Inter- and intrafractional positional uncertainties in pediatric radiotherapy patients with brain and head and neck tumors.
ABSTRACT To estimate radiation therapy planning margins based on inter- and intrafractional uncertainty for pediatric brain and head and neck tumor patients at different imaging frequencies.
Pediatric patients with brain (n = 83) and head and neck (n = 17) tumors (median age = 7.2 years) were enrolled on an internal review board-approved localization protocol and stratified according to treatment position and use of anesthesia. Megavoltage cone-beam CT (CBCT) was performed before each treatment and after every other treatment. The pretreatment offsets were used to calculate the interfractional setup uncertainty (SU), and posttreatment offsets were used to calculate the intrafractional residual uncertainty (RU). The SU and RU are the patient-related components of the setup margin (SM), which is part of the planning target volume (PTV). SU data was used to simulate four intervention strategies using different imaging frequencies and thresholds.
The SM based on all patients treated on this study was 2.1 mm (SU = 0.9 mm, RU = 1.9 mm) and varied according to treatment position (supine = 1.8 mm, prone = 2.6 mm) and use of anesthesia (with = 1.7 mm, without = 2.5 mm) because of differences in the RU. The average SU for a 2-mm threshold based on no imaging, once per week imaging, initial five images, and daily imaging was 3.6, 2.1, 2.2, and 0.9 mm, respectively.
On the basis of this study, the SM component of the PTV may be reduced to 2 mm for daily CBCT compared with 3.5 mm for weekly CBCT. Considering patients who undergo daily pretreatment CBCT, the SM is larger for those treated in the prone position or smaller for those treated under anesthesia because of differences in the RU.
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ABSTRACT: 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.Journal of the Egyptian National Cancer Institute 06/2011; 23(2):55-60.