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S186
ESTRO 36 2017
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A. Baker
1
, T. Hague
2
, Y. Tsang
1
, P.J. Hoskin
1
1
Mount Vernon Hospital, Department of Radiotherapy,
Northwood Middlesex, United Kingdom
2
Mount Vernon Hospital, Department of Radiotherapy
Physics, Northwood Middlesex, United Kingdom
Purpose or Objective
Plan of the day (PoD) ART for cervical cancer patients can
potentially reduce toxicity and the risk of geometrical
miss but may be resource intensive. In order to implement
accurate PoD for these patients this study aimed to assess
the accuracy of adaptive online plan selection and linac
resource impact.
Material and Methods
An initial patient cohort had planning CTs acquired with
an empty and full bladder and an intermediate MRI. CTVs
were outlined on each of the datasets to include uterus
and proximal vagina, from which an ITV and PTV were
defined with further nodal volumes as required. VMAT
plans were created depending on the amount of uterine
movement, with a further plan using the previous standard
technique as a backup.
Online daily CBCT was performed for all patients with
additional kV planar images used for nodal positioning in
one patient and for pelvic tilt in another. Plan selection
following online registration using a combination of bony
anatomy and soft tissue was performed by 2 members of
the project team (observers) who had attended an
anatomical training session and had a range of experience
with female pelvic CBCT analysis. A 3mm margin between
the visible target anatomy and the PTV contour was
allowed for intrafraction motion. This was assessed
through the addition of weekly post-treatment CBCTs. In-
room time (patient enter to exit) was recorded at each
session and patients were booked into the departmental
20 minutes time slot for ART.
A consensus standard PoD was agreed offline by an
experienced clinician and RTT. Offline analysis was
performed to measure concordance with the consensus
standard PoD and the online decision.
Results
A total of 100 online PoD evaluations plus 600 offline
evaluations, by 6 observers, were used for the analysis.
The median concordance between the consensus standard
PoD and the online plan selection was 98%. Where poor
concordance was observed between online plan selection
and the consensus standard PoD, a safe larger volume
option was chosen online. Post-treatment CBCT’s showed
target anatomy was covered in all but 1 case. In-room
timing ranged from 10 – 30mins with a median time of
19mins. The median score of the 4 observers offline
compared to the consensus standard was 86%. The range
between individuals was 76%- 96% and between patients
was 78 – 96%.
Conclusion
High online concordance of 98% with the consensus
standard PoD demonstrates that the initial training
equipped the team with appropriate knowledge to
perform accurate plan selection. A combination of 2
observers online achieve closer results to the consensus
standard rather than individually. The joint decision
making can be performed within the standard
departmental ART time slot of 20 minutes. The CBCT data,
consensus standard PoD and anatomy training can be used
as part of the assessment programme for future RTT
observers. Greater confidence in choosing smaller volume
plans needs to be built to achieve the full potential of
ART.
OC-0354 Dosimetric impact of anatomical changes in
photon and particle therapy for pancreatic cancer
K. Crama
1
, A.C. Houweling
1
, J. Visser
1
, K. Fukata
2
, C.R.N.
Rasch
1
, T. Ohno
2
, A. Bel
1
, A. Van der Horst
1
1
Academic Medical Center, Department of Radiation
Oncology, Amsterdam, The Netherlands
2
Gunma University, Gunma University Heavy Ion Medical
Center, Maebashi, Japan
Purpose or Objective
Radiotherapy of pancreatic tumors is toxic due to the high
dose to surrounding organs-at-risk (OARs). Irradiation with
charged particles is characterized by a sharp dose fall-off
around the target area. Compared to photon therapy,
OARs can be further spared while delivering a high dose to
the tumor. Treatment planning studies in pancreatic
cancer patients have shown this benefit of charged
particle
therapy
over
photon
therapy.
However, intra- and interfractional changes may greatly
affect the robustness of particle therapy. Past studies only
investigated differences in planned dose; studies
comparing the robustness of different modalities have not
been published yet.
We compared the dosimetric impact of interfractional
anatomical changes (i.e. body contour differences,
gastrointestinal gas volume changes and setup errors) in
photon, proton and carbon ion therapy for pancreatic
cancer patients. Intrafractional changes were not taken
into account in this study.
Material and Methods
Photon, proton and carbon ion treatment plans (36 Gy, 12
fractions) were created for 9 patients. For the particle
therapy plans, the relative radiobiological effectiveness
was taken into account. To simulate daily online setup
correction, the CBCTs were rigidly registered (only
translations) to the planning CT using fiducial markers.
Fraction dose calculation was then made possible by
deformable registering the planning CT to each of the 12
CBCTs. Gastrointestinal gas was delineated on each CBCT
and copied to the deformed CT, a relative density override
was
applied
for
dose
calculation
(0.01).
Fraction doses were accumulated rigidly. To compare
planned and accumulated dose, for each radiotherapy
modality, dose volume histogram (DVH) parameters of the
planned and accumulated dose were determined for the
internal gross tumor volume (iGTV), internal clinical
target volume (iCTV) and OARS (duodenum, stomach,
kidneys, liver and spinal cord).