RADIATION FIELD DIAGRAMS TO THE EYE:

Patient Supine with Direct Laterals:

• Note that because of divergence the opposite eye receives part of the dose
• This is the typical dose distribution for a whole brain setup

Patient Prone with Direct Laterals:

• Note that Asymmetrical Jaws where used and the Upper Jaws are closed down preventing divergence

Figure A represents a Wedged Pair and Figure B represents a Wedged Pair with Pencil Beam Blocks:

Figure A is an example of the Pencil Beam Block with Bolus and Figure A shows an Anterior Field which is weighted with a lateral field at 1:4.5 (this means that if the total dose is 200 cGy the Anterior will receive a dose of 36.3 cGy and the Lateral will receive a dose of 163.7 Cgy; a 1:4.5 ratio):

TOLERANCE DOSES WHEN TREATING THE BRAIN:

CENTRAL NERVOUS SYSTEM

Central Nervous System : The portion of the nervous system consisting of the brain and spinal cord.

• Brain : ends at the medulla oblongata
• Spinal Cord : starts at the cervical spine which is located in the foramen magnum.  The Live spinal cord extends from Formamen Magnum(C1) to L2 while the vertebrae continue till the end of the Sacrum
• At the level of C7 is the spiney process which can be seen and palpated.  This is a good landmark to localize C7 or the beginning of T1

Anatomy of the Brain:

Cerebrum: made up of 2 hemispheres which each has 4 lobes:

• Frontal lobe
• Temporal lobe
• Occipital lobe
• Parietal lobe

 

The Left and Right Hemispheres are separated by the Corpus Callosum:

Corpus Callosum : The arched bridge of nervous tissue that connects the two cerebral hemispheres, allowing communication between the right and left sides of the brain.  It is filled with fluid.

 

Meninges : The three membranes that cover the brain and spinal cord and is filled with Cerebral Spinal Fluid (CSF); going from skull to the brain, the membranes are:

• dura mater (the toughest of the three)
• arachnoid (so called because it resembles a web)
• pia mater (delicate and highly vascular).

 *Cerebro Spinal Fluid flows in the Sub-arachnoid space which is in between the arachnoid and pia mater and ends at S2*

 

Cerebro Spinal Fluid : a liquid that is comparable to serum but contains less dissolved material, that is secreted from the blood into the lateral ventricles of the brain by the choroid plexus, circulates through the ventricles to the spaces between the meninges about the brain and spinal cord, and is resorbed into the blood through the subarachnoid sinuses, and that serves chiefly to maintain uniform pressure within the brain and spinal cord.  Acts as a shock absorber.  (It is actually diluted lymphatic fluid)

Ventricles : any of the interconnecting cavities in the brain.  We have 2 lateral ventricles and 2 lower ventricles.  Filtrate from the lymphatic system enters the ventricles and eventually becomes Cerebro Spinal Fliud.

Primary Brain Tumors:

Gliomas (astrocytomas): primary brain tumors of the forebrain

• tumors of the glial cells which are the sticky cells of the nerves; hold neurons together
• diagnosis based upon GRADE; either they are :
  • LOW GRADE 1 or 2 ; Benign
  • HIGH GRADE 3 or 4 ; Malignant

Ependymomas: A central nervous system neoplasm made up of relatively undifferentiated ependymal cells(cells that line the Cerebro Spinal Fluid Sac and ventricles)

 

Oligodendro Cells : fatty cells that protect the brain

 

Astrocytoma Grade 4 (Glioblastoma Multiforme): the worst most deadly tumor known to man; It is made up of highly undifferentiated (anaplastic) cells

Metastases are rare with primary brain tumors because of the blood brain barrier; for metastases to occur they must spread by the lymphatics or the blood.

Grade and Histology are extremely important to diagnosis; Stage is not important

Direct invasion is common