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Systems, 3 fields, multi-field optimization, NUPO/PCS
V13.7.14) and compared with the clinical VMAT plans
(Varian Medical Systems, RapidArc, 2 arcs, 6MV photons,
Acuros V13.6.15). All patients had a simultaneous
integrated boost with a fractionation scheme of 35 x
1,55/2 Gy for PTV-elective/PTV-boost. For all plans, the
focus was on sparing of all individual salivary and
swallowing structures. Patients were categorized
according to the amount of additional OAR sparing
achieved with proton plans represented by the sum of
mean doses to (1) all swallowing structures, (2) parotid
gland(s) and (3) submandibular gland(s), if the mean dose
was <45Gy for any of these structures using IMPT and/or
VMAT.
Results
The range for the sum of the three mean doses for photon
plans was 37.2-180.8Gy (median 115.4), and for proton
plans 21.5-161.9Gy (median 92.1). Differences in total
mean dose between photon and proton plans ranged from
1.2-37.2Gy (median 16.9). Assuming a threshold of 10Gy
sparing in sum of mean doses (i.e. proton plan achieved at
least 10Gy reduction in total mean dose): 82.5% of proton
plans achieved this; threshold 15Gy: achieved by 67.5%
proton plans; threshold 20Gy: achieved by 40.0% proton
plans; threshold 25Gy: achieved by 17.5% proton plans.
Conclusion
The vast majority of patients had a dosimetric benefit
with protons as assessed by mean dose to swallowing and
salivary gland structures. The results show that the
number of patients who would be selected/referred for
protons is highly sensitive to the choice of threshold. This
has implications for activity levels in proton and photon
departments.
EP-1833 Bowel doses in cervical cancer patients
treated with a full bladder during radiotherapy.
A. Schouboe
1
, E. Kjaersgaard
1
, N. Jensen
1
, L. Fokdal
1
, L.
Nyvang
2
, M. Assenholt
2
, J. Lindegaard
1
, K. Tanderup
2
, A.
Vestergaard
2
1
Aarhus University Hospital, Department of Oncology,
Aarhus N, Denmark
2
Aarhus University Hospital, Department of Medical
Physics, Aarhus N, Denmark
Purpose or Objective
Large inter-fractional organ motion is a challenge in cervix
cancer radiotherapy. To reduce inter-fractional organ
motion, a bladder filling instruction aiming for a
comfortably full bladder has been introduced with daily
Cone Beam CT (CBCT). The purpose of the study is to
evaluate the correlation between bladder filling variation
and bowel dose.
Material and Methods
Eight consecutive patients with locally advanced cervical
cancer treated with chemo radiotherapy, were included in
the study. Dose planning and treatment was performed in
supine position. Prescribed dose was 45-50 Gy in 25-30
fractions to the pelvis, 5 fractions per week. Daily CBCT
with bony fusion and couch correction was performed. A
bladder filling protocol was applied with verbal and
written instructions, advising patients to drink 450 ml of
water in 15 minutes post micturition, 1 hour prior to each
treatment. All patients were retrospectively re-planned
with a uniform dose of 45 Gy in 25 fractions using VMAT.
Target and organs at risk were delineated on CT images
for dose planning and on every CBCT (n=210). The clinical
target volume (CTV) encompassed the gross tumour
volume (GTV), cervix, parametria, uterus, upper vagina
and the nodal CTV. A PTV margin of 1.5cm was applied for
the GTV, cervix, and uterus and 5mm for the nodal CTV
and parametria. Outer extension of bowel loops were
delineated including sigmoid in one volume (Bowel) on
each of the CBCTs. Based on a bony fusion, all structures
were transferred to the planning CT to asses bowel V30 Gy
and V43Gy for each fraction. Changes in bowel V30 and
V43 of bowel were evaluated as a function of changes in
bladder volume compared to the parameters on the
planning CT.
Results
A large variation in bladder filling was observed and was
found to be patient dependent. Large variation in bladder
volume and uterus position resulted in large variation in
V30 (Table 1). A linear correlation was found between
bowel V30 and V43 and bladder volume. Inter-fractional
bowel motion was observed due to changes in bladder
volume. Linear regression showed that with an increase of
100 cc in bladder volume, the bowel V30 is decreased by
a mean of 58 cc (range: 10 – 87 cc). For one patient, the
position of the uterus was not affected by bladder filling
changes and there was no clear correlation between
bladder volume changes and bowel irradiation (Fig 1).
Conclusion
A correlation between increased bladder filling volume
and dose to the bowel were established. Based on these
results it is recommended to focus on a uniform bladder
filling during planning CT and radiotherapy in order to
reduce the dose to the bowel during treatment.