CRANIOSPINAL IRRADIATION

1. REJIL RAJAN

2.  The concept of CSI was advanced by Dr Edith Paterson (wife of Ralston Paterson).
 Before this the patients of Medulloblastomas were treated with posterior fossa or
whole brain radiation
 She advocated the treatment of the entire neuraxis – bringing the concept of CSI
 Paterson and Farr reported that with the use of cranio-spinal irradiation in 27
patient resulted in a 3 yr survival of 65% (Acta Radiologica – 1953)

4. SPINAL CORD
lower border
of L1(adults)
or S1 (children)
Sub Arachnoid
Space : lower
border of S2
(adults) or
S3-S4 (children)

5. Tumours prone for CSF Spread
Tumours in proximity of CSF drainage pathways are prone for CSF spread

6.  Medulloblastoma
 Germinoma
 Pineoblastoma
 Supratentorial PNET
 Anaplastic ependymoma

7. Management of tumours
• Medulloblastoma
• Pineoblastoma
• Anaplastic Ependymoma
• Supratentorial PNET
• Germinoma : RT alone
(CSI + primary boost
vs.
Reduced volume RT + boost)
Surgery
(Maximum safe
resection)
+
post op RT
(CSI + PF boost)
± chemotherapy

8.  CSI is a very complex technique
 Goal is to achieve uniform dosage throughout the
subarachnoid space, encompassing the entire
intracranial vault and spinal canal.
 Fundamental is
 the use of opposed lateral fields including the cranium and
upper cervical spinal canal,
 matching a posterior spinal field including the full spinal
subarachnoid space with cranial field
 in larger children, the upper posterior spinal field matching
with a separate lower posterior spinal field

9.  Medulloblastoma forms the most common indication for CSI
 In medulloblastoma , CSF Dissemination is known in 20 - 30 % of
cases, producing a risk of metastases along the neuraxis.
 Posterior fossa, spinal cord, ventricular walls & supratentorial
region including the cribriform plate form the main sites of relapse
 Being a radiosensitive tumour, RT is curative in upto 70 % of
average risk patients

10.  Patient positioning and immobilization difficult, especially
in paediatric cases (may require anaesthesia)
 Large, irregular target volume
 Critical structures, with special importance to paediatric
cases, who are potential long term survivors
 Problems of matching junctions between the divergent brain
and spinal cord fields

11.  Pituitary
 Eyes / Lens
 Cochlea / Inner ear
 Parotid
 Oral cavity
 Mandible
 Thyroid
 Larynx
 Heart
 Lungs
 Oesophagus
 Liver
 Kidneys
 Gonads (Testes / Ovaries)

12. Phase I : Craniospinal radiotherapy
(two parallel opposed lateral cranial fields
orthogonally matched with the posterior spinal
field to cover the entire length of the spinal cord)
Phase II : Posterior fossa boost
(whole posterior fossa irradiation or conformal
boost to tumour bed)

13.  Detailed history & operative notes.
 General physical & complete neurologic examination
(ophthalmoscopy included)
 Gadolinium enhanced pre-op MRI of the brain & spine.
 Immediate post-op MRI brain for residual disease status.
 Delayed post-op MRI of the spine (if pre-op scans not done).
 CSF cytology

14.  Target Volume:
 Entire brain and its meningeal coverings with the CSF
 Spinal cord and the leptomeninges with CSF
 Posterior fossa – boost
 Energy
 4-6 MV linac or Co60
 Portals
 Whole Brain: Two parallel opposed lateral field.
 Spine: Direct Posterior field
 Scheduling of radiotherapy:
 Starting time : within 28 days following surgery
 Duration of treatment : 45 to 47 days

15. Positioning
Immobilization
Simulation
Field arrangement
Matching of CSI
Aligning of spinal field
Implementation of plan

16.  Prone:
 Direct visualization of light fields for spine field setup
 Supine:
 More comfortable for the patient.
 In-anaesthetic patient
 Head position
 Slightly extended and the shoulders pulled down
to avoid beam divergence into the mandible & dentition.
 Facilitates the use of a moving junction between the cephalad
border of post. Spine field and the lower borders of cranial
fields.

18. 1.Orfit(Thermoplastic devices)for immobilization of the head, cervical
spine & shoulder
2.Small children –inverted full body plaster cast with facial area open
for access for anesthesia
3.Alpha cradle
4.Vac lok
5.CSI board: Lucite base plate fitted on which is a sliding
semicircular lucite structure for head-rest & chin-rest.
Slots from A to E to allow various degrees of extension of neck, so as
to avoid exit of superior border of the spinal field through the oral
cavity.
Thermocol wedge for supporting the chest wall.

20.  Spinal field simulated first, as easier to match divergence of spinal field
with the cranial field by means of collimator rotation
 SSD technique
 Field width ~ 4 cm in small children to 6 - 7 cm in adults to cover the
lateral spinal roots
 Gap of 5 mm between spinal and cranial fields
 Issues :
 Level of craniospinal junction higher (C1 - C2) or lower (C5 - C7)
 Determination of the lower border of termination of thecal sac
 Total length of the patient’s thecal sac, to decide whether 1 or 2
spinal fields required

21.  Higher level - C1 / C2 interspace is routinely practised, since overdose
at cord is low as compared to low junction
 Lower level - lowest level in the neck with exclusion of the shoulders in
the lateral fields (from C5 to C7), lowers the exit dose to thyroid,
mandible, larynx & pharynx
Cause of overdose at the neck region (Halperin IJROBP 1996) :
 Narrow neck separation than cranium & dose prescription at midplane
of brain
 Couch rotation towards gantry leads to decreased treatment distance.

22.  Traditional recommendation for lower border of spinal field is at inferior
edge of S2 (myelogram & autopsy studies)
 It adequately treats majority of patients.
 8.7% patients have termination below S2-S3 interspace.
 MRI has been advocated to set the lower border of the spinal field as it
accurately determines the level of termination of the thecal sac & the
extent of neuraxial disease if present
 Helps to minimize gonadal toxicity as radiation scatter to ovaries or
testes is dependent on how close the lower border is to the gonads
(IJROBP, 1998, vol 41)

24.  In children, one field is often sufficient to cover the entire length of cord,
single craniospinal junction
 In adults - two fields required; cranio-spinal & spinal-spinal junction
 Usually, 2 spinal fields required if length > 36 cm
 Junction between two spinal fields generally placed at L2 - L3
Field matching at both the junctions critical
1. Cranio-spinal junction : various techniques; described subsequently
2. Spinal-spinal junction : no gap / fixed gap / calculated gap can be
employed for matching as central axes of both the beams are parallel

25.  Proponents of no gap argue that as medulloblastoma is a
radiosensitive tumor, small reduction in dose per fraction or
total dose to part of TV, owing to a gap, may produce
significant
difference in cell kill over a fractionated course of CSI, seen
as
local recurrences (Tinkler, IJROBP 1995)
 Proponents of gap argue that no gap risks overdose at the
junction & cervical spine & may result in disabling late
toxicity

26.  Many institutes use a fixed gap ranging from 5 mm - 10 mm
 A customized gap calculated for each patient depending on field
length & depth of prescription, is more appropriate
S = ½ L1. d/SSD1 + ½ L2 . d/SSD2

27.  After gap calculation, the
spinal fields are simulated
 SSD = 100 cm
 Width - vertebral body + 1
cm to include the
intervertebral foramina;
usual width 5 - 7 cm

29.  Ensure that the spine field is
not exiting through oral cavity
 Mark the divergent boundary
of
the superior margin of spinal
field (red line) on the lateral
aspect of neck to provide a
match line for the lateral
cranial
field (blue line)
 Open length of field to a
maximum length and mark
inferior border

30.  If adequate coverage inferiorly
(upto 4th sacral foramen),
nothing
else required. If not, additional
spinal field required
 Gap of 1 cm between the two
spinal fields, as evident from
marker of the lower border of
upper spinal field

31.  Encompasses entire spinal canal in
single field with better homogeneity
 Higher PDD and grester penumbra
 Higher doses to all anteriorly placed
normal structures (mandible,
oesophagus, lungs, heart, thyroid
gland, gonads. Doses to gonads and
thyroid gland with the use of an
extended SSD technique may impact
on sterility, thyroid dysfunction &
carcinogenesis
 Hence not routinely recommended.

32.  AP width includes entire
skull with 2 cm clearance
 Superiorly, clearance of ~ 4
cm to allow for symmetric
field reduction while doing
junction shift
 Inferiorly, the border is
matched with superior
border of spinal field
(typically placed at C3 - C4
junction)
 to provide adequate
margin to posterior fossa
tumour
 to facilitate matching with
the spinal field

33.  Critical to appropriately shape the shielding
particularly in
frontal (cribriform plate) & temporal regions as
recurrences due to inadequate coverage are known.
 SFOP defines the inferior border of lateral brain field to
be 5mm below the orbital roof.
 Lens can be adequately shielded using MLC’s of smaller
leaf width (lower penumbra).

36. Shielding

37. Techniques
 Collimator rotation (7 - 10°) to match divergence of spinal
field with the cranial field
 Couch rotation (~ 6°) to match divergence of cranial field
with the spinal field
 Half beam block
 Asymmetric jaws
 Penumbra trimmers
With use of these, no collimator or
couch rotation required

43. Matching Cranio-Spinal Junction
Collimator and Couch Rotation

44. Collimator Rotation with hemiblock of cranial fields

45.  Cranial field is set up so that caudal field margin is
parallel with the diverging superior margin of the spinal
field
Collimator angle = tan-1 {½ L1/SSD}
L1 is spinal field length.
 To match the diverging cranial fields with the diverging
spinal field the couch must also be rotated in addition to
the collimator rotation.
Couch angle = tan-1 { ½ L2/SAD}
L2 is cranial field length
Matching Cranio-Spinal Junction

46.  Owing to lateral scatter of photons & electrons, a gap on skin as
defined by the light beam will be reduced by 1-2mm at depth
(Tatcher, 1989, IJROBP).
 At doses relevant for medulloblastoma, a 5mm overlap at 4 MV
photons can result in 30 to 40% overdose i.e. 14Gy for 36Gy
prescribed dose, which may exceed cord tolerance
(Hopulka, 1993, IJROBP)
 Systematic error during radiotherapy delivery could further lead to an
overlap or gap. Acceptable systematic set up error for CSI is 2 mm
 Concurrent CT recently being used for high risk patients can also
result in long term neurotoxicity.

47.  Moving the junction / Feathering after every 5 to 7 fractions smoothes out
any overdose or underdose over a longer segment of cord
 Done every few fractions (every 7 #)
 Can be done either cranially or caudally.
 Cranial inferior collimator is closed & spinal superior collimator is
advanced by the same distance superiorly (if junction to be shifted
cranially).
 Similarly, lower border of superior spinal field & superior border of
inferior spinal field are also shifted superiorly, maintaining the
calculated gap between them.
 Usually shifted by 1 to 2 cm each time.

51.  Anterior: posterior clinoid process (avoid pituitary)
 Posterior: internal occipital protuberance
 Inferior: C1-C2 interspace
 Superior: Midpoint of foramen magnum & vertex or
1
cm above the tentorium (as seen on MRI)
Field arrangement - two lateral opposing fields
3DCRT boost to the pre-op tumor bed with margins

54.  Reduced dose RT (i.e., 23.4 Gy CSI, 54 Gy to the posterior fossa)
combined with chemotherapy gives similar results.
 Hyperfractionation : 36Gy/36# @ 2#/day CSI, followed by 32Gy/32#
to CTV conformal boost has been reported to have similar survival
with better neurocognitive outcome & acute toxicity less than
concurrent CTRT protocol.
Dose Medulloblastoma &
other PNETs
Germinoma
CSI 30-36Gy/18-21#/4
wks
30Gy/18#/4 wks
PF boost 18-20Gy/10-11#/2.5
wks
15Gy/8#/1.5 wks

55.  CSI for average - risk disease
(age >3 yrs, M0 status, and residual <1.5 cm2)
 Standard dose CSI: 35-36 Gy/21-20#/4 weeks @ 1.67-1.8 Gy/#
 Reduced dose CSI: 23.4 Gy/13#/2.5 weeks @1.8 Gy/# (+ adj CT)
 Very reduced dose CSI: 18 Gy/10#/2 weeks @ 1.8 Gy/# (+ adj CT)
 Boost for average - risk disease
 Standard dose CSI : PF or TB boost: 19.8 Gy/11#/2 weeks
 Reduced dose CSI: Tumour bed boost: 32.4 Gy/18#/3.5 weeks
 Very reduced dose CSI: Tumour bed boost: 39.6 Gy/22#/4.5 weeks
 Total tumour bed dose: 54-56 Gy/ 30-33#/ 6.6.5 weeks
(conventional #)

56.  CSI for high-risk disease
(age <3 yrs, M+ status, and residual >1.5 cm2)
 Standard dose CSI: 35-36 Gy/21-20#/4 weeks @ 1.67-1.8 Gy/#
 Higher dose spinal RT: 39.6 Gy/22#/4.5 weeks @1.8 Gy/#
 Boost for high-risk disease
 Whole posterior fossa boost: 19.8 Gy/11#/2 weeks
 Boost for gross focal spinal deposit: 7.2-9 Gy/4-5#/1 week