Comparison of mega-voltage cone-beam computed tomography prostate localization with online ultrasound and fiducial markers methods

Authors

  • Gayou Olivier,

    1. Department of Radiation Oncology, Allegheny General Hospital, Pittsburgh, Pennsylvania 15212 and Drexel University College of Medicine, Allegheny Campus, Pittsburgh, Pennsylvania 15212
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    • a)

      Author to whom correspondence should be addressed. Electronic mail: ogayou@wpahs.org

  • Miften Moyed

    1. Department of Radiation Oncology, Allegheny General Hospital, Pittsburgh, Pennsylvania 15212 and Drexel University College of Medicine, Allegheny Campus, Pittsburgh, Pennsylvania 15212
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Abstract

The online image-guided localization data from 696 ultrasound (US), 598 mega-voltage cone-beam computed tomography (MV-CBCT), and 393 seed markers (SMs) couch alignments for patients undergoing intensity modulation radiotherapy of the prostate were analyzed. Daily US, MV-CBCT and SM images were acquired for 19, 17 and 12 patients, respectively, after each patient was immobilized in a vacuum cradle and setup to skin markers as the center of mass. The couch shifts applied in the lateral (left-right/LR), vertical (anterior-posterior/AP), and longitudinal (superior-inferior/SI) directions, along with the magnitude of the three-dimensional (3D) shift vector, were analyzed and compared for all three methods. The percentage of shifts larger than 5mm in all directions was also compared. Clinical target volume-planning target volume (CTV-to-PTV) expansion margins were estimated based on the localization data with US, CB, and SM image guidance. Results show the US data have greater variability. Systematic and random shifts were 1.2±6.8mm (LR), 2.8±5.1mm (SI) and 1.0±5.9mm (AP) for US, 1.0±3.9mm (LR), 1.3±2.5mm (SI) and 0.3±3.9mm (AP) for CB, and 1.0±3.4mm (LR), 0.0±3.4mm (SI) and 0.5±4.1mm (AP) for SM. The mean 3D shift distance was larger using US (8.8±6.2mm) compared to CB and SM (5.3±3.4mm and 5.2±3.7mm, respectively). The percentage of US shifts larger than 5mm were 34%, 31%, and 38% in the LR, SI, and AP directions, respectively, compared to 18%, 6%, and 16% for CB and 14%, 10%, and 20% for SM. MV-CBCT and SM localization data suggest a different distribution of prostate center-of-mass shifts with smaller variability, compared to US. The online MV-CBCT and SM image-guidance data show that for treatments that do not include daily prostate localization, one can use a CTV-to-PTV margin that is 4mm smaller than the one suggested by US data, hence allowing more rectum and bladder sparing and potentially improving the therapeutic ratio.

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