 th fraction and returned to
0.74 at 3 months after treatment. Partial recovery was
observed at 3 months after treatment. All differences
between each pair of consecutive measurements (such as
between 10th and 20th fractions and 20 th and 30th fractions)
were statistically significant (p < 0.05). The mean volume
change for ipsilateral parotid gland decreased from -
15.27% at 20th fraction to -37.49% at 30th fraction and
partially recovered to -23.14% in 3 months. There were no
significant differences between ipsilateral and
contralateral groups despite the changes in the ipsilateral
side being relatively greater. The centroid displacement
followed a similar pattern, which moved medially and
superiorly by an average of 0.30 cm and 0.18 cm
respectively at 30th fraction. The changes in ipsilateral
gland were slightly greater than the contralateral side.
Conclusion
In radiotherapy of NPC patients with PPS involvement, the
parotid gland shrank by about 1/3 towards the end of the
treatment course. DSC and percentage volume changes of
both ipsilateral and contralateral parotid glands
decreased during the radiotherapy course and partially
recovered in 3 months after treatment. This trend was also
seen in the displacements of centroids and the medial and
lateral borders of the gland. A re-planning was suggested
at around 15th to 20th fraction so as to reduce the dose to
the parotid gland due to the detected movement of this
structure during the radiotherapy course.
EP-1823 DVH- and NTCP-based dosimetric comparison
of different margins for VMAT-IMRT of esophageal
cancer
S. Münch 1, M. Oechsner 1, S.E. Combs1,2, D. Habermehl1
1Klinikum rechts der Isar- TU München, Radiation
Oncology, München, Germany
2Institute of innovative radiotherapy iRT, Hermholtz
Zentrum, Oberschleißheim, Germany
Purpose or Objective
To cover the microscopic, longitudinal tumor spread in
squamous cell carcinoma of the esophagus (SCC),
longitudinal margins of 3-4 centimetres are used for
neoadjuvant and definitive radiotherapy (RT)
protocols. Therefore, RT of SCC is often done with large
treatment volumes, which lead to high doses to the organs
at risk (OAR). However, while the promising results of the
CROSS-Trial, that used longitudinal margins of 4 cm,
defined a new standard for neoadjuvant chemoradiation
(CRT), a smaller margin of 3 cm might be reasonable,
especially for early tumor stages.
Purpose of this study was to compare the dose distribution
to the organs at risk for different longitudinal margins
using a DVH- and NTC-based approach.
Material and Methods
10 patients with SCC of the middle or the lower third, who
underwent CRT at our institution were retrospective
selected. Three planning target volumes (PTV) were
created for every patient, with an axial margin of 1.5 cm
to the gross target volume (GTV) (primary tumor and PET-
positive lymph nodes), analogous to the protocol of the
CROSS-trial. The longitudinal margins were 4 cm, 3 cm and
2 cm, respectively. Contouring and treatment planning
was performed with the Eclipse 13 planning system (Varian
Medical Systems, Palo Alto, CA, USA). For every PTV,
volumetric modulated arc therapy (VMAT) plans were
optimized. Dose calculation was performed using the AAA
algorithm (version 10.0.28) and heterogeneity correction.
All plans were normalized to a median prescribed PTV dose
of 41.4 Gy with a daily dose of 1.8 Gy. Dose to the lungs,
heart and liver were evaluated and compared. Differences
of dose parameters were tested for significance with t-
test for paired samples.
Results
Median tumor length was 6 cm with a range of 3 to 10 cm
and 8 of the 10 patients (80%) had lymph node metastasis.
When using a longitudinal margin of 3 cm instead of 4 cm,
all dose parameters (Dmin, Dmax, Dmean, Dmedian and
V5-V35), except Dmax could be significantly reduced for
the lungs. Regarding the heart, a significant reduction was
seen for Dmean and V5, whereas no significant difference
was seen for Dmin, Dmax, Dmedian and V10-V35. When
comparing a longitudinal margin of 4 cm to a longitudinal
margin of 2 cm, not only Dmin, Dmax, Dmean, Dmedian
and V5-V35 for the lungs, but also Dmax, Dmin and V5-V35
for the heart were significantly reduced. Nevertheless, no
difference was seen for the median heart dose. In
addition, the risk of pneumonitis was significantly reduced
by a margin reduction of 3 cm and 2 cm.