Abstract
The purpose was to quantify the setup margin for pediatric patients with neuro-blastoma using cone beam CT imaging (CBCT) and ultrasound localization. Ten patients, with a median age of 4.3 years (1.8 to 7.9) underwent daily pretreatment localization CBCT and every other day post-treatment CBCT to calculate interfractional and intrafraction movement. Localization was based on CBCT to treatment planning CT registration in the lumbar spine region. Each subject was treated in the supine position under IV general anesthesia using intensity-modulated radiation therapy. Patients were repositioned based on the daily pretreatment CBCT. Required setup margins based on inter- and intrafraction positioning errors were calculated based on weekly and daily imaging scenarios. Four patients had ultra-sound localization of the kidneys performed before the CBCT. Correlation between daily CBCT and ultrasound was investigated. A lateral, longitudinal and vertical setup margin of 5.4, 5.6, and 5.9 mm is required without daily CBCT. When daily CBCT was incorporated, the setup margin was reduced to 1.5, 2.1, and 1.7 mm. There was no correlation between the suggested ultrasound shifts and the shifts based on the CBCT. Daily localization based on CBCT of the lumbar spine can reduce the required setup margin for neuroblastoma patients, thereby reducing normal tissue exposure for this young patient population. The internal margin needs further investigation before PTV reduction can be made. Ultrasound localization was highly variable and not correlated to CBCT shifts.