Assessment of set up accuracy using an evacuated cushion for proton beam therapy
Roberts, D. Foden, Philip Elliff, L. Allen, K. Lowe, Matthew Eccles, Cynthia L
Roberts, D.
Foden, Philip
Elliff, L.
Allen, K.
Lowe, Matthew
Eccles, Cynthia L
Citations
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Abstract
Purpose or Objective
Effective, reproducible patient positioning is paramount
for radiotherapy. With the introduction of a new proton
beam therapy (PBT) service in January 2019, evacuated
cushions (vacbags) have been implemented for patients
receiving treatment to the limbs, abdomen, thorax and
pelvis. This work reports on an audit reviewing set up
times, image review times and positional reproducibility
to confirm the suitability of the vacbag as an
immobilisation tool in PBT.
Material and Methods
The first ten patients treated on vacbags were included in
this study. Patients were initially imaged with kilovoltage
(kV) 2-dimensional (2D) image pairs as a gross error check
(GEC), followed by a conebeam CT (CBCT) from which
positional errors were corrected and verified with a final
2DkV image before proceeding to treatment. Images were
compared to the planning CT to determine translational
and rotational displacements. Treatment and image
review times were also reported to audit the new service.
Results
Compared to the planning CT, translational displacements
were satisfactory with 95% <5mm and 100% <10mm.
Greater variation was seen in rotational displacements,
however, on average only 3% of all exceeded the 2⁰
tolerance. Agreement between 2D and 3D imaging were
within the institutions defined GEC tolerance of <1cm and
<2⁰. This agreement was reached in 99% of translational
and 91% of rotational displacements. A Bland-Altman
analysis demonstrated that there was no statistical
significance for the bias between the 2D and 3D
measurements. In total 181 CBCT images were assessed
against confirmation 2D images. Agreement was achieved
in 89.1% of translational (2mm tolerance) and 94.3% of
rotational (1⁰ tolerance) orientations.
Treatment delivery times in minutes (mins) were reduced
from a mean of 55 mins for the first patient to 47 mins for
the most recent patient. Average Image review time for
the GEC 2D orthogonal images also reduced from 6 mins to
3 mins, however the trend for review time of CBCT
remained an average of <10 mins (range 7 mins – 12mins).
Conclusion
Initial results show that vacbags are suitable for accurate
patient positioning and minimising translational
displacements. Rotational displacements appeared more
random, especially pitch. As such it has been determined,
the use of a 6⁰ of freedom bed, combined with daily
imaging, is paramount for accurate set up. Future
investigations will include a comparison of this data to
external institutional data and further review of the
effects of contour change on patient position and set up accuracy.
Description
Date
2020
Publisher
Collections
Keywords
Type
Meetings and Proceedings
Citation
Roberts D, Foden P, Elliff L, Allen K, Lowe M, Eccles C. PO-1832: Assessment of set up accuracy using an evacuated cushion for Proton Beam Therapy. Radiotherapy and Oncology . 2020 Nov;152:S1022.