- Cranial and lateral limits are outside the patient in the air.
- For modeling of the caudal borders on the simulator, it is important to include the base of the skull with branches of the cranial nerves as these are covered by the meninges to an extent (be especially aware of the lamina cribrosa). Usually, the brain stem is included farther down than for solid tumors. At the same time, the lenses should be blocked and the eyes as much as possible. For curative treatment, it must sometimes be accepted that tolerance doses for lenses are exceeded.
- Field shaping from side to side including eye blocks will obtain a "German helmet" configuration and reaches farther down the cervical column with good margins to the spinal canal (for example to discs C2/C3 or C3/C4 level).
- Orbital cavities can be included if there is known lymphoma involvement of the eye ball or optic nerves.
- For treatment which is planned with CT, the entire brain is modeled including the outer liquor space as CTV. For practical purposes, the inside of the bone in the cranium and the spinal canal is the limit.
- Be very precise when contouring the skull. The dura sacs which follow the nerve roots in the different foramens must be included in the CTV. CTV includes also the upper cervical column such that the field limits later remain at levels with for example C3 or C4.
- The orbital cavities may be included in CTV if there is known lymphoma in the eye ball or optic nerves.
- Field shaping will be two opposing lateral fields. Field shaping should be checked with regards to skeletal structures at the base of the skull before approval just to ensure good coverage of the lamina cribrosa and other foramens in the cranial nerves.
CT dosage plan
Fractionation and total dose depend on multiple factors, among others, the type of lymphoma and the protocol is followed. For a treatment plan intended to cure the disease the following is recommended:
For PCNSL, Oslo University Hospital follow the protocol from MSKCC, which is 1.80 Gy x 25 for patients under 60 years subsequent to 5 courses of high-dose MTX, if the patient is not in complete remission. For involvement of the eye, these are included with 1.8 Gy x 20. If the patient is in complete remission, 1.2 Gy x 30 is given with 2 fractions daily. Patients over 60 are not routinely given radiation therapy.
For lymphoblastic leukemia/ALL, treatment is administered according to study protocols, for example NOPHO in children, preferably 2 Gy x 9-12.
For other lymphomas, fractionation is determined individually.
For palliative treatment, 3 Gy x 10 is most often used.