March 2012 - Institute of Physics and Engineering in Medicine

SCOPE | MEETING REPORTS
ELECTRONS: DOSIMETRY,
PLANNING AND TREATMENT
JAMES WILLIAMS AND SUSAN BUCKLEY ABM University Health Board
AUSTIN COURT, BIRMINGHAM 10th November 2011
AROUND 80 DELEGATES FROM across the UK and
Ireland gathered on a misty autumn morning for an IPEM
meeting on electron dosimetry, planning and treatment.
Electrons can sometimes seem to be the Cinderella
modality of radiation treatment, with the difficulties of
accurate computerised treatment planning restricting
many clinical users to relatively simple calculations.
However, the excellent range of speakers and the number
of delegates at this meeting shows there is much interest
in electrons and their clinical applications.
After welcoming remarks from meeting organiser
Nina Burton (Cheltenham General Hospital), the opening
session took the theme of the clinical use of electron
beams and each talk highlighted the unique challenges
presented by electrons in providing accurate and precise
treatment. Gareth Baugh (Arden Cancer Centre,
Coventry) presented the results of an investigation into
the different techniques of delivering total skin electron
therapy to a specially designed phantom across 10
national radiotherapy centres. The variation in dose
uniformity discovered by his work led him to recommend
the adoption of the Stanford technique by all centres.
One of the most common clinical applications of
electrons is to provide a boost dose to the tumour bed in
breast treatments. Amanda Welsh (Royal Shrewsbury
Hospital) highlighted how new surgical techniques have
meant that using the surgical scar to locate the tumour
bed is no longer sufficiently accurate. Instead, a new
technique uses surgical clips visible on planning CT
images and virtual simulation. An audit of 30 patients
planned with both techniques showed that the extra time
spent planning and setting up with the new technique is
more than worth the gain in treatment accuracy.
Geometrical accuracy is one challenge, accurate
dosimetry is another, and Pavel Dvorak (The London
Clinic) presented a very interesting talk examining
monitor unit calculations for breast boosts using the
electron Monte Carlo (eMC) algorithm in the Eclipse
planning system, conventional flat phantom
measurements and a purpose-built breast-phantom
(figure 1). The results demonstrated the effect that surface
curvature has on electron measurements, such that
applying flat-surface measurements to curved-surface
clinical sites may lead to significant underdosing. This
provoked a discussion on whether doses should be
reported in terms of the technique or algorithm used to
calculate them, which continued into the short coffee
break held in the exhibitor’s room.
ELECTRON BEAM TREATMENT PLANNING
After the mid-morning break, the second session
contained three talks examining electron beam treatment
planning. Monte Carlo modelling has long promised to
improve accuracy in electron planning, and several
commercial systems are now available. Ralph Roberts
(Oxford Radcliffe Hospitals NHS Trust) examined the
eMC algorithm in Eclipse for lung and bone
inhomogeneities, Martin Green (Clatterbridge Centre
for Oncology, Wirral) examined the macro Monte Carlo
algorithm in Eclipse for lung and nose phantoms and
Louise Gately (Clatterbridge Centre for Oncology,
Wirral) compared clinical dose distributions calculated
using the electron pencil beam algorithm in the Plato
system to the eMC model in Oncentra MasterPlan. Each
talk investigated the accuracy of the different algorithms
and models against measured data and other systems
(figure 2). Each algorithm has its own strengths and
weaknesses which may affect their accuracy in clinical
use. Clinical decisions need to be made with an
understanding of the accuracy of the predicted dose
distributions, and each talk highlighted the importance
of commissioning measurements in developing local
understanding of the accuracy of planned doses. The
talks and following questions gave those attending from
centres not using computerised planning an opportunity
to discover more about the time and effort needed to
commission electron planning. A more general
discussion on the future for computerised planning
followed the morning session and took delegates into
lunch, with a longer opportunity to browse the
exhibitor’s stands.
Any postprandial lethargy following a delicious lunch
buffet was soon dispersed by Mark Bailey’s (National
Physical Laboratory, Teddington) entertaining tour of
electron beam dosimetry and the 2003 IPEM Code of
Practice. As previous talks had shown, standardisation
of technique is an important method of maintaining
treatment accuracy, and the grandaddy of
standardisation must be the Code of Practice, ensuring
all UK centres trace their dosimetry back to the national
standard set at NPL. The effectiveness of the Code of
Practice could be demonstrated by the national variation
in dose measurements. Fortunately the UK has one of the
most robust national and regional audit programmes for
radiotherapy, with excellent evidence to show that all
departments are meeting the standard. Much of the
credit for the strong audit programme is due to Steve
Bolton (The Christie NHS Foundation Trust,
Manchester), Chair of the National Interdepartmental
Audit Group, who presented a talk discussing the results
of the most recent National Electron Audit (2009–2010).
In each participating centre, three parameters (beam
energy, output and measured dose for a planned
treatment) were measured for three clinical beam
energies. Results were very encouraging, with beam
energy measurements largely within ±3 per cent and
output within ±2 per cent. The planned cut-out fields
saw greater variation and the results of three
departments which saw an underdose of >10 per cent
were discussed. Each department reinvestigated their
32 | MARCH 2012 | SCOPE