ICRU Report 71 outlines the procedures for prescribing, recording, and reporting electron beam therapy, highlighting key benefits such as dose uniformity for superficial tumors and minimal collateral damage to deeper tissues. The report also details methods for reporting dose information, recommendations for quality assurance, and addresses the challenges associated with irregular field shapes and dose determination in heterogeneous tissues. Additionally, it discusses critical parameters for effective treatment such as isodose distributions and collimation techniques.

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.