RADIOLOGICAL PHYSICS GROUP, CHELSEA
Head of
Medical Physics Chelsea:
D
R Dance PhD
Head of Radiological Physics
Group and RPA:
S
H Evans MSc
Radiation Protection and Staff Dosimetry
Service
SH Evans, IA Castellano, BE
Pratt, S Dhanse, L Martinez , S Chima, A Spencer, RE Doull, E
McDonagh
We continue
to provide an approved dosimetry service to the Royal Marsden,
The Institute of Cancer Research and the Royal Brompton and Harefield
NHS Trust (Brompton site). This service has now been extended
to the Harefield site. We also continue to provide a calibration
service for contamination monitors throughout the Trust. Radiation
protection shielding for the new PET suite and for the relocation
and upgrade of the systemic iodine suite have been evaluated.
Attention remains focused on the requirements of the Ionising
Radiations Regulations 1999 (IRR99) and the Ionising Radiations
(Medical Exposures) Regulations (IRMER). The Medical Exposures
Committee and the Trust’s Radiation Protection Committee
continue to oversee the implementation of the IRMER and the IRR99.
During the year the group has focused on ensuring departments
are fully compliant with the new regulations, particularly with
the auditing requirements under the Clinical Governance framework.
We also supply extremity dosimeters to Brighton NHS Trust and
a hospital in Portugal. Staff are monitored mainly using film
badge dosimeters. The radiographs formed when the films are exposed
to radiation are visually inspected by our staff prior to densitometric
evaluation of the films’ densities. Over 15,000 dosimeters
were processed and evaluated last year.
A member of staff
measuring films Film
badges
The
following is a list of some of the main features of our dosimetry
service:
- personalised badges for positive ID of
staff
- maximum 10 day reporting of doses
- immediate response (1 working day) to
evaluation of doses to staff exposed accidentally
- radiation protection advice on the significance
of staff doses
- results available on hard copy or electronic
media
- visual and densitometric evaluation of
films
- flexibility in response to changes
in needs
- individual and personal approach
- doses measured down to 0.01mSv
Some of the other main functions of the Group
are outlined in the following sections.
Comparison
of Personal Dosemeters for the Measurement of Low Radiation Doses
TO Faleye, CL Skinner, SH Evans
Source of funding:The RoyalMarsden
The dose thresholds of personal monitoring film, thermoluminescent
dosimeters (TLDs) and electronic personal dosemeters (EPDs) were
measured at various photon energies. Film was found to have a
dose threshold of 0.04mSv. The dose thresholds of TLDs were found
to be 0.02mSv and 0.002mSv depending upon the phosphor type. Above
0.1mSv, the average deviation from the applied dose of all dosemeters
was better than 15%. Below 0.1mSv, the response for film badges
were unreliable. The PDs did not respond adequately to pulsed
high doserates.
Diagnostic
Radiology Physics Quality Assurance Service
SH Evans, IA Castellano, CL
Skinner, L Martinez, K Bell, ET McDonagh
Source of funding: The Royal Marsden
We continue to supply a diagnostic radiology quality
assurance service to a number of hospitals. Recent advances in
X-ray equipment have expanded the scope of our work to include
spiral CT scanners, advanced cardiac units and digital X-ray equipment
such as computed radiography systems and digital mammography units.
The sophistication of such equipment requires up-to-date knowledge
of technological and clinical developments. In 2002 we commissioned
a 4-slice CT scanner; an upgrade to an electron beam CT scanner;
two full field digital mammography units; a simulator with a flat-panel
imager; and two mobile units. We are also involved in writing
technical specifications for new equipment, and evaluating the
models offered prior to purchase. This information is then used
to decide on the manufacturer that best meets the clinical requirements.
In 2002 we prepared the technical specification for an upgrade
to an electron beam CT scanner, and assessed three 16-slice CT
scanners. The scope and frequency of the tests comply with the
recommendations set out in IPEM report 77: "Recommended standards
for the routine performance testing of diagnostic X-ray imaging
systems". The Group aims to provide a very high level of
service incorporating thorough measurements and good liaison with
staff in the X-ray departments to explain our results and any
action that needs to be taken. In addition, we follow up our recommendations
and liaise with service engineers. We have recently been updating
our reporting methods and have developed a range of spreadsheet
templates to provide direct data entry into a notebook PC that
also facilitates preliminary analysis of measurements. Patient
dosimetry is performed according to national protocols where these
are available, and our own protocols are developed where necessary.
Particular attention is given to high-dose procedures such as
examinations using fluoroscopy. Results are also compared with
national and European reference levels when these are available.
It is encouraging to note that our patient dose measurements are
below the reference levels published by the NRPB. Such information
provides objective evidence of differences in practices and equipment.
Radiation Protection
Training
SH Evans, IA Castellano Smith, B Pratt, CL Skinner, E Santusova.
Source of funding : RMT, external delegates
Proper training of staff is essential for both
staff and patient protection against unwanted exposure to ionising
radiation. We will be running our eleventh annual Radiation Protection
Training Course (RPTC) this year. It lasts one week and has gained
both national and international recognition. IRMER (Ionising Radiation
(Medical Exposure) Regulations 2000)) courses have been run to
satisfy the legal training requirement for key staff concerned
with radiation protection of the patient both for brachytherapy
and unsealed source therapy.
Radiographic Techniques
and Lesion Detection in Digital Mammography
DR Dance, RA Hunt, IA Castellano;
in collaboration with W Huda, SUNY, Syracuse, New York. Source
of funding: US Department of Defence Breast Cancer Research Program,
The Royal Marsden
The Royal Marsden
has X-ray equipment which can capture digital mammograms and can
produce image quality to match or exceed that for conventional
screen/film mammography. The equipment has exciting potential
for improvement of the small cancer detection rate. We are making
a combined experimental and theoretical study of how best to utilise
digital mammographic systems. We aim to quantify the improvement
in image quality and to identify imaging configurations (X-ray
spectrum and exposure) which will produce the desired image quality
at the smallest breast dose. Experimental observer studies by
our USA collaborators have used the ACR phantom and an anthropomorphic
phantom to study the detection of small cancers. The results have
been successfully compared with the predictions of our theoretical
model of the imaging system. Both approaches are being combined
to find operating configurations which can reduce breast radiation
dose whilst matching or improving image quality.
Patient
Dose Optimisation in Computed tomography
IA Castellano, DR Dance, CL
Skinner, P Evans. Source of funding:The Royal Marsden
Computed tomography (CT) is probably the greatest
single source of man-made medical radiation and the relationship
between radiation dose to the patient and image quality must be
optimised. This is particularly imperative for innovative screening
procedures such as fast volume acquisition scans of smokers to
detect lung cancer at an early stage. The first step in the process
is the estimation of patient doses. We have developed a computer
model to simulate the electron beam CT scanner, which is used
for such scans. The radiation doses calculated agree with experimental
measurements to within 10%. We have demonstrated that, for this
scanner, patient doses are1mSv for a high resolution scanning
technique, but as much as 7 mSv for a volume scan. Therefore great
care must be exercised when choosing a scanning protocol for screening
procedures.
Comparison of Monte Carlo calculations with experimental measurements
at various locations within the AAPM head and body phantoms, using
the 3 and 6 mm slice modes.
Towards a Simple Method for Evaluating
Patient Doses in Computed Tomography
DR Dance, IA Castellano, CL
Skinner; in collaboration with P Aviles, E Vano, Universidad Complutense
de Madrid, Spain. Source of funding: EU, The Royal Marsden
We are developing methods of estimating
radiation dose for computed tomography examinations using thermoluminescent
dosimeters (TLD) placed on the patient's skin. This approach is
more practical than the standard methodology, which uses large
plastic phantoms and ionisation chambers. We have found that the
skin dose depends strongly on the vertical position of the patient
within the CT scanner gantry. We have developed a simple computational
model which can accurately predict this vertical variation, and
which can be used to correct dose measurements to those for a
standard patient position. This work is directly applicable to
CT fluoroscopy, a new technique for CT-guided biopsies that can
potentially deliver high skin doses. We have demonstrated that
patient skin doses can be considerably greater than those estimated
using plastic phantoms, and thus the probability of causing skin
erythema is greater than at first anticipated.
Unification of Physical and Clinical
Requirements for Digital X-ray Imaging
DR Dance, RA Hunt, SH Evans, IA Castellano,
CL Skinner; in collaboration with G Alm Carlsson, M Sandborg,
G Ullman, Linkoping; groups from Braunschweig, Lausanne, Liverpool,
Malmo, Magdeburg, Neuherberg Source of funding: EU, The Royal
Marsden
This study is part of a large Fifth Framework
EU funded collaboration to develop methods of quantifying and
improving image quality in digital radiography of the breast and
chest. Our contribution is a theoretical study of the influence
of system design on image quality and radiation dose. We use Monte
Carlo computational methods and a voxel phantom to simulate the
radiographic examination. During the first year of this new project
our main emphasis has been on the extension and validation of
our existing computer model of the mammographic examination to
simulate digital imaging systems. The resulting code can calculate
the complete radiographic image as well as the necessary dosimetric
quantities. It has been validated by comparison with measurements
on our General Electric 2000D digital mammographic unit. Good
agreement between measured and calculated pixel values and relative
noise levels has been obtained.
Use of the Thomas Jefferson University
Breast Phantom for Breast Dosimetry
DR Dance, RA Hunt; in collaboration
with ADM Maidment and PR Bakic, Thomas Jefferson University; G
Alm Carlsson, M Sandborg, Linköping. Source of funding: EU,
The Royal Marsden
Our computer model of X-ray mammography has
been extended to make use of realistic high resolution models
of the breast, recently developed at Thomas Jefferson University.
It allows for the first time dosimetry for individual breast tissues,
and studies of the dependence of image properties on breast structure.
The realistic breast phantoms have a voxel size of 200 or 400
micron and simulate in three dimensions adipose and fibro-glandular
tissues, Cooper's ligaments, ducts and skin. The computer program
calculates the dose to each tissue and image properties such as
energy imparted and noise per pixel. In a preliminary study, doses
to each tissue have been calculated for a phantom 50mm thick and
a conventional Mo/MO spectrum. Doses (normalised to 100 for adipose
tissue) are: fibroglandular tissue 92; ducts 87; skin 284; Cooper's
ligaments 147. Variations in the scatter-to-primary ratio due
to tissue inhomogeneity are 25%.
Calculated images of the
Thomas Jefferson breast phantom. The upper image has been calculated
using primary photons only. The lower image has reversed polarity
and has been calculated with the attenuation in the ducts enhanced
so that the ductal tree can be visualised.
Use of Scattered X-rays
for the Detection and Characterisation of Breast Abnormalities
DR Dance, SH Evans; in collaboration
with groups from CLRC Daresbury, Cranfield, Manchester, Nottingham.
Source of funding: The Royal Marsden
We are part of a group led by the national synchrotron radiation
laboratory at Daresbury studying the scattering and refraction
of X-rays by breast tissue. The work is exploring the imaging
potential of scattered x-rays for both the detection and characterisation
of tumours. Images taken using the synchrotron radiation source
in Trieste showed remarkable tumour contrast when a novel technique
known as diffraction enhanced imaging was used. The practical
potential of this and related techniques is continuing to be explored.
Development of an International
Code of Practice for Patient Dosimetry in Diagnostic Radiology
DR Dance; in collaboration
with G Alm Carlsson, Linköping; L De-Werd, Wisconsin; H-M
Kramer, Braunschweig; K-H Ng, Malaysia, F Pernicka, Vienna Source
of funding: International Atomic Energy Agency, The Royal Marsden
Medical X-ray examinations contribute
greatly to the population dose from man-made radiation sources.
A key stage in the process of controlling this dose is the standardisation
of the procedures for instrument calibration and for dose measurement
in the clinic. We are participating in a working group supported
and co-ordinated by the International Atomic Energy Agency in
Vienna, that is developing an international code of practice for
instrument calibration and practical measurements in the clinic.
Influence of Operator Protocols on Effective
Doses for Paediatric Computed Radiography
S Dhanse, IA Castellano, SH Evans.
Source of funding: The Royal Marsden
The effect of operator protocols on effective dose
for different aged infants, undergoing a chest x-ray using computed
radiography has been investigated using a Monte Carlo program
(PCXMC). Simulated doses received by new-born, 1year old and 5
year old infants were calculated. The effective dose was largely
independent of the age of the infant for a given exposure protocol.
In comparison to a well collimated x-ray field, the effective
doses increased by 157%, 101% and 157% for the 0, 1, and 5 year
old respectively when the collimators were open both laterally
and longitudunally
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