Simulating, contouring,planning and evaluation of radiotherapy plans for brain tumours.

1. Plan Evaluation in
Primary Brain Tumours
Dr. Ashutosh Mukherji
Associate Professor,
Department of Radiotherapy, RCC, JIPMER

2. Indications for Radiotherapy
ASTROCYTOMAS:
•Surgery is the mainstay of treatment, with 80% of low grade cerebral,
cerebellar and spinal cord tumours; and 40% of diencephalic tumours
amenable for complete surgical resection.
Indications for radiotherapy:
•In low grade tumours following incomplete resection in areas where
tumour progression would compromise neurological functions.
•Patients with progressive and / or symptomatic unresectable disease
Radiotherapy is otherwise not indicated after complete resection in low
grade astrocytomas.
•Post operative radiotherapy is indicated IN ALL CASES OF HIGH GRADE
GLIOMAS
•In optic nerve gliomas, progressive disease on chemotherapy or after
surgery in children older than 10 years.

3. Indications for Radiotherapy (contd)
Brainstem Gliomas
•Rare patients with high grade
gliomas.
•Patients with low grade
gliomas with progressive post
op.
•Surgically inaccessible
tumours.
•Diffusely infiltrating gliomas
of the pons.
ASTROCYTOMAS OF SPINAL CORD:
•Older children with low
grade gliomas with post op
progressive disease
•Surgically inaccessible
tumours.
•Diffusely infiltrating / high
grade gliomas.

4. Indications for Radiotherapy (contd)
•
EPENDYMOMA: Post op RT indicated in all cases. Radiotherapy can be
avoided in only the following situations: (a) Ependymoma of spinal cord post
complete resection and (b) In selected supra-tentorial ependymomas
involving non-eloquent areas resected with wide margins.
•
CHOROID PLEXUS TUMOURS: Atypical tumours; Lepto-meningeal seedings
(CSI to be given); Post op residual disease.
•
CRANIOPHARYNGIOMAS / SELLAR TUMOURS: Sub-totally resected tumour or
recurrence after total excision.
•
PINEAL TUMOURS: Sub-totally resection; or leptomeningeal seedings.
•
GERM CELL TUMOURS / GERMINOMAS: All cases.

5. Radiotherapy technique
•
For larger lesions opposed lateral fields
provide appropriate coverage.
•
For smaller PTV a 3-field technique can
provide a highly conformal dose
distribution.
•
IMRT can further improve dose
conformality, but increase in treatment
time is to be considered if anaesthesia
is also being used in children.
•
SRT uses a hypofractionated treatment
schedule, & may be preferred for
children.

6. Patient positioning: A neutral
head position with the patient
supine is easily reproducible.
Noncoplanar beams can be used
to avoid entry and exit dose to
organs at risk (OAR).

7. •
Immobilization: Variability of setup not more than 2-3 mm with thermoplastic mask.
More accurate and/or rigid head positioning and immobilization can be obtained by a
modified stereotactic head frame with noninvasive multiple-point head fixation.
•
Patient is placed in the positioning device, and scanned, typically with radio-opaque
reference markers placed at the setup isocenter.
•
Verification films taken before treatment; and include orthogonal radiographs to
verify the isocenter, + films of any custom-shaped portal fields.

8. •
•
•
•
Simulation: patient is immobilised and scanned with three radio-opaque reference
markers placed on the thermoplastic mask.
A prone setup may be considered for posterior fossa tumors.
An optimum beam arrangement typically consists of 3 to 7 non opposed shaped
beams. When applicable, the contralateral uninvolved hemisphere of the brain
should be spared as much as possible.
A true vertex beam should be avoided, if possible, due to exit dose into the body;
a 5 to 10 degree gantry rotation should be considered instead.

9. •
•
Beams: Most lesions can be treated well with 4-6 MV photons
In case of infra-tentorial tumours, the posterior fossa must be boosted, with
the anterior border covering the posterior clinoids and the superior border
including 1 cm above halfway between the foramen magnum and vertex
(superior extent to tentorium cerebelli). Inferiorly, the field should cover the
foramen magnum.
IMRT: improves dose delivery to target
volumes and reduces dose to the doselimiting OARs within the cranium. IMRT
in craniospinal axis irradiation can
homogenize
dose
and
improve
conformality.

10. Cranio-Spinal Irradiation:
•OARs to be contoured include brainstem, temporal lobes, middle and inner ear.
Decreasing dose delivered to cochlea and 8th cranial nerve in pediatric patients
significantly reduces risk of hearing loss and is recommended for all patients
receiving posterior fossa irradiation .
•The two opposed beams should be angled anteriorly. The inferior border of the
initial cranial field is placed around C2-3. Depending on length, the spine is
treated through one or two posterior fields.
•The field length of upper spinal field is maximised (40 cm at 100 cm SSD) and
lower spinal field is minimized, to simplify junction shifts. The caudal border of
the lower PA spine field is set inferior to S3 by a length equal to the two-field
shifts, and then blocked back to S3.

11. •
Accurate matching of upper border of the spine field to the lower border
of the cranial field is required to avoid overlap in the upper cervical cord.
All junction lines are moved 0.5 to 1.0 cm every 8 to 12 Gy to avoid
overdosing or under dosing segments of the cord.

12. Radiotherapy doses and volumes
•
In general, low grade tumours require doses between 50 -54 Gy in
conventional fraction sizes of 1.8 Gy per day; while high grade tumours
require 60 Gy @ 2 Gy per day.
•
Ependymal tumours and PNET doses may differ; maximally between 5054 Gy total dose. CSI dose is usually restricted to maximum 36 Gy (high
risk) and 23.4 Gy (low risk).
•
Germ cell tumours respond to doses of about 25 Gy; while lymphomas
may require doses upto 30 Gy @ 1.5 Gy per day.

14. Contouring Guidelines
•
Target volumes will be based upon CT or MRI.
•
The initial gross tumor volume (GTV1) is defined and then margin added
for clinical target volume (CTV1) and planning target volume (PTV1).
•
Reducing PTV margins to modify organ at risk (OAR) dose is not advised.
The OAR must be defined along with a planning risk volume (PRV) which is
usually OAR plus 3 mm.

15. •
GTV: Includes pre-op tumour
volume on T2-Weighted MRI
for phase 1 for low grade and
T1 weighted post op contrast
MRI for high grade lesions.
•
CTV:
•
PTV: 0.5 cm margin to CTV

16. •
In the event that an OAR is in immediate
proximity to a PTV such that dose to the
OAR cannot be constrained within
protocol limits, a second PTV defined as
the overlap between the PTV1 and the
particular PRV of concern, is created.
•
Dose to the PTV overlap must be as
close as permissible to microscopic
disease dose while not exceeding the
OAR dose limit.
•
The boost gross tumor volume (GTV2) is
also defined along with boost clinical
target volume (CTV2) and boost
planning target volume (PTV2).
1

18. Goals of treatment planning
• prescription dose conforms to target volume
– normal tissues are not excessively irradiated
• PTV receives uniform dose
• doses to OARs do not exceed tolerance values

19. Goals of treatment planning: PTV
• to ensure 100% PTV is covered by 95% of the
prescription dose
• in other words, underdose to any part of PTV
shall not exceed 5% of prescription dose
• to ensure overdose to any part of PTV shall
not exceed 7% of prescription dose

20. Dose statistics provide quantitative information on the volume
of the target or critical structure, and on the dose received by
that volume. These include:
- Minimum dose to the volume
- Maximum dose to the volume
- Mean dose to the volume
- Dose received by at least 90-95% of the volume
- Volume irradiated to at least 90-95% of the prescribed dose.

21. The following tools are used in the evaluation of the
planned dose distribution:
- Isodose curves
- Orthogonal planes and isodose surfaces
- Dose distribution statistics
- Differential Dose Volume Histogram
- Cumulative Dose Volume Histogram

22. Isodose chart or CT
based 2D plan
evaluation

23. • Equivalent Square Field size (EFS) of each field is calculated. Any
blocks used are deducted from this area by calculating the EFS
of the blocked area also. The percent depth dose of each beam
(PDD) is calculated using the EFS and prescription depth from
standard tables.
• Then using standard formulae and percentage depth dose
tables, the treatment time or number of MUs for each beam is
obtained.
• This is multiplied with field weightage and wedge factors or tray
factors (for blocks, etc) to correct it for beam weightage as well
as for attenuation with use of modifying devices.

24. •
The obtained isodose lines are then
checked to see if they include the entire
area under the field size as well as the
extent of the 90% and 100% isodose line.
•
Usually the 100% isodose line is shifted
more towards the field surfaces with a
narrow waist at the isocentre and it is
usually reasonable to prescribe the dose
to 90% isodose line.
•
Dose is prescribed to the isodose which
best covers the full target volume.

25. • In CT based 2D planning, dose is calculated for a given section
and the isodose lines over the target volume reviewed.
• The isodose covering the target volume best with least dose
variation in centre (higher isodose lines) is chosen.
• The disadvantage with this method is that the entire field size
is assumed to match this one CT slice. This can be partially
offset by choosing a few more slices representative of change
in body contour and sequencing them as per their order in
actual patient field length.

26. In 3D Planning, plan evaluation will involve:
Checking
•Normalisation and normalised prescription dose.
•Global dose maxima
•Dose volume parameters of PTV and OARs.
•Plan Sum of phase wise plans.
•Brain and target volume coverage by prescribed doses and 50%
isodose.
•Hot and cold spots and their location and value
•Conformity and homogeneity indices
•DVH shape and distribution
•Beam’s eye view and DRRs

27. Normalization
• after the dose
calculation is over,
the dose at some
point has to be
normalized to 100%
• this point can be
anywhere in the grid
• user’s choice

28. Normalization Options

29. Check Global Dose Maximum
• what is its value?
– not more than 107% for 3-D CRT
– can be higher for IMRT, but within 115%
• where is it located?
– it should be within CTV
– preferably within GTV

31. Qualitative Evaluation
Methods

32. Slice-by-slice evaluation
• review dose distributions in all slices
• whether a selected isodose level adequately
covers the target volume or not
– one has to select as high a isodose level as possible
• identification of hot spots / cold spots
– location of these spots

37. Dose Cloud
Dose Cloud
Other display tools

38. Quantitative Evaluation
Methods

39. Dose statistics for volumes
OAR
PTV

40. Dose statistics for volumes
• minimum dose
– strong correlation between target minimum dose and
clinical outcome
– high percentage of the dose maximum
• maximum dose
– useful tool for critical structures
– typically tolerance dose
• mean dose
– indicator of dose uniformity within the target volume
– should be very close to maximum dose

41. For Target Volumes
• Target volume maximal dose ideally should
not be more than 5-7% of the prescribed
dose and minimum dose to the target
volume should not be less than 5% of
prescribed dose.
• Inhomogeneity within target volume kept to
± 10% of the prescribed dose. ICRU 83 report
is used for describing IMRT has described
D98%, D50%, and D2%. (Dmax, Dmedian
and Dmin)
• Dmax are checked in the dose colour wash in
each slice to note its location; whether it is
within the PTV as well as volume of the brain
getting a dose 50% of prescription dose.

42. For OARs
• In case of serial OARs, their
Dmax is checked as to whether it
is limited to within tolerance
doses.
• In parallel OARS Dmean is seen
for analysis. Dmax is also noted
to check for any undue hot
spots.
• Check plan sum of all phases of
the treatment plan to ensure
once more that all dose
parameters
are
within
prescribed limits.

43. Dose Volume Histogram (DVH)
•
most important evaluation tool
for 3-D planning
•
graphical summarization of 3-D
dose distribution
•
Represents
a
frequency
distribution of dose values
within a defined volume that
may be the PTV itself or a
specific organ in the vicinity of
the PTV

44. Types of DVH
• cumulative
– most used
• differential
• dose and volume axes can
be absolute or relative
– 4 combinations

45. Cumulative DVH
•
The computer calculates
the volume that receives
at least the given dose and
plots this volume (or
percentage volume) versus
dose.
•
Basically the area under
curve is the volume of
tissue getting a dose and
the smaller this area, the
better for an OAR.
The tail of the curve
should not ideally taper
too much to the right as
this will mean a smaller
volume getting a higher
dose.
•

46. As high a isodose level as possible…
• should cover a large volume as well (>95%)
• provisional selection of this isodose level for dose
prescription

47. Let us prescribe at 96%...
Prescription Dose 5500 cGy
• what difference does this make?

48. What did we do?
Dose
Plan
prescription Normalization
100%
96%
5225 cGy 5500 cGy
• 5225 cGy is 95% of 5500 cGy
• entire PTV should get 5225 cGy

49. Differential DVHs
• nice tool for assessing PTV dose uniformity

50. Limitations of DVH
• no spatial information
– where the hot / cold spot occurred
– whether it occurred in one or several disconnected
regions
• DVHs cannot be the sole criterion for
evaluating / disclosing the best plan
• interpretation of the plot can be subjective!

51. Possible scenarios
Body
PTV
miss
Body
PTV
RI
RI
Body
PTV
Body
PTV
RI
RI
spill

52. Perfectly conformal plan…
• how much of PTV is covered?
• how much is the spill?
• how is the uniformity within PTV?

53. Coverage Factor
• tells you how much you miss on PTV
volume of PTV covered by RI
volume of PTV
volume of overlapping region
volume of PTV
ideal value = 1
miss
Body
PTV
RI

54. Conformity Index (CI)
Body
PTV
RI
•
•
•
•
tells you how
much you spill
outside PTV
spill
ratio of
/
ratio of
/
ideally = 1; but expect around 1.3 to 1.5
presently, inverse of the ratio is followed

55. Homogeneity Index (HI)
• measure of uniformity within PTV
• expressed as the ratio D2/D98
– D2 is the maximum dose received by at least 2% of
the PTV
– D98 is the maximum dose received by at least 98%
of the PTV

56. Homogeneity Index (HI)

57. Homogeneity Index (HI)
• D2 / D98 = 5830/5463 = 1.067
• for a typical 3-D CRT plan, it is around 1.07
• for IMRT it should be ≤ 1.15
• D5 / D95 has also been used

58. A word of caution ...
• these are ratios...
• and hence are relative
values...
• one has to be cautious in
interpreting...
• for a 10 cc PTV, CI = 2 is
acceptable
• but for a 400 cc ?

59. IMRT plan evaluation
• not very different from 3-D CRT
– IMRT – most refined form of 3-D CRT
• trade-off between conformity and
homogeneity
– if priority is conformity, accept increased
inhomogeneity
– priority is homogeneity, accept decreased
conformity

60. HOT and COLD spots
• three questions to ask about all hot and cold
spots
– volume?
– magnitude?
– location?
• is there a consensus to any of these questions
in any tumor site?

61. Cold spots: Recommendations
• volume: <1% of PTV
• magnitude: underdosing exceeds 5% of the
prescription dose
• location: periphery of PTV; never acceptable
within the CTV

62. Hot spots: Recommendations
• volume: <15-20% of the PTV
• magnitude: overdosing exceeds 15% of the
prescription dose
– <15% volume at the 110% dose level
– <1% volume at the 115% level
• location: within the CTV (preferably GTV); not
acceptable on the periphery of PTV

63. Summary
Normalize
correctly
Check Global Max
Select as high a
isodose level as
possible
Prescribe
95% dose covers
100% PTV?
Absolute dose
DVH analysis
CI and HI
Documentation

64. • Thus benefits of conformal
radiotherapy lie in:
 ensuring protection of normal
tissues and…….
 achieving dose escalation to
tumor volume.
• Technology has given us new
tools to hit targets……….
• But to use it correctly depends
on us.

65. Thank you