paediatrics Brussels 17

Childs Nerv Syst

ative three-dimensionally acquired post-Gd T1-weighted data and thin section T2-weighted MR imaging data sets formatted in the transverse plane and registered to the CT study enable the radiation oncologist to contour the preoperative extent of disease and the postoperative tumor bed appreciating the full extent of disease and the postoperative shift of normal tissues. Other data sets representing alternative MR sequences may be registered and used as needed. It has also been found useful to repeat MR imaging immediately prior to radiation therapy which can be useful to clarify significant changes noted on the MRI obtained immediately postoperatively. The MR studies for RT planning, whenever feasible, should be obtained as close as possible to the start of treatment and about the time of simulation to account for changes in ventricular volumes, the operative site, and extra-axial fluid collections. The CT scan is the primary data set for radiation therapy planning and required to account for tissue heterogeneity in the planning process. We also suggest that the cochleae, spinal cord, and skin contour originate from the CT scan owing to the small size (cochleae) or critical nature (spinal cord) of these structures. The MR data set is used for the target volumes (GTV, CTV, PTV) and critical normal tissue structures in the head and neck (thyroid) and the entire brain, eyes, optic nerves, optic chiasm, pituitary, hypothal- amus, and temporal lobes [ 23 ]. Radiation oncologists generally accept the need for higher doses of radiation to treat ependymoma but remain concerned about normal tissue effects. Indeed, the dose to the spinal cord and brainstem are first among concerns when irradiating young children. Other normal tissue volumes or critical structures include the cochlea, hypothalamic-pituitary unit, optic chiasm, and temporal lobes. In recent years, algorithms for handling dose to these critical structures and defined dose limits have become available. For the purposes of treatment planning an infratentorial tumor, the upper aspect of the spinal cord begins at the inferior border of the foramen magnum and should be contoured on the treatment planning CT. For consistency in reporting the spinal cord should be con- toured on a number of images to be determined by the image section thickness. We have recommended 30 images at 2 mm section thickness. The treatment should be planned without compromising the dose prescription and to mini- mize inhomogeneity that would have the spinal cord receiving >1.8 Gy/day. If the cumulative treatment dose may exceed 54 Gy to more than 10% of the protocol defined spinal cord structure, the spinal cord should be excluded from the treatment after 54 Gy and receive no more than 1.25 Gy per fraction at any point. No myelopathy has been reported using these guidelines [ 24 ].

Forward planned three-dimensional radiation therapy follows target and normal tissue volume contouring with beam ’ s eye view treatment planning and the placement of multiple, noncoplanar individually shaped treatment beams pointed at the target yet avoiding critical normal tissues when feasible. The positioning of the beams, the number, shape and weight of beams, the exposure of normal tissues, and the accepted level of conformity is empiric yet limited by tumor size, location, patient positioning, and other factors coincident with the overall treatment plan. Intensity- modulated radiation therapy follows the same process before arriving at the iterative process of inverse planning to achieve predetermined levels of target volume coverage and adhere to operator imposed normal tissue constraints. Fifty-four grays has been widely considered as the minimum dose required for local tumor control with gross residual and tumor bed concentrations of microscopic disease; higher doses are considered to be more efficacious based on first principles of radiation therapy and our understanding that local failure dominates as a component of first failure. More recent series have employed 59.4 Gy at 1.8 Gy/day for all patients except those under the age of 18 months who have undergone gross-total resection who have been treated with 54 Gy. These dose requirements question the utility of craniospinal irradiation for metastatic ependymoma given that neuraxis doses are limited to 36 – 39.6 Gy. Most would consider that there is a difference in the level of microscopic tumor concentration in the subclinically involved neuraxis versus the resected tumor bed which requires a higher dose. The treatment planning objectives for conformal radia- tion therapy are to ensure target volume (PTV) coverage, minimize inhomogeneity, respect normal tissue tolerances, and c, and hypothalamic-pituitary unit. The full spectrum of conformal treatment techniques including forward or inversely planned conformal radiation therapy (intensity- modulated radiation therapy) is capable of achieving these goals. Proton beam radiation therapy also falls under the same rubric. Patients who receive conformal radiation therapy may be treated in the supine or prone position. A treatment planning CT is required and contrast is optional. The planning procedure should be performed as close to the start of treatment as possible because the possibility of postoperative changes in normal tissues. The CT scan should be of high resolution, certainly smaller section thickness that the planning target volume margin. In 2009, ≤ 2 mm is considered the standard. Registration of MR to CT is now a requirement for treatment planning to determine the extent of disease and to visualize the postoperative tumor bed, especially for posterior fossa tumors where the performance of CT is low. Because ependymoma has variable enhancement pre- and postoper-