Archive for the ‘Radiology’ Category
Facial Nerve
June 11th, 2011 | 
Author: Facial nerve injuries are rarer than trigeminal nerve complications, which is partly attributed to the fact that the trigeminal is a somatosensory nerve. In standard fractionation head and neck cases, the facial nerve routinely receives over 70 Gy without complication. Multiple fraction regimens treating cases of vestibular schwannoma have resulted in facial nerve preservation rates of 94% to 100%.With modern stereotactic radiosurgery planning techniques employing MRI and a margin dose less than or equal to 13 Gy, the risk of permanent facial weakness has been reduced to less than 1% in most series. This is a clear improvement over older series showing complication rates of approximately 30% for patients who received marginal doses as high as 20 Gy using CT planning. As with the trigeminal nerve, Beegle and colleagues found the facial nerve to have an approximate six-fold increase in complication risk for every 2.5 Gy above 12.5 Gy prescribed to the tumor margin. Only marginal dose correlated with facial weakness in a review of 190 subjects described by Flickinger and colleagues involving modern radiation techniques, with no subject experiencing facial weakness with doses less than 15 Gy.
RADIATION BIOLOGY
A comprehensive discussion of the tissue effects of radiation is beyond the scope of this article. However, a brief overview of this topic is necessary to fully understand radiation effects on the auditory and vestibular systems. Tissue effects of radiation are dependent on a number of factors. Megavoltage X rays in the therapeutic range interact with tissue primarily by way of the Compton effect. The Compton mass attenuation coefficient is independent of the atomic number and depends only on the number of the electrons per gram of the interacting material. The attenuation of beam is related to density thickness (density of material multiplied by the thickness) expressed as g/cm2, thus a relative decrease in attenuation leads to increased penetration in air-filled spaces or air cavities, such as the lungs and temporal bones.
Deposition of radiation energy in tissue results in cell injury and death. Most of radiation’s
tissue effect is thought to be a result of the damage to DNA. This occurs both directly and indirectly. The latter, the predominant mechanism, involves ionization of surrounding water molecules to form free radicals, which, in turn, result in doublestrand breaks in DNA. This injury may result in cell death during mitosis, induction
of apoptosis (ie, programmed cell death), recovery, and cell cycle arrest or terminal differentiation through activation of repair pathways that may also play a role in tumor suppression (eg, activation of p53).
The radio-response of a tissue depends on the inherent sensitivity of the cells, the kinetics of cell population, total dose, dose per fraction, and time-dose fractionation.
Cells with fast turnover rate or higher mitotic activity exhibit more sensitivity to radiation, subjecting to cell death in attempting subsequent mitosis. This is the basis for therapeutic RT (ie, relatively greater damage to highly reproductive tumor cells). Further, RT fractionation offers the potential for greater differential sparing of normal tissues and killing of tumor cells.
These factors also determine the unwanted manifestations of RT. Skin and mucosa, which cycle quickly, manifest more significant early, transient, and inflammatory changes. Organ dysfunction are often manifest by cell lines with slow turnover (eg, radionecrosis of bone). In some tissues, such as inner-ear hair cells, functional progenitor cells may be lacking, resulting in greater organ system dysfunction. It is unclear how RT leads to long-term dysfunction in cells, such as neurons and innerear hair cells, which lose mitotic activity after differentiation. Such cells are dependent on supporting cells (eg, glia) and small blood vessels. Though a wide range of morphologic changes in neural tissue in response to RT have been observed. the effect of RT on neurons cannot be distinguished from the effect on the supporting cells and the vasculature.
Finally, radiation dose to the target and surrounding tissues is controlled by the choice of modality (eg, conventional external beam, IMRT, or SRS) and treatment plan. In SRS, doses are prescribed to the tumor margin. The maximal dose within the target area may vary tremendously, depending on the specifics of the treatment plan. For example, single or multiple isocenters may be used. For further information, please see the Treatment Planning/Radiation Delivery Section in the Gamma Knife Radiosurgery for Vestibular Schwannoma chapter of this publication.
RADIATION THERAPY FOR VESTIBULAR SCHWANNOMA
The options for delivery of radiation therapy for VS include SRS, FSRT, intensitymodulated radiotherapy (IMRT), three-dimensional conformal radiotherapy (3DCRT) and, rarely, conventional RT. SRS uses multiple, convergent, nonparallel, noncoplaner radiation beams to deliver a high dose of irradiation to a small target volume while delivering low doses to the surrounding normal tissues. This can be accomplished with either a gamma knife (GK-SRS) or a linear accelerator. Depending on the choice of prescription isodose line covering the target, up to 50% or more variation may be seen between the peripheral dose and the maximum dose. FSRT offers the advantages of SRS and conventional RT. Like SRS, FSRT allows the conformation of the irradiation dose precisely to the target volume. In addition, it allows the fractionated delivery of the dose, which is thought to result in differential sparing of cranial nerves relative to the tumor. VS typically presents with auditory or vestibular dysfunction. This dysfunction is thought to result from any one, or a combination, of the direct compression of the eighth cranial nerve, labyrinthine vasculature, or the development of poorly understood changes within the inner ear fluids. Symptoms may include tinnitus, ataxia, dizziness, unsteadiness, vertigo, a sensation of fullness in the ear and gradual hearing loss.37 Most patients present with hearing loss (98%), tinnitus (70%), and disequilibrium (67%).38 Thus, it can be difficult to distinguish the auditory and vestibular effects of RT for VS from those caused by the natural progression of VS. As hearing is the most readily measured parameter of auditory and vestibular function, hearing preservation serves as the best indicator of RT impact on the auditory systems, particularly for VS. Hearing preservation rates, according to the Gardner Robertson criteria, ranging from 50% to 70%, and local control rates ranging from 95% to 100% have been reported with GK-SRS and LA-SRS for VS. Immediate SNHL post-SRS for VS has been rarely reported. SNHL within 3 months of SRS may result from neural edema or demyelination. Most SNHL has been reported at 3 to 24 months after SRS. Foote and colleagues reported a median time to the onset of SNHL after GK-SRS among subjects with sporadic VS to be 18 months (range, 8.9–30 months). In subjects with neurofibromatosis type 2 (NF-2)-associated VS treated with SRS, Linskey and colleagues observed progressive SNHL with a median time of onset to be 4 months (range, 3–12 months). Subach and colleagues51 reported a median time of onset of SNHL at 4 months (range, 3–15 months) with none documented before 3 months after SRS. Many RT and tumor parameters have been found to affect auditory outcomes in the treatment of VS. These parameters include involved nerve length, total dose, fractionation, and tumor type.
Stereotactic Radiotherapy
SRT is a technique that attempts to combine the biologic advantages of fractionation (ie, delivering multiple small doses of radiation) with the physical advantages of the precision immobilization seen in SRS. This technique is performed on a linear accelerator with a relocatable stereotactic frame such as with the Novalis (BrainLab Incorporated, Westchester, Illinois) or Radionics (Burlington, Massachusetts) systems or with frameless systems that use optical tracking such as Cyberknife (Accuray Incorporated, Sunnyvale, California). Much of published experience with SRT has been with the Gill-Thomas-Cosman relocatable head ring. Treatments can be delivered with multiple noncoplanar arcs or with fixed fields. Multiple noncoplanar arcs imply that a finely collimated linear accelerator delivers the radiation dose while rotating around the patient, whereas with fixed fields, the beam is fixed but delivered through multiple points that converge on the target lesion.
As a rule, the precision for SRT is less than that of SRS, with a relocatable accuracy of approximately 2 mm,3 and often safety margins of 2 to 5 mm are added to the planning volume to account for this uncertainty.In recent years, the development of intensity-modulated radiation therapy (IMRT) and image-guided radiation therapy (IGRT) has refined SRT further. IMRT is the ability to modulate the intensity of the radiation beam through the treatment volume to allow for maximum sparing of normal tissues with improved dosimetry to the target. IGRT is the use of daily imaging of the target volume before treatment delivery to account for patient movement to assure target accuracy. One could argue that these techniques have replaced SRT in that they are imaging- and not coordinate (or frame)-based.
However defined, these techniques are used widely in many radiation oncology departments and have allowed for treatments with SRT-like precision to be performed on many of the same intracranial targets as SRS. One posited advantage of SRT relative to SRS is that because of the ability to fractionate the treatment, targets larger than 3 cm/or those involving or adjacent to critical structures such as the brainstem or optic apparatus can be treated with SRT without undue risk of complications. These size and dose constraints are much more limiting in single-fraction SRS. Further, these developments are being applied to extracranial targets, including tumors in the lungs, pancreas, liver, prostate, and spine.