POR TANTO:

• The external acoustic meatus are bilaterally symmetric; they participate in the

definition of anatomic reference planes in the head (Reid's baseline and Frankfort horizontal plane, connecting points in the two external acoustic meatus and one anterior infraorbital edge). Unless marked at simulation, the external acoustic meatus may be difficult to see on lateral projections because of overlying temporal bone structures.

• The two lateral parts of the anterior cranial fossa, the two anterior parts of the middle cranial fossa floors, and the two mandibular angle points, with their lateral locations, represent appropriate reference points.

• On a lateral projection radiograph, the sella turcica is centrally located and marks the lower border of the median telencephalon and diencephalon. The hypothalamic structures are located an additional 1 cm superior to the sellar floor, and the optic canal runs, at the most, 1 cm superior and 1 cm anterior to that point.

• The pineal body (tentorial notch) usually sits approximately 1 cm posterior and 3 cm

superior to the external acoustic meatus.

• The cribriform plate, the most inferior part of the anterior cranial fossa, is an important reference point for the inferior border of whole-brain irradiation fields. In most

patients, little distance is found between the lateral projections of the lens and the most inferior part of the cribriform plate. Without a good head fixation device, it may not be possible to both block out the lens and include the cribriform plate for a prescribed dose in both locations.

Treatment Setup

• The head should be positioned so that its major axes are parallel and perpendicular

to the central axis incident beam and the treatment table; the most common errors are rotation of the head and longitudinal axis deviation (tilting).

• Reproducibility of head positioning is achieved with a fixation device such as the

Fixster stereotactic device (table-fixed reference plate attached to a plastic turban plus mouthpiece). Other devices are an individually made mouthpiece attached to a table frame or a table-fixed thermoplastic net mask.

Irradiation of Entire Intracranial Contents

• Whole-brain irradiation is administered through parallel-opposed lateral portals, which

should always be individualized.

• The inferior field border should be 0.5 to 1.0 cm inferior to the cribriform plate, middle cranial fossa, and foramen magnum, all of which should be distinguishable on simulation or portal localization radiographs.

• The anterior border must be about 3 cm posterior to the ipsilateral eyelid for the

diverging beam to exclude the contralateral lens; however, this supplies the posterior ocular bulbs with only about 40% of the prescribed dose. A better alternative is to angle the beam about 5 or 7 degrees (100 cm or 80 cm source to axis distance midline, but also field-size dependent) against the frontal plane, so that the anterior beam border traverses the head in a frontal plane about 0.5 cm posterior to the lenses (about 2 cm posterior to eyelid markers). This arrangement provides full dose to the posterior parts of the ocular globe (15).

Treatment Volume in Brain Tumors

• Small portals are used for boost treatment or when risk of spread is low.

• For glioblastoma multiforme, it initially was recommended that large volumes or even

the entire intracranial contents should be irradiated because of their diffuse nature. However, in 35 patients who had a CT scan within 2 months of an autopsy, 78% of recurrences were within 2 cm of the margin of the initial tumor bed and 56% were within 1 cm or less of the volume outlined by the CT scan (13). No unifocal tumor recurred as a multifocal lesion. These findings were confirmed by others (37).

• In a review of CT scans and pathologic sections of 15 patients with glioblastoma multiforme, if radiation treatment portals had been designed to cover the contrast- enhancing volume along with a 3-cm margin around the edema, they would have covered all histologically identified tumor in all cases (12).

• Relatively generous margins (i.e., 3 cm) and inclusion of all radiographic evidence of

tumor and associated edema must be the rule in designing treatment portals.

• Figure 15-3A illustrates the initial portal used for the treatment of a grade 3 multifocal malignant astrocytoma with some brain edema to deliver 45 Gy, and the reduced portal to deliver an additional 14.4 Gy.

View Figure

Fig. 15-3: A: Simulation film of head outlines frontal and thalamic tumor (T) with associated

edema (E). Solid line depicts initial portal used to deliver 45 Gy to brain with opposing lateral fields (combination of 6- and 18-MV photons). Broken line outlines reduced volume irradiated through left lateral portal to deliver additional 14 Gy to midplane of brain with 6-MV photons. B: Isodose curves demonstrate dose distributions. (From Wara WM, Bauman GJ, Sneed PK, et al. Brain, brainstem, and cerebellum. In: Perez CA, Brady LW, eds. Principles and practice of

radiation oncology, 3rd ed. Philadelphia: Lippincott–Raven, 1998:777–828, with permission.) Irradiation Techniques

• Bilateral or medial cerebral hemispheric tumors are best treated with parallel-opposed

portals (Fig. 15-3B).

• If the tumor is asymmetric or lateralized, combinations of dual-photon energies (6 MV

and 20 MV) provide better dose distributions, yielding higher tumor doses and diminishing the dose to the uninvolved normal brain (Fig. 15-4) (5).

• Frontal lesions encompassing only the anterior parts of the lobe can be treated with

anterior and lateral isocentric perpendicular beams; the dose distribution can be optimized with wedges in either or both beams (Fig. 15-5).

• Midcerebral tumors (posterior frontal or anterior parietal) are best treated with parallel-opposed anterior and posterior portals and lateral portals, all isocentric and with or without wedges.

• Posterior parietal or occipital lesions can be treated with posterior and lateral isocentric beams, both suitably wedged for dose homogenization.

• Lesions in the temporal lobe tip are difficult to treat with other than lateral portals unless the patient is flexible enough to tuck the chin against the chest so that a sagittal beam does not traverse the lens. In that case, an added lateral portal may result in an acceptable local dose distribution, which could be further improved by a posterior parallel-opposed field.

• Craniopharyngiomas and pituitary, optic nerve, hypothalamic, and brainstem tumors

isocentric three-portal, four-portal, rotation, or arc-rotation treatment techniques. Stationary beams give adequate dose homogeneity in and around the sella turcica. The three-field technique consists of parallel-opposed lateral portals and an anterior vertex portal. The lateral portals may be wedged to compensate for the declining anteroposterior dose gradient from the anterior portal. The four-field box technique uses both lateral and sagittal parallel-opposed portals. A 360-degree rotation

technique can be used if fixation is adequate to avoid geographic misses. Because of the shorter distance from the anterior surface, the cylindrical dose distribution

becomes flattened posteriorly. Arc rotation with reversed edges enables an elliptoid dose distribution.

• Brainstem lesions are adequately treated with parallel-opposed lateral portals

combined with a posterior midline portal that does not irradiate the eyes (make certain that the ocular lenses are not in the irradiated volume).

• Unilateral cerebellar lesions also can be covered by appropriately wedged posterior

and lateral portals.

• Pineal lesions are often treated with parallel-opposed lateral portals.

• Superficial lesions (superior sagittal sinus meningiomas) can be treated with parallel-

opposed isocentric tangential fields or half-beam block to avoid divergence of beams to the normal brain.

View Figure

Fig. 15-4: Isodose curves illustrate increased tumor dose in unilateral cerebral hemisphere tumor treated with unequal beam weight (greater on side of lesion). (From Cooley G, Gillin MT, Murray JF, et al. Improved dose localization with dual energy photon irradiation in treatment of lateralized intracranial malignancies. Int J Radiat Oncol Biol Phys 1991;20:815–821, with permission.)

View Figure

Fig. 15-5: Isodose curves illustrate treatment of patient with frontal parietal tumor with anteroposterior and lateral portals using wedges to improve dose homogeneity. (From Wara WM, Bauman GJ, Sneed PK, et al. Brain, brainstem, and cerebellum. In: Perez CA, Brady LW, eds. Principles and practice of

radiation oncology, 3rd ed. Philadelphia: Lippincott–Raven, 1998:777–828, with permission.) Three-Dimensional Conformal Irradiation

• 3-D conformal therapy increasingly is used to treat primary and metastatic brain

tumors.

• When International Commission on Radiation Units Report No. 50 (14) is used for 3-

D treatment planning, the gross tumor volume encompasses the enhancing tumor and surrounding edema on CT or MRI scans. The clinical target volume adds 1 to 3 cm, depending on histologic grade. The planning target volume adds 0.5 to 1.0 cm.

• Multiple planar and noncoplanar fields (a minimum of five to six) encompassing the

tumor and surrounding edema with appropriate margin (planning target volume) are used—sometimes with static or dynamic wedges and multileaf collimation—to deliver 60.0 to 64.8 Gy in 1.8-Gy fractions. The treated volume should be encompassed by the 95% isodose volume (Fig. 15-6).

• Marks et al. (21) described some of these techniques; they believed that noncoplanar

beams were preferable to coplanar beams when the target was in the central regions of the head.

• Sometimes the CT scan defines abnormalities not always perceptible on MRI studies.

Integration of MRI and CT scan data may be necessary for optimal 3-D treatment planning of brain tumors.

View Figure

Fig. 15-6: Lateral helmet (whole-brain) field with Cerrobend blocking (stripes) and spinal fields. The cranial field central ray is stationary such that the superior field border splays the cranial vault at least 5 cm superior to the vertex. The inferior cranial field border traverses the lowest possible cervical vertebra, which allows a moving junction of 1 cm for each 10-Gy tumor dose. Abutment adaptation to the superior spine field border is achieved by rotating the helmet field 9 or 11 degrees (100 or 80 cm source to axis distance) with 30-cm superior spine field height) against the transverse plane through the body. With ideal head fixation, the anterior block border is about 0.5 cm inferior to the projection of the cribriform plate, 3 cm posterior to the ipsilateral eyelid surface (1.5 cm between eyelid and posterior lens surface + 1 cm to protect the contralateral lens from the diverging beam + 0.5-cm safety margin), and 0.5 cm inferior to the middle cranial fossa floor and approximately bisects the cervical vertebral bodies. Spine beams for neuraxis irradiation abut the cranial field. The superior spine beam has a stationary central ray in a transverse plane of the body, which enables optimal reproducibility of simultaneous movement of superior and inferior junctions after each 10 Gy. If possible, the superior beam should reach to the L1-2 space to avoid junctions over the inferior part of the spinal cord. The inferior beam has a stationary inferior border at S-3 because the dural sac ends at S-2. The central ray and superior border must move with step junctions unless the beam is angled. For optimal junction abutment, the inferior beam may be angled 18 or 22 degrees (100 or 80 cm source to axis distance with 30-cm field height) against the transverse plane through the body. Without angling, the junctions must be gapped according to junction dose summations. (From Wara WM, Bauman GJ, Sneed PK, et al. Brain, brainstem, and cerebellum. In: Perez CA, Brady LW, eds.

Principles and practice of radiation oncology, 3rd ed. Philadelphia: Lippincott–Raven, 1998:777–828,

with permission.)

• Brain tumors that may require irradiation of both the CNS and the entire subarachnoid space (neuraxis) include medulloblastoma, high-grade posterior fossa ependymoma, and other CNS tumors with metastases.

• The patient may be irradiated with "boost," "helmet," and "spine" fields, in that order.

• The boost is an individual portal arrangement that depends on tumor size and

localization. We always attempt to provide a 2-cm or greater 3-D margin around the presurgical (CT-enhanced tumor) volume. Wedges, angles, and multiple portals or rotational fields may be used. The patient is fixed in a prone position with the head (ideally) aligned and the neck as straight as possible.

• The helmet field simulation is prepared first by manufacture of a fixation device (a

combination of trunk- and head-fixation devices). Eyelid markers are necessary.

• Parallel-opposed large lateral fields are simulated with the central ray in the pineal region. The inferior field border is allowed to reach the most inferior cervical vertebra without traversing the ipsilateral shoulder. When the junctions are moved, this field can be conveniently decreased without a change in the position of the isocenter.

• The gantry can be angled up from the horizontal position so that the eyelid markers

coincide. This allows the ocular bulb behind the lens to reach full-dose levels because the field is no longer allowed to diverge from either direction. The collimator should be angled to accommodate the superior diverging spine field. By abutment, this avoids gap junctions in the cervical spinal cord.

• Blocks are drawn on the radiograph so that the irradiated volume includes the

olfactory groove (cribriform plate), the orbits 3 cm posterior to the eyelid markers (2 cm if gantry is angled), the middle cranial fossa plus more than 1-cm margin, and the posterior halves of the odontoid process and the included cervical vertebral bodies (Fig. 15-7). A posterior block is optional; if used, it should project through the tips of the cervical spinous processes and follow the external contour of the occipital bone. It should not be allowed to turn anteriorly with the skull contour.

• The superoinferior dimension of the helmet field is decreased by 2 cm for every 10

Gy of tumor dose to allow for a 1-cm movement of the junction in the cervical region. Because the central ray is placed so high in the head, splaying of the cranial vault by the superior field border continues, even with decreased field sizes.

• In adults, the spine fields are usually one superior and one inferior field. The superior field has a stationary central ray location. The moving junction at each 10 Gy of additional irradiation dose moves with the superior spine field. The field width should be adjusted so that the lateral field borders are at least 1 cm lateral to the lateral edge of each ipsilateral pedicle. For scoliotic patients, it may be necessary to cut tailored blocks. Unless angled (or with a block below S-3), the central ray for the inferior field must be moved with the moving junction because the inferior border of that field must stay at the S-3 level (the dural sac and subarachnoid space end at S-2).

• If helmet field rotation and inferior spine beam angling are not used, the junction must be "gapped." The dimensions of the gaps between adjoining fields must be

determined individually by dose calculation summations. This is helped by

measurements from lateral radiographs on which the patient's midsagittal plane is marked. When the field borders are placed to optimize the gap conditions,

consideration also must be given to divergence angles and to the individual attenuation characteristics of each treatment machine. All simulation radiographs must be indicated for midplane magnification, field size, and source to skin distance (source to axis distance) setup parameters.

View Figure

Fig. 15-7: Example of 3-D treatment planning conformal irradiation technique in patient with large glioblastoma multiforme of frontal temporal region. Anteroposterior/posteroanterior (A) and lateral (B) portals. C: Reduced field to boost gross tumor. D: Virtual simulation illustrates fields used. E: 3-D isodose curve (55 Gy to edema with 2-cm margin and 64.8 Gy to primary tumor with 3-cm margin). (From Wara WM, Bauman GJ, Sneed PK, et al. Brain, brainstem, and cerebellum. In: Perez CA, Brady LW, eds. Principles and practice of radiation oncology, 3rd ed. Philadelphia: Lippincott–Raven,

1998:777–828, with permission.)

Brain Irradiation during Pregnancy

• The fetal dose from irradiation of brain tumors during pregnancy was determined

using phantom measurements with thermoluminescent dosimeters in two patients

(34). For the first patient, both clinical and phantom measurements estimated fetal dose to be 0.09% of the tumor dose (0.06 Gy for a tumor dose of 68 Gy). Internal scatter contributed 20% of the fetal dose, leakage 20%, collimator scatter 33%, and block scatter 27%. For the second patient, the estimated fetal dose was 0.04% of the tumor dose (0.03 Gy for a tumor dose of 78 Gy); leakage contributed 74% of the fetal dose, internal scatter 13%, collimator scatter 9%, and wedge scatter 4%.

• When indicated, brain tumors may be irradiated to high doses during pregnancy,

resulting in fetal exposure of less than 0.10 Gy. This may confer an increased but acceptable risk of leukemia in the child, but has no other deleterious effects to the fetus after the fourth week of gestation.

Chemotherapy

• Primary chemotherapy is rarely used as the sole treatment for intracerebral

malignancies.

• Nitrosoureas, vincristine, diaziquone (AZQ), cisplatin, and procarbazine have been used in conjunction with surgery and irradiation.

• For tumors with leptomeningeal spread or at high risk of CSF involvement, direct

intrathecal injection of chemotherapeutic agents into the CSF space has been used. A limited number of agents (thiotepa, methotrexate, cytosine arabinoside) are suitable for intrathecal injections (17).