Geometrical uncertainty margins in 3D conformal radiotherapy in the pediatric age group

To evaluate set-up variation of pediatric patients undergoing 3D conformal radiotherapy (3DCRT) using electronic portal image device (EPID), in an effort to evaluate the adequacy of the planning target volume (PTV) margin employed for the 3DCRT treatment of pediatric patients.

Set-up data was collected from 48 pediatric patients treated with 3DCRTfor head and neck (31 patients), abdomino-pelvic (9 patients) and chest (8 patients) sites during the period between September 2008 and February 2009. A total of 358 images obtained by EPID were analyzed. The mean (M) and standard deviation (SD) for systematic and random errors were calculated and the results were analyzed.

All images were studied in anterior and lateral portals. The systematic errors along longitudinal, lateral and vertical directions in all patients showed an M equal to 1.9, 1.6, and 1.6mm with SD of 1.8, 1.4, and 1.8mm, respectively; (head and neck cases: M equal to 1.5, 1.2, and 1.6mm with SD 1.4, 1.2, and 1.8mm; chest cases: M equal to 2.5, 1.8, and 0.8mm with SD 2.7, 1.7, and 1.2mm, abdomino-pelvic cases: M equal to 2.9, 2.8 and 2.3mm with SD 1.6, 1.2, and 2.3mm). Similarly, the random errors for all patients showed SD of 1.9, 1.6, and 1.8mm, respectively (head and neck cases: SD 1.7, 1.3, and 1.5mm; chest cases: SD 1.2, 1.9, and 2.5mm; abdomino-pelvic cases SD 2.5, 2, and 2.4mm, respectively). Using Van Herk's formula the suggested (PTV) margin around the clinical target volume (CTV) of 5.5mm appears to be adequate.

The ranges of set-up errors are site specific and depends on many factors.