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The optic chiasm dose should be managed in a very
similar fashion to the spinal cord and should be defined on
CT or MR and appearing on at least two successive images.
If the cumulative treatment dose may exceed 54 Gy, the
chiasm should be excluded from the treatment after 54 Gy
and receive no more than 1.25 Gy per fraction at any point.
These guidelines also allow for the coverage of the target
volumes to be compromised after 54 Gy in selected cases.
Each cochlea should be contoured separately on the CT
data as a circular structure within the petrous portion of the
temporal bone. The size and position of the contoured
cochlea should be confirmed by viewing the structures in
three dimensions using the treatment planning system and
on two successive CT images. The mean dose to the
cochleae should be limited to 35 Gy. At these levels, the
risk of hearing loss is less than 5% ].
The brainstem is central to the irradiated volume in
patients with posterior fossa tumors, and while major side
effects from radiation therapy have not been widely
reported, investigators remain very concerned about the
long-term effects of irradiation especially for children who
suffer neurological effects from tumor and surgery. Recent
data suggest that factors impeding neurologic recovery in
children with ependymoma treated with high-dose postop-
erative radiation therapy do not include radiation dose,
rather, the volume of tumor and clinical and treatment
factors related to tumor and surgery. Given the safety
profile of radiation therapy as administered in recent trials
and plans to further reduce the target volume for radiation
therapy, the risk of side effects involving the brainstem
should be further diminished ].
Temporal lobe and whole brain doses of radiation
therapy are correlated with cognitive outcome corrected
for the age of the patient at the time of irradiation. This
important knowledge has driven investigators to find new
ways to reduce dose to normal tissues (shrinking target
volume margins) or this high-dose volume of irradiation
using conformal methods. Evaluating patients with ependy-
moma and considering radiation effects should not be
absent the potential effects of hydrocephalus , ].
Hypothalamic dose volume effects have been modeled
for patients with ependymoma suggesting that the risk of
endocrinopathy is low for most patients and that while even
low doses to the hypothalamus result in a risk for growth
hormone deficiency, other endocrinopathies are even less
common if baseline assessments prove to be normal.
Preexisting endocrine deficiencies in these children corre-
late with ventricle size (hydrocephalus) at diagnosis ,
].
Result
The peer-reviewed scientific literature contains numerous
references to highly focused focal irradiation for intracra-
nial ependymoma in children; however, there is only one
perspective conformal series. The primary measure of
success for conformal radiation therapy is local tumor
control corrected for extent of resection which is the most
important prognostic factor. There are several contemporary
series that utilized fully or to a large extent conformal
radiation therapy. Local tumor control has been estimated at
68 –89% when measured at 3– 5 years (Table ) , ].
The rates of local control in the modern series are
considerably higher than those inferred from historic series
where event-free survival and not local control rates have
been reported (Table ) .
IMRT
The use of IMRT in very young children has raised concern
about extraneous dose to normal tissues. Mansur et al. ]
found that IMRT lowered peripheral doses near the target.
This was attributed to reduced internal scatter due to
smaller effective field sizes. The thyroid was given as an
example of a critical peripheral organ near to the targeted
volume. The peripheral dose was similar for both IMRT and
three-dimensional CRT indicating that peripheral dose was
difficult to predict by monitor units which are often
significantly greater for IMRT.
Table 1 Local tumor control estimates from contemporary reports using postoperative irradiation
Series Time period Patients GTR (%) Local control
MacDonald-PBRT 2000–2006 17 76 86% at 2 years
Schroeder-IMRT 1994–2005 22 77 68% at 3 years
Massimino-HFRT 1993–2001 46 74 70% at 4 years
Merchant-CRT/IMRT 1997–2007 153 85 89% at 5 years
PBRT proton beam radiation therapy, IMRT intensity-modulated radiation therapy, HFRT hyperfractionated radiation therapy, CRT conformal
radiation therapy, GTR gross-total resection
Childs Nerv Syst