paediatrics Brussels 17

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-

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

Childs Nerv Syst