CSIREJIL RAJAN

HISTORY 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)

CSF DRAINAGE PATHWAY

SPINAL CORD lower border of L1(adults)or S1 (children)

Sub ArachnoidSpace : lower border of S2 (adults) or S3-S4 (children)

Tumours prone for CSF Spread

Tumours in proximity of CSF drainage pathways are prone for CSF spread

INDICATIONS Medulloblastoma Germinoma Pineoblastoma Supratentorial PNET Anaplastic ependymoma

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

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

INTRODUCTION

RATIONALE 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

WHAT MAKES CSI CHALLENGING ?

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

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

FIELDS TO ENCOMPASS TARGET VOLUMEPhase 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)

PRE-RT EVALUATION 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

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

PLANNING STEPSPositioning ImmobilizationSimulation Field arrangementMatching of CSIAligning of spinal field Implementation of plan

PATIENT POSITION

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.

IMMOBILIZATION METHOD

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.

CSI BOARD

SIMULATION OF SPINAL FIELD 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

PLACEMENT OF CRANIO - SPINAL JUNCTION

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.

TERMINATION OF THECAL SAC

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)

VALUE OF MRI

ONE OR TWO SPINAL FIELDS ?

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

GAP OR NO GAP BETWEEN SPINAL FIELDS

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

GAP : FIXED OR CALCULATED 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 appropriateS = ½ L1. d/SSD1 + ½ L2 . d/SSD2

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

SIMULATION OF SPINAL FIELD

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

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

EXTENDED SSD TECHNIQUE

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.

SIMULATION OF CRANIAL FIELD 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

SHIELDING 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).

SFOP GUIDELINES

Shielding

MATCHING CRANIO-SPINAL JUNCTION

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 orcouch rotation required

Matching Cranio-Spinal Junction

Collimator and Couch Rotation

Collimator Rotation with hemiblock of cranial fields

MATCHING CRANIO-SPINAL JUNCTION

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

JUNCTION : FIXED OR MOVING

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.

JUNCTION SHIFT 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.

POSTERIOR FOSSA BOOST 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

SFOP GUIDELINES

POSTERIOR FOSSA BOOST

DOSE PRESCRIPTION

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

DOSES AND VOLUMES FOR MEDULLOBLASTOMA

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 #)

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

DOSES AND VOLUMES FOR MEDULLOBLASTOMA