ICRU 71 REPORT, Summarized presentation of international commission of radiation units & measurements regarding electron beam therapy.

1. ICRU REPORT 71Dr Dhiman Das
2nd yr PGT
Dept of Radiotherapy
Medical College & Hospital,Kolkata

2. ICRU REPORT 71
PRESCRIBING,RECORDING & REPORTING ELECTRON BEAM
THERAPY

3. Why unique?
• In clinically useful range of energies (6-20 MeV)
in treating superficial tumors (<5 cm deep) it
offers—
1. Dose uniformity in the target volume.
2. Sharp dose falloff beyond target.
3. Minimal dose to the deeper tissues.

4. MAIN USES
a) Treatment of skin & lip cancers.
b) Chest wall irradiation for breast cancer.
c) Boost dose to nodes.
d) Head & Neck irradiation.

5. Nuggets in ICRU 71
• Treatment volume same as ICRU 50 & 62.
• Characteristic of electron beam.
• Physical & dosimetric data.
• Reporting of electron beam.
• Specific recommendations for reporting for non-
reference conditions-small & irregular beam,
oblique beam incidence & presence of
heterogeneities .
• Quality assurance

6. Physical
characteristi
cs of
electron beam
• R p-The point at which the
tangent at the steepest
point(the inflection point)on
the almost straight descending
portion of the depth-dose curve
meets the extrapolated
bremsterlung background(Dₓ)
• E p,o=C1+C₂Rp+C₃R²p
• Rt=depth on the beam axis, of a
given isodose relevant for the
treatment.
• Broad Beam-central axis depth
dose distribution can be considered
independent of the field size, when
further increased.
 Lax & Brahme-Field size
diameter>Rp represent broad
beam situation.

7. Effect of field size on central-
axis depth-dose curves
7MeV 13MeV 20MeV

8. DOSE
GRADIENT
• G- A measure of dose fall-off.
• G=
𝑅𝑝
𝑅𝑟−𝑅𝑞
• Lower value of G(2.5)is observed
in most accelerators.
• Higher value of G (3.3) –an
accelerator with optimized design
for the treatment head.
• G<2.3 for broad beams (5-30
MeV) indicates the flattening &
the collimating system is of poor
design & that unnecessary large
volume of normal tissues are
irradiated in a Single beam
technique.
• G decreases for small fields.

9. Oblique
beam
incidence
 Changes-
1. ↑Surface dose.
2. ↑dose at the
maximum along the
beam axis.
3. ↓penetration of the
therapeutic depth
dose.
4. ↑range of penetration
of a low dose
component.
 Isodose distribution has a
“wedge” form & the

10. collimation
 Electron beam is
generally
collimated with
external
applicators, placed
directly on the
skin or close to
skin.
 Without using
external
applicators large
penumbra results.

11. Collimation
contd..
a) OLD TYPE-
 1 scattering foil.
 Large energy & angular spread.
b) CONVENTIONAL TYPE-
 2 scattering foils.
 Much less energy & angular
spread.
c) NEW DEVELOPMENT-
 A scanning beam.
 Leaf collimator is used(for both
photon & electron).
 Air is replaced by He.
 It is seen (karlsson et al 1992)
using He in place of air reduces
penumbra by about 40%.
a. b. c.

12. Drawbacks of electron
beam therapy
1. Irregular field shapes are laborious to set up.
2. Difficulties in using adjacent electron & photon
beams.
3. Dose determination & computation difficult,
particularly in case of heterogenicities.

13. REPORTING e-BEAM
• In LEVEL 1- 3 values constitute the minimum
information to be reported.
1) Dose @ ICRU Reference Point.
2) Maximum & 3) minimum dose to PTV.
• In LEVEL 2&3-
1) More accurate information regarding 2) & 3).
2) DVH for PTV, Some cases CTV,GTV, PRV to be
reported.

14. REPORTING e-BEAM
• Reporting is very similar
to ICRU 50 & 62.
• ICRU Reference Point
(general criteria)-
1. Dose @this point should be
clinically relevant.
2. Point should be easy to
define in a clear &
unambiguous way.
3. Where the dose can be
accurately determined.
4. In a region where there is
no steep dose gradient.
• To fulfill these criteria…
a) Always @the center (or in
the central part) of the
PTV.
b) If 1 beam is used in e
beam therapy, whenever
possible,the point should
be @ the beam axis,@ the
level of the peak dose.
• Dose @ ICRU Reference
point is ICRU Reference
dose.

15. ICRU Reference point @max depth
dose
ICRU Reference point & max depth
dose are @ different area.

16. contd…
• Ideally the PTV should be covered by the TV.
• As it is not be the case, proportion PTV enclosed
by the TV should be recorded…like
Portion of the PTV receiving 95,90 & 85% of the ICRU
Reference dose is referred as PTV95, PTV90, PTV85.

17. Dose at Reference
Points
Issues to be considered:-
• Geometric factors.
• Dose computation
techniques.
ICRU Reference Point is at the
center/at the central part of PTV-
• No steep dose gradient.
• Relatively homogenous
dose around Reference
Point.
• Dose @ the point is
equal /close to the dose
delivered to large
portion of PTV.

18. • Depending on the algorithm used & voxel size there
may be statistically significant fluctuation in
dose calculation.
• In such cases average dose value within a sphere
(usually 1cm in diameter) centered around the ICRU
Reference point is calculated,

19. Reporting dose
distribution to OAR
• Probability of late effects depends on-
a.Dose level.
b.Fractionation.
c.Absolute volume &/or fration of the OAR
irradiated.
• For each OAR the Maximum dose should be
reported.

20. A single electron beam
Level 1 Level 2 & 3

21. Small & irregularly
shaped beams
Specialities
• Depth of max dose moves
towards surface.
• ↓Depth of therapeutic range.
• ↑Relative surface dose.
• Dose fall-off shallower.
• ↓Dose rate.
Changes ↑with ↓energy
Level 1,2 & 3
• Same as single e beam

22. Extended SSD
Specialities
• ↑SSD leads to-
o↑Surface dose.
oModerate change in
depth-dose region,
except build up
region.
oLoss in beam
flatness.
oWider penumbra.
Level 1-3
• same

23. Heterogeneities
Effect of heterogeneities Level 1
• If only level 1 is available,
use of e beam to be
reconsidered.
LEVEL 2
• Peak absorbed dose in water to
be determined for an incident
beam in a homogeneous phantom.
• An accurate dose distribution
(corrected for
heterogeneities) should be
obtained, so that radiation
oncologist can decide the ICRU
Reference Point.

24. Bolus
Used to..
• ↑Skin dose.
• Compensate for surface
irregularities/oblique
beam incidence.
• Match the beam
penetration with the
shape of the PTV.
LEVEL 1-3
• SSD should be clearly
stated.
• ICRU Reference Point
should be in the tissue,
not in the bolus.
• Others being same.

25. Electron
plus photon
beam
Used for-
Coaxial beams
1.More homogenous
irradiation.
2.Better skin sparing &
normal tissue sparing.
Adjacent beams-
• A CTV may have to be
covered by several
PTVs & for each PTV an
ICRU Reference Point
has to be defined.

26. Combination of e beams
Adjacent parallel
• Used when PTV is too large.
• Special care to be taken at the
field junctions.
Parallel opposed
• Less frequently used.
• Usualy beams of high energy is
(about 40 MeV) is used.
• Significant over/under dosing
can result from beam energy
<30 MeV.