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Simulated Body Shape for Dose Determination in Phototherapy
D.McG.Clarkson,Department of Clinical Physics and
Bioengineering,UHCW NHS Trust, Coventry.
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ISSUES
Complex pattern of UV irradiance over a body surface.
Bodies come in all shapes and sizes.
Choice of timed dose or automated internal dosimetry.
How effective is internal dosimetry
system –
if
utilised?
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ISSUES
If checking cabinet irradiance, how is this best done –
direct or
indirect?
How good is measurement technology accuracy -
calibration and
traceability?
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Measurement Framework
Moseley H.
Scottish UV dosimetry
guidelines, "ScUViDo". Photodermatol
Photoimmunol
Photomed. 2001 Oct;17(5):230-3.
Taylor DK et al, Guidelines for dosimetry
and calibration in ultraviolet radiation therapy: a report of a
British Photodermatology
Group workshop. Br J Dermatol. 2002
May;146(5):755-63.
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Measurement Techniques
Martin CJ, Currie GD, Pye
SD,The importance of radiometer angular response for ultraviolet
phototherapy dosimetry. Phys Med Biol. 1999 Apr;44(4):843-55.
Fulljames
CA, Welsh AD.Measurement of patient dose in ultraviolet therapy
using a phantom. : Br J Dermatol. 2000 Apr;142(4):748-51
Currie GD, Evans AL, Smith D, Martin CJ, McCalman
S, Bilsland
D
An automated dosimetry
system for testing whole-body ultraviolet phototherapy cabinets.
Phys Med Biol. 2001 Feb;46(2):333-46.
Martin CJ, Clouting H, Aitken
A A study of the correction factor for ultraviolet phototherapy
dose measurements made by the indirect method. Br J Dermatol. 2003
Dec;149(6):1227-31.
Clarkson DM, The use of simulated body shape for determination
of patient dosimetry
within whole body treatment cabinets, Phys. Med.
Biol. 2006, 51, N51-N58
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Unit ConstructionBody shape constructed by L. Franks of
Radiotherapy Physics Section, Walsgrave Hospital.
Styrofoam used extensively within Radiotherapy Physics
Unit fabricated within three sections.
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Body Shape Advantages
Safety aspect of not being present in cabinet when measurements
made.
Reproducible positions makes comparisons more relevant.
Potential to verify internal cabinet dosimetry
systems.
Potential for inter cabinet Comparisons.
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Male(1)
Female (2)
Mean (1)+(2)
Body Shape
Mean height(cm)
175.1 161.9 168.5 172.5
Mean weight (Kg)
83.8 70.1 76.95 -
Est. Body surface area(m2)
1.994 1.783 1.889 1.67
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Item Value
Total Height 172.5 cmMaximum Breadth 53.5 cmTotal weight 4.815
Kg
Total Volume 0.1605 m2
Density Styrofoam 30 Kg/m-3
Major axis cross section area
0.617 m2
Minor axis cross section
0.343 m2
Material Styrofoam(building insulation)
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Measurement System Bentham
dmc150 spectroradiometer
Detector DH-10 photomultiplier
Calibration light source CL6-H
Detector D6 diffuser f2 error 0.84% at 400 nm linked to 2 m
fibre optic cable
Slit width 1 nm
Measurement interval 1 nm
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Single parameter value for dosimetry.
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Surface Uniformity
∑fi
(h)Ai
(h)Wi
(h) dh
--------------------------
i = 0 to H
∑Ai(h) dh
Where fi
(h) = fraction of peak irradiance at height h
Ai
(h) = body shape area at height h per cm of height
Wi
(h) = arbitary weighting (
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Variation of console/dmc150 reading with orientation
of body shape
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Coefficient of Overdosing
MAX(reference -
console irradiance value)/mean
Where reference > console irradiance value
Coefficient of Underdosing
MAX(console irradiance value –
reference)/mean
Based on e.g. Rotation of body shape in cabinet in units of 90
degrees
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Movement Compensation
Describes gradient of function of reference measurements and of
indicated console values. For two sites Ref1
and Ref2 -
eg front and rear of patient, a Positional Compensation term
(PcxRef) can be described as :-
PcxRef1
= (dRef1
/dx -
dCon/dx)
PcxRef2
= (dRef1/dx -
dCon/dx)
Where Con is console indicated value.
These terms can be understood in the sense of x axis in
direction normal to entrance door. One side of patient appears
to
have some
form of compensation -
the other does not. These are constants of the physical design
of the cabinet and the physical characteristics of the body
shape.
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Movement Compensation Sideways
No compensation present
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Considerations
•Body shapes for ‘child’ -
‘teeny’ -
‘adult’.
•Surface reflectance -
application of paint an advantage -
5% reflectance target in UV
•Profile of detector.
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Summary
A rigid lightweight body shape has proved of value for routine
Quality Control issues.
The scope of such devices should encourage their development
within the framework of a
specific medical device standard for whole body treatment
cabinets for ultra violet radiation.
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