Stereotactic Radiotherapy for Malignancies Involving the Trigeminal and Facial Nerves

Abstract

Involvement of a cranial nerve caries a poor prognosis for many malignancies. Recurrent or residual disease in the trigeminal or facial nerve after primary therapy poses a challenge due to the location of the nerve in the skull base, the proximity to the brain, brainstem, cavernous sinus, and optic apparatus and the resulting complex geometry. Surgical resection caries a high risk of morbidity and is often not an option for these patients. Stereotactic radiosurgery and radiotherapy are potential treatment options for patients with cancer involving the trigeminal or facial nerve. These techniques can deliver high doses of radiation to complex volumes while sparing adjacent critical structures. In the current study, seven cases of cancer involving the trigeminal or facial nerve are presented. These patients had unresectable recurrent or residual disease after definitive local therapy. Each patient was treated with stereotactic radiation therapy using a linear accelerator based system. A multidisciplinary approach including neuroradiology and surgical oncology was used to delineate target volumes. Treatment was well tolerated with no acute grade 3 or higher toxicity. One patient who was reirradiated experienced cerebral radionecrosis with mild symptoms. Four of the seven patients treated had no evidence of disease after a median follow up of 12 months (range 2-24 months). A dosimetric analysis was performed to compare intensity modulated fractionated stereotactic radiation therapy (IM-FSRT) to a 3D conformal technique. The dose to 90% (D90) of the brainstem was lower with the IM-FSRT plan by a mean of 13.5 Gy. The D95 to the ipsilateral optic nerve was also reduced with IM-FSRT by 12.2 Gy and the D95 for the optic chiasm was lower with FSRT by 16.3 Gy. Treatment of malignancies involving a cranial nerve requires a multidisciplinary approach. Use of an IM-FSRT technique with a micro-multileaf collimator resulted in a lower dose to the brainstem, optic nerves and chiasm for each case examined.

Multicriteria optimization of the spatial dose distribution

“…Likewise, we also demonstrated that it is possible to selectively cover a region by studying in which local regions to relax the dose constraints in order to shape a boost volume inside the prostate. A similar trade-off between PTV coverage and OAR sparing is typical for planning treatments close to the optic nerves, 24,25 where the planner is frequently willing to compromise coverage locally, but not globally. Again, we illustrated for scenario 2 that the dose distribution can be locally modified based on information where constraints limit progress and an assessment of whether relaxing these constraints is clinically acceptable.…”

“…[17][18][19] For example, with the advent of improved functional imaging, "dose painting" approaches taking into account tumor foci or nerves have been studied more intensely. [20][21][22][23][24][25] Extending this idea we show how the stepwise optimization framework can be used to achieve multicriteria control over the spatial dose distribution. First, we present the underlying optimization model.…”