International Journal of Radiation Oncology*Biology*Physics
Physics ContributionInter- and Intrafractional Positional Uncertainties in Pediatric Radiotherapy Patients With Brain and Head and Neck Tumors
Introduction
The trend to irradiate young children with brain tumors and the promise of recent advancements has increased the acceptance of radiation therapy (RT) for pediatric patients. Because RT has wide ranging side effects in pediatric patients 1, 2, 3, 4, 5, investigators have focused on reducing normal tissue irradiation. One method is to reduce the target volume margins that are used in RT planning.
The volume and margin definitions of the International Commission on Radiation Units and Measurements Reports 50 and 62, 6, 7 have been adopted in pediatric clinical trials for brain and head and neck tumors—namely, gross tumor volume (GTV), clinical target volume (CTV), and planning target volume (PTV). The purpose has been to use three-dimensional treatment planning and delivery systematically in clinical trials and study ways to reduce treatment effects using disease-specific target volume margins. Only the CTV should receive the prescription dose; however, because of temporal variation in the position, shape, and size of the CTV, an internal margin (IM) must be added. In addition, because of uncertainties in the daily patient positioning, patient intrafractional motion, dose calculation, and beam delivery, a setup margin (SM) is also required. The combination of the IM and SM define the PTV. The need to estimate and minimize the IM and SM in a variety of clinical settings and for children of all ages has become increasingly important as highly conformal radiation therapy, including intensity modulated radiation therapy (IMRT) and proton therapy, have entered the mainstream.
The PTV margins currently specified for children with brain and head and neck tumors enrolled on institutional and cooperative group trials are largely empiric. As the definition of the GTV is refined and the CTV is systematically reduced for specific diseases, the margin chosen for the PTV will become an increasingly important means to minimize dose to normal tissues and the potential risk factor for treatment failure. Given the importance and complexity in determining the appropriate PTV margin, a number of studies have proposed patient population-based formulas for the patient-related portion of the setup margin 8, 9, 10, 11 to allow for systematic quantification of setup uncertainties based on statistical methods. Among the sites under study, intracranial 12, 13 and head and neck 14, 15, 16, 17 sites are of particular interest; however, only two small studies have been published focusing on pediatric localization 18, 19.
We developed a protocol to quantitatively assess localization and refine PTV margin definitions for pediatric patients with head and neck and intracranial tumors. The goal was to estimate the patient-related components of the SM and provide guidelines for target volume definitions in clinical trials. In this report, we estimated setup uncertainty (SU) and residual uncertainty (RU). The SU represented interfractional positioning differences, and the RU represented intrafractional patient motion. We used daily pretreatment cone-beam computed tomography (CBCT) and alternate day posttreatment CBCT to calculate SU and RU. The acquired data were modeled using different imaging regimens 20, 21 to determine the influence of imaging frequency on the SM.
Section snippets
Patient cohort
One hundred patients with brain tumors (n = 83) and tumors involving the head and neck region (n = 17) were included in this study. The cohort included 54 male and 46 female patients with a median age of 7.2 years (range, 1.0–25.3 years). Only two patients were older than 21 years of age at the time of irradiation. They were included because they were diagnosed with pediatric brain and musculoskeletal tumors. General anesthesia (GA) was required for 46 (median age, 4.8 years; range, 1.0–19.1
Results
For the 100 patients included in this report, 2,362 pretreatment and 1,092 posttreatment localization CBCTs were acquired. The median (± SD) number per patient was 29 ± 8.7 and 12.5 ± 4.1, respectively. Figure 2 shows images from a 1-cGy IBL-CBCT and treatment planning CT for a 4-year-old patient. Figure 3 shows the offsets for the pretreatment CBCT and posttreatment CBCT of the patient.
The average setup uncertainty (SU) of the three directions for the L1 localization was 3.6 mm. For the 2-mm
Discussion
On the basis of current U.S. treatment standards, we estimate more than 1,000 children with primary brain, head and neck, and orbital tumors will receive fractionated external beam radiation therapy (RT) each year as part of their initial management. The treatment of these patients and the requirements for daily localization and verification is complicated by the extremes of age, level of cooperation, and clinical and treatment factors associated with their tumor type, tumor location, and
Conclusion
CBCT can be used to minimize positional uncertainty in pediatric patients and reduce dose to normal tissue. When daily CBCT is used for pediatric patients with brain and head and neck tumors, a SM of 2 mm appears to be appropriate. The SM may be increased for patients treated in the prone position or further reduced when general anesthesia is used. When weekly CBCT is used, the SM should be increased to 3.5 mm.
Although additional uncertainties in the SM may be attributed to the hardware and
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This research was partially funded by a grant from Siemens Medical Systems and by support from the American Lebanese Syrian Associated Charities (ALSAC). This work was presented in part at the 2009 AAPM conference in Anaheim CA.
Conflict of interest: none.