Distribution of Brain
Metastases in Relation to the Hippocampus: Implications for Neurocognitive Functional
Preservation
Amol Ghi University of Wisconsin, Madison, WI IJROBP
2007;68:971
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| Conformal avoidance whole brain plan
and corresponding dose-volume histogram in which the hippocampal region is conformally
avoided and the gross visible brain metastases are selectively boosted to a minimal
peripheral dose of 63 Gy. The 73 Gy (yellow), 63 Gy (red), 32.3 Gy (green), 10 Gy (sky
blue), and 8 Gy (blue) isodose lines are shown. The structures shown in the dose-volume
histogram from left to right are as follows: eyes (dark aquamarine), hippocampus (white),
hippocampus + 5 mm (light aquamarine), planning target volumes (PTV)—left-temporal
metastasis (black), PTV—left-parasagittal metastasis (yellow), and
PTV—cerebellar metastasis (blue). |
With the advent of
intensity-modulated radiotherapy, the ability to limit the radiation dose to normal
tissue offers an avenue to limit side effects. This study attempted to delineate the
distribution of brain metastases with relation to the hippocampus for the purpose of
exploring the viability of tomotherapy-guided hippocampal sparing therapy potentially to
reduce neurocognitive deficits from radiation.
Methods and Materials: The pre-radiotherapy T1-weighted, postcontrast axial MR images of
100 patients who received whole brain radiotherapy, stereotactic radiosurgery, or a
radiosurgical boost following whole brain radiotherapy between 2002 and 2006 were
examined. We contoured brain metastases as well as hippocampi with 5-, 10-, and 15-mm
expansion envelopes.
Results: Of the 272 identified metastases, 3.3% (n = 9) were within 5 mm of the
hippocampus, and 86.4% of metastases were greater than 15 mm from the hippocampus (n =
235). The most common location for metastatic disease was the frontal lobe (31.6%, n =
86). This was followed by the cerebellum (24.3%, n = 66), parietal lobe (16.9%, n = 46),
temporal lobe (12.9%, n = 35), occipital lobe (7.7%, n = 21), deep brain nuclei (4.0%, n =
11), and brainstem (2.6%, n = 7).
Conclusions: Of the 100 patients, 8 had metastases within 5 mm of
the hippocampus. Hence, a 5-mm margin around the hippocampus for conformal avoidance whole
brain radiotherapy represents an acceptable risk, especially because these patients in the
absence of any other intracranial disease could be salvaged using stereotactic
radiosurgery. Moreover, we developed a hippocampal sparing tomotherapy plan as
proof of principle to verify the feasibility of this therapy in the setting of brain
metastases. |