Stereotactic Radiotherapy for Optic Nerve and Meningeal Lesions

DEFINITIONS

Stereotactic irradiation is defined as the accurate delivery of high doses of radiation to a stereotactically defined target (the tumor) in such a way that the dose fall-off outside the target volume is very sharp. The radiation prescription can be given in one of two ways: (1) when the treatment is delivered in a single fraction, the term ‘‘stereotactic radiosurgery’’ (SRS) is used, and (2) when the treatment is delivered in a fractionated manner over a number of weeks, it is called ‘‘stereotactic radiation therapy’’ (SRT). For patients with salvageable vision, the latter is more appropriate because fractionation optimizes the effect of radiation on cycling tumor cells and minimizes the damage to late responding neural tissue such as the optic nerve.

METHODS OF STEREOTACTIC RADIATION

THERAPY

The three essential components of SRT are precise immobilization, precise tumor localization, and conformal treatment planning and delivery. Each will be described briefly as follows.

Precise Immobilization

To allow for fractionation of stereotactic irradiation, several devices that provide a relocatable coordinate system have been developed. Generally, these are rigid thermoplastic mask and=or bite block systems with a repositioning accuracy of 1.0 mm both axially and in the superior to inferior direction (Fig. 1).

Precise Tumor Localization

In SRT, the location of the tumor to be irradiated must be defined to within 1 mm accuracy in three-dimensional (3D) space. The task of tumor localization starts with acquisition of thin-slice, contrast-enhanced axial computed tomographic

Figure 3 Co-registered axial images from a CT and MRI scans.

planning computer, which enables the reconstruction of the images in coronal, sagittal, or other defined planes (Fig. 3).

Conformal Treatment Planning and Delivery

Stereotactic radiation therapy may be delivered using either photons (most common) or charged particles. Due to its characteristic Bragg peak, a proton beam offers an advantage in dose reduction at depths greater than the target tumor. However, due to the high cost of construction and the considerable physics and technical support required, proton therapy is available in only a few centers worldwide. Conversely, due to wide availability of the linear accelerator (LINAC), SRT is more commonly delivered by high-energy photons generated from a LINAC modified for SRT application. Hence, the following discussion on treatment planning considerations and delivery is limited to the LINAC-based technology.

The minimal dose=fractionation that is required to prevent progression of an optic nerve astrocytoma or meningioma is 45 Gy administered in 25 fractions over 5 weeks. Therefore, the primary aim of treatment planning is to deliver this dose to the periphery of the tumor with the central

maximum dose not exceeding 50 Gy in order to limit the risk of radiation optic neuropathy and retinopathy to less than 5%. Secondary considerations of treatment planning are to minimize the dose to the lens and the pituitary gland as damage to these organs, albeit correctable, has a low dose threshold. Lastly, minimizing the dose of radiation to the mesial temporal and orbitofrontal lobes of the brain would be prudent, as the threshold for damage to the limbic system is unknown and the effects are likely complex and long lasting.

Since a LINAC is originally designed to irradiate targets up to 40 cm in size, tertiary collimation is required to restrict the radiation beam down to a maximum size of approximately 4 cm for SRT use. As an optic nerve tumor is usually very elongated in shape, the most simple method is to use an array of multiple noncoplanar fixed beams, each of which conforms to the cross-section of the tumor in the ‘‘beam’s eye view.’’ Tertiary collimators fabricated from Cerrobend, a low melting point alloy consisting of 50% bismuth, 26% lead, 13% tin, and 10% cadmium, are the most cost-effective solution. However, because each collimator has to be manually mounted onto the treatment head of the LINAC before each beam is used, it is impractical to use more than 5–6 beams per treatment session. As a result, this leads to ‘‘dose peaks’’ along the entrance path of each beam (Fig. 4).

The introduction of the micro-multileaf collimator (mMLC) represents a big advancement in the technology of tertiary collimation. Depending on its manufacturer, an mMLC usually consists of 20–30 pairs of computer-controlled Tungsten leaves, each of which is usually 2–5 mm in width (Fig. 5).

Even in its most basic application, the mMLC allows for a rapid and automated deployment of a large number of static beams (Fig. 6). A more advanced application is the use of dynamic arc treatment—a dynamic shaping of the beam during gantry rotation, thus simulating an infinite number of static beams distributed in the most optimal distribution (Fig. 7). This results in greatly diminished entrance ‘‘dose peaks’’ compared to the dose distribution of Fig. 4.