Evaluating setup accuracy of a positioning device for supine pelvic radiotherapy

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dc.contributor.author Belay, Eskadmas Yinesu
dc.date.accessioned 2012-01-11T07:47:59Z
dc.date.available 2012-01-11T07:47:59Z
dc.date.issued 2012-01-11
dc.identifier.uri http://hdl.handle.net/10539/10964
dc.description MSc., Faculty of Science, University of the Witwatersrand, 2011 en_US
dc.description.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. en_US
dc.language.iso en en_US
dc.subject Cervix uteri (cancer) en_US
dc.subject Generative organs, female (cancer) en_US
dc.title Evaluating setup accuracy of a positioning device for supine pelvic radiotherapy en_US
dc.type Thesis en_US


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