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

brain_met_ghia_807.jpg (56641 bytes)

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.