Evaluating setup accuracy of a positioning device for supine pelvic radiotherapy
Date
2012-01-11
Authors
Belay, Eskadmas Yinesu
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Aim: This study aimed at evaluating the accuracy of the treatment setup margin in
external beam radiotherapy in cervical cancer patients treated supine with or without
the CIVCO “kneefix and feetfix”TM immobilizing devices.
Methods and materials: 2 groups of 30 cervical cancer patients each, who were
treated supine with two parallel opposed fields or a four-field “box” technique were
selected randomly. The treatment fields were planned with a 2 cm setup margin
defined radiographically. The first group was treated without any immobilization and
the second group was treated with the “kneefix and feetfix”TM immobilization device.
Both groups of patients were selected from the patients treated on one of two linear
accelerators (linac), which had weekly mechanical quality control (QC). All patients
had pre-treatment verifications on the treatment machine in which a megavoltage Xray
film was taken to compare with the planning simulation film. Both films were
approved by the radiation oncologist managing the patient. In this study the position
of the treatment couch as at the approved machine film was taken as the intended or
planned position for the immobilized patients. The digital readouts of the daily
treatment position of the couch were recorded for each patient as the absolute X
(lateral), Y (longitudinal), and Z (vertical) position of the couch from the record and
verify system interfaced to the treatment machine.
A total of 1241 (582 for the immobilized and 659 for the non-immobilized patient
group) daily treatment setup positions were recorded in terms of the X, Y and Z
coordinates of the couch corresponding to the Medio-lateral (ML), Supero-inferior
(SI) and Antero-posterior (AP) directions of the patient, respectively. The daily
translational setup deviation of the patient was calculated by taking the difference
between the planned (approved) and daily treatment setup positions in each direction.
Each patient’s systematic setup error (mi) and the population mean setup deviation
(M), was calculated. Random ( ) and systematic ( ) setup errors were then calculated
for each group in each direction. The translational setup variations found in the AP,
iii
ML, SI directions were compared with the 2 cm x 2 cm x 2 cm Planning Target
Volume (PTV). Couch tolerance limits with the immobilization device were
suggested based on the ± 2SD (standard deviation) obtained for each translational
movement of the treatment couch.
Result: The random and systematic errors for the immobilized patient group were
less than those for the non-immobilized patient group. For the immobilized patient
group, the systematic setup error was greater than the random error in the ML and SI
direction as shown in Table I.
Table I: The random and systematic errors in the setup in the Antero-posterior
(AP), Medio-lateral (ML) and Supero-inferior (SI) directions and the suggested
couch tolerance limits for both patient groups.
Almost all treatment setup positions had less than 2 cm variation in the AP setup for
both patient groups however; one third of the immobilized positions had more than 2
cm variation in the setup in the ML and SI directions.
Conclusion: The “kneefix and feetfix”TM immobilizing device resulted in a minor
improvement in both the random and systematic setup errors. The systematic setup
errors need to be investigated further. There are measurable patient rotations of more
than 2 cm in the setup margin with the immobilizing device and this should be
confirmed with an imaging study. The 2 cm margin in the ML and SI directions
Immobilized patient group Non-immobilized patient group
AP (cm) ML (cm) SI (cm) AP (cm) ML (cm) SI (cm)
Random
error (!)
0.30 1.35 1.26 0.37 2.74 7.83
Systematic
error (")
0.19 1.55 1.64 0.33 1.70 8.11
Suggested
couch
tolerance
limits
(±2SD)
0.70 4.04 4.08 0.88 4.76 N/A
iv
established at simulation should not be changed for these patients. A 1 cm tolerance
in the AP setup margin could be introduced at this institution.
Description
MSc., Faculty of Science, University of the Witwatersrand, 2011
Keywords
Cervix uteri (cancer), Generative organs, female (cancer)