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Absolute radiation dosimetry in photochemotherapy
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1978 Phys. Med. Biol. 23 1124 (http://iopscience.iop.org/0031-9155/23/6/008) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 130.237.165.40 This content was downloaded on 02/10/2015 at 09:51
Please note that terms and conditions apply.
PHYS. MED. BIOL., 1978, Vol. 23, No. 6, 1124-1129.
Printed in Great Britain
Absolute Radiation Dosimetry in Photochemotherapy B. L. DIFFEY, P H . D . ~and A. V. J. CHALLONER,
PH.D.$
tDepartment of Medical Physics, Kent and Canterbury Hospital, Canterbury, Kent CT1 3NG, U.K. $Department of Photobiology, Institute of Dermatology, Homerton Grove, London E 9 6BX, U.K. Received 24 November 1977, in$nal f o r m 18 April 1978 ABSTRACT.Theconstruction and properties of adetector which can be used to measure absoluteirradiance from extended UV sources are described. It is shown how the use of this detector coupled with a knowledge of the relative spectral emission from a photochemotherapy unit can lead t o accurate estimation of absolute spectral irradiance incident upon a patient’s skin.
1. Introduction
Thetreatment of psoriasis by photochemotherapy (PUVA) is rapidly expanding.Therationale of thisform of treatment is the oral or topical administration of the photoactivedrug,8-methoxypsoralen, followed by exposure to high-intensity,long-wave,ultravioletradiation. I n order to achieve a satisfactory clearing of the psoriatic lesions, it is essential to have reliable control of the dose of ultraviolet radiation given to the patient. Some dermatologistsapproach the probleminapurelyempiricalfashionrelying solely on their observation of patient reactions to gauge appropriate treatment. I n thiswaythey employ theirpatients as biological monitors and use no physical or chemical systems to quantitate the radiation output of the PUVA machine. The majority of workers depend on a device supplied by the manufacturer tomonitor the output, butin general they are unawareof the relationship between what is being measured and the dose received by the patient. Finally, there is a group of dermatologists who seek support from physicists in thehope that more careful measurement might resultin improved treatment. A clinician would notadminister methoxsalen to a patientwithoutdue regard to the clinical state of the patient and the pharmacology of the drug. It is reasonable to suggest, therefore, that he should be cognisant of the uv dose received by the patient and the ultraviolet spectrum incident upon the patient, even though the relationship between the incident spectrum and the activewavelengths causing regression of psoriatic lesions is, a t present, uncertain. We believe that proper uv dosimetry is important,both for successful treatment at thepresent time, and thatin the future, should problems of, say, carcinogenesis become evident, there will exist adequate records of treatment so that retrospective studies of dose and effect can be readily correlated. I n a previous paper (Challoner and Diffey 1977) we discussed the problems of uv dosimetry in photochemotherapy but were unable to suggest a satisfactory 0031-9155/78,’0006-1124$01.00
@ 1978 The Institute of Physics
Absolute Radiation Dosimetry in P?wtochemotherapy
1125
solution to theproblem as at thetime there was no suitable detectoravailable. This lack of appropriate instrumentation was also mentioned by Lakshmipathi, Gould, MacKenzie, Johnson and Frain-Bell (1977). In thepresent communication we describe the construction and properties of a recently developed detector (FT32 Wide Angle Thermo-electric detector, Rank Hilger Ltd.) followed by a discussion on how i t may be usecl to give an absolute measure of irmdiance incident upon a patient.
2.
Construction of the detector
The principle of operation of the detector is based on the Seebeck or thermoelectric effect whereby an EMF is generated when heat is applied to thejunction of two dissimilar metals. Construction of practical devices of this nature was first described by Schwarz (1952). The present device consists of a platinum receiving element 2-155 0.02 mm in diameter, blackened for maximum absorption of radiation and mounted above a precious metal support assembly of great heatcapacity. A compensated element of similar climensions to the receiving element is mounted adjacent to the support assembly in order to overcome variations in ambient temperature which may cause clrift. I n order to increase sensitivity the components are encased in an evacuated glass envelope. The receiving element is situated within 0-5 mmof a SiO, winclow, 1 mm thick, which is attached to the glass envelope by means of a vacuum seal prior to evacuation.The use of SiO, as a window material ensures uniform transmission of radiation from 250 to 2200 nm, thus thedevice has a response which is effectively independent of wavelength within this waveband. The final low pressure is achieved using a barium getter. The evacuated envelope is mounted with the receiving element centrally positioned behind the aperture and enclosed within an aluminium housing with a 5-pin socket at therear for electrical connection to a voltmeter. a maximum angle of acceptance of 150' with a typical Thedetectorhas
Fig. 1. The complete assembly of the detector.
1126
B. L. Diffey and A . V . J . Challoner
sensitivity of 15.0 mVmW-l and a typical approximate response time better than 0.1 S for 63% full scale deflection. A photograph of the complete assembly is shown in fig. 1.
3. Angularresponse of the detector
The detector was mounted on a turntableon an optical bench and irradiated with a high intensity, point source mercury arc lamp which had an arc size of 0.25 mm x 0.25 mm. The output from this lamp was 'cept constant by using a stabilised power supply. The response of the detector was noted as it was rotated on its turntable about the normal a t 10" intervals up to 70". The resultsare shown in fig. 2, where it may be seen that the response of the
io
20
30
LO
50
60
70
Incident angle,8 Ideg)
Fig. 2. The angular response of the detector.
detector is within the experimental error of 2% of a cosine response a t angles up to 50". At angles greater than 50" the response falls more rapidly than that predicted for a cosine response due to increased reflection of radiation from the quartz window. However, it should be borne in mind that the detector will be used to measure the radiance a t a distance of, say, 20 cm from the midpoint of an extended source of 2 m in lengthwhich results in a subtended angle of 78". The intensity falling on the detector from angles at which there is a deviation from a cosine response (50" to 78") is 16% of the total intensity, and even a t these angles the detector still approximates closely to a cosine response. Consequently, it is estimated that the reading will be approximately 3% smaller than fora true cosine detector; an error which canreadilybe tolerated in this application.
Absolute Radiation Dosimetry in Photochemotherapy
1127
Measurement of absoluteirradiance If the detector is placed in front of an extended source, it will measure radiation with uniform response in the range 250-2200 nm by virtue of its wavelength andangular response. Lettheirradianceincidentuponthe detector be R mW cm-2. If the relativespectral emission of the source is wavelength of the known,such that S(h) is the relative intensity per unit emission spectrum a t wavelength h, thentheabsolutespectralirradiance, I(X),is given by 4.
I@)
=
R S ( h ) / fS(h)dX (mWcm-2nm-1)
(1)
where the integration is over all wavelengths present in the emission spectrum, not just in the UVA region. As an example of the way in which the present detector may be used to give absolute dosimetry, the following measurements were carried out.The detector was placed in a Waldmann PUVA 4000 Photochemotherapy Unit in a position approximating that of a patient’s skin. The unit hadbeen in clinical use for about 500 h.Thetotalirradiance measuredby the detector was 9.62 mW cm-2. A Beckman DBG Spectrophotometer with a l nm bandwidth was then used to determine the relativespectral emission from the lamps. The analogue signal from the spectrophotometer was digitised by means of a voltage-to-frequencyconverter andtheresultant pulses accumulatedina scalar. Every ten seconds the contents of the scalar were recorded on paper tape and the cycle repeated.Duringthe 10 S data integrationperiod, the wavelength change on the spectrophotometerwas 1.70 nm. It was necessary to correct the recordedspectrum for the variationwithwavelength of the sensitivity of the spectrophotometer.This was achievedbyrecording the spectrum of a 200 W quartz-iodine lamp of known spectral irradiance and deriving a correction function which could be applied to the PUVA spectrum. The lamp had been calibrated by the suppliers against a standard obtained from the National Bureau of Standards, Washington, using a method described by Stair, Schneider and Jackson (1963). Thephotomultipliertube employed intheBeckmann DBGSpectrophotometer was such that the efficiency of detection fell rapidly a t wavelengthsgreater thanabout 650 nm. Since the thermoelectricdetector will respond to wavelengths up to 2500 nm, it was necessary to make an estimate of the fraction of the measured irradiance in the near infrared region. This was achieved by using a Schott red glass filter (type RG63O), 3 mm thick and 50 mm in diameter. This filter transmits uniformly from 650 nm to beyond 2500 nm with a 50% internal transmittance a t 630 nm and negligible transmittance a t wavelengths below 600 nm. Measurements with this filter indicated was due to wavethat 27 f 3% of the irradiance measured by the detector lengths greater than 600 nm. This means that the value used for R in eqn (1) should in fact be 7-02 mWcm-2 (i.e. 9.62 x 0.73). Finally, the nature of the measurements necessitated modifying eqn (1) from its continuous form to the discrete form. The spectral irradiance in the wavelength interval h, - AX12 $0
B. L.Diffey and A . V . J . Challoner
1128
hi
+ Ah12 is now given as
I
I(&,Ah)= RX(i) C S ( i ) A h(mWcm-2nm-1) where
i=l
h, = the central wavelength in the ith timing period, AA = the wavelength increment during each data integration period (Ah = 1.70 nm), R = the irradiance incident upon the detector for wavelengths up to 600 nm ( R = 7.02 mW cm-2), S ( i )= the number of pulses collected during the ith timing period corrected for nonlinearity of spectrophotometer response, N = the number of timing periods ( X - 200). This procedure enables the spectral distribution of radiation at thepatient’s skin to be determined in absolute units as shown in fig. 3.
JLl l
300
I
350
Fig. 3. Absolute spectralirradiancefrom Unit.
L,
l I
LOO 450 500 Wavelength lnmi
a Waldman
11 550 PUVA
\
600
4000 Photochemotherapy
5. Discussion
Knowledge of I(X),the absolute spectral irradiance a t each wavelength in the emission spectrum, means that it is possible to compound I ( h ) with any chosen action spectrum for the regression of lesions in psoriasis and arrive a t a biologically effective dose. It follows that measurement of I(X) will enable retrospective calculations of the ‘biological dose’ to be carried out when more information is available on the action spectrum in photochemotherapy. At thepresent time it is customary to use wavelength dependent photometers, or ‘black-ray’ meters, as a means of quantifying the irradiance from photochemotherapyunit’s.These devices can be calibrated bytheQuantum Metrology Division at the NationalPhysicalLaboratory, where afiltered medium-pressuremercury arc source is used asthecalibrationspectrum.
Absolute Radiation Dosimetry
in Photochemotherapy
1129
This results in effectively monochromatic radiation a t 365 nm. However, the spectral sources used in practice for PUVA treatment are a continuum from about 320 t o 400 nm accompanied by several lines in the visible (see fig. 3). This means that the reading on the black-ray meter in mW cm-2 represents an effective irradiance a t 365 nm. Calculation of the true irradiance will demand knowledge of both the spectral emission of the lamp and the spectral response of the detector. It is also important that theaction spectrum forthe regression of psoriatic lesions falls to zero whilst the spectral response of the detector is still finite, Since this action spectrum is uncertain at the present time, it is suggested that the technique outlined in this paper, using a detector with a wavelength response which uniformly covers the whole emission spectrum, is to be preferred. At the present time there is nopressurefromdermatologists engaged in PUVA treatment to pursue the problem of basic uv dosimetry. This may be because they are unaware of the limitations of black-ray meters or because they do not regard more elaborate measurement as being necessary. We are not in a positionto sayhow beneficial a fundamental approach to uv dosimetry will be in the long term, but experience from radiotherapy has shown that carefulmeasurementleads to a betterunderstanding of the biological mechanisms a t work and ultimately t o improved treatment for the patient. Acknowledgements are due to M r . J. R. Corfield for his critical evaluation of the manuscript and Mr. D. S. Day for his helpful interest in the development of the wide-angle detector. We are also grateful to Mi. C. P. Wells for assisting with the instrumentation.
RBsunlE DosimBtrie de radiation absolue en photochimioth6rapie Description de la construction et des propri6tAs d’un d6tecteurqui peut servir a mesurer l’irradiance absolue de sources d’ultraviolets non ponctuelles. Les auteurs montrent que l’usage de ce dbtecteur, joint21 une connaissance de 1’6missionspectrale relative d’une unit6de photochimiothbrapie, peut mener a une Bvaluation pr6cise de l’irradiance spectrale absolue qui frappe la peau d’un malade.
ZUSAMMEXFASSCRG Dosimetrie der absoluten Bestrahlung in der Photochemotherapie Konstruktion und Eigenschaften eines Detektors werden beschrieben, der zur Messmg der absoluten Beleuchtung von ausgedehnten Ultraviolettquellen verwendet werden kann. Es wird gezeigt, wie die Verwendung dieses Detektors werbunden mit der Kenntnis der relativen Spektral. aussaat einer Photochemotherapieeinheit eine genaue Bestimmungder auf die Haut eines Patienten einfallenden absoluten Spektralbeleuchtung ergeben kann.
REFERENCES CHALLONER,A. V. J., and DIFFEY,B. L., 1977, Br. J . Dermatol., 97, 643. LAKSHMIPATHI, T., GOULD,P. W., MACKENZIE,L. A., JOHNSON, B. E., and FRAINBELL,W., 1977, Br. J . Dermatol., 96, 587. SCHWARZ, E., 1952, Research, 5, 407. STAIR,R.,SCHNEIDER, W. E., and JACKSON, J . K., 1963, Applied Optics, 2, 1151.
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My IOPscience
Absolute radiation dosimetry in photochemotherapy
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1978 Phys. Med. Biol. 23 1124 (http://iopscience.iop.org/0031-9155/23/6/008) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 130.237.165.40 This content was downloaded on 02/10/2015 at 09:51
Please note that terms and conditions apply.
PHYS. MED. BIOL., 1978, Vol. 23, No. 6, 1124-1129.
Printed in Great Britain
Absolute Radiation Dosimetry in Photochemotherapy B. L. DIFFEY, P H . D . ~and A. V. J. CHALLONER,
PH.D.$
tDepartment of Medical Physics, Kent and Canterbury Hospital, Canterbury, Kent CT1 3NG, U.K. $Department of Photobiology, Institute of Dermatology, Homerton Grove, London E 9 6BX, U.K. Received 24 November 1977, in$nal f o r m 18 April 1978 ABSTRACT.Theconstruction and properties of adetector which can be used to measure absoluteirradiance from extended UV sources are described. It is shown how the use of this detector coupled with a knowledge of the relative spectral emission from a photochemotherapy unit can lead t o accurate estimation of absolute spectral irradiance incident upon a patient’s skin.
1. Introduction
Thetreatment of psoriasis by photochemotherapy (PUVA) is rapidly expanding.Therationale of thisform of treatment is the oral or topical administration of the photoactivedrug,8-methoxypsoralen, followed by exposure to high-intensity,long-wave,ultravioletradiation. I n order to achieve a satisfactory clearing of the psoriatic lesions, it is essential to have reliable control of the dose of ultraviolet radiation given to the patient. Some dermatologistsapproach the probleminapurelyempiricalfashionrelying solely on their observation of patient reactions to gauge appropriate treatment. I n thiswaythey employ theirpatients as biological monitors and use no physical or chemical systems to quantitate the radiation output of the PUVA machine. The majority of workers depend on a device supplied by the manufacturer tomonitor the output, butin general they are unawareof the relationship between what is being measured and the dose received by the patient. Finally, there is a group of dermatologists who seek support from physicists in thehope that more careful measurement might resultin improved treatment. A clinician would notadminister methoxsalen to a patientwithoutdue regard to the clinical state of the patient and the pharmacology of the drug. It is reasonable to suggest, therefore, that he should be cognisant of the uv dose received by the patient and the ultraviolet spectrum incident upon the patient, even though the relationship between the incident spectrum and the activewavelengths causing regression of psoriatic lesions is, a t present, uncertain. We believe that proper uv dosimetry is important,both for successful treatment at thepresent time, and thatin the future, should problems of, say, carcinogenesis become evident, there will exist adequate records of treatment so that retrospective studies of dose and effect can be readily correlated. I n a previous paper (Challoner and Diffey 1977) we discussed the problems of uv dosimetry in photochemotherapy but were unable to suggest a satisfactory 0031-9155/78,’0006-1124$01.00
@ 1978 The Institute of Physics
Absolute Radiation Dosimetry in P?wtochemotherapy
1125
solution to theproblem as at thetime there was no suitable detectoravailable. This lack of appropriate instrumentation was also mentioned by Lakshmipathi, Gould, MacKenzie, Johnson and Frain-Bell (1977). In thepresent communication we describe the construction and properties of a recently developed detector (FT32 Wide Angle Thermo-electric detector, Rank Hilger Ltd.) followed by a discussion on how i t may be usecl to give an absolute measure of irmdiance incident upon a patient.
2.
Construction of the detector
The principle of operation of the detector is based on the Seebeck or thermoelectric effect whereby an EMF is generated when heat is applied to thejunction of two dissimilar metals. Construction of practical devices of this nature was first described by Schwarz (1952). The present device consists of a platinum receiving element 2-155 0.02 mm in diameter, blackened for maximum absorption of radiation and mounted above a precious metal support assembly of great heatcapacity. A compensated element of similar climensions to the receiving element is mounted adjacent to the support assembly in order to overcome variations in ambient temperature which may cause clrift. I n order to increase sensitivity the components are encased in an evacuated glass envelope. The receiving element is situated within 0-5 mmof a SiO, winclow, 1 mm thick, which is attached to the glass envelope by means of a vacuum seal prior to evacuation.The use of SiO, as a window material ensures uniform transmission of radiation from 250 to 2200 nm, thus thedevice has a response which is effectively independent of wavelength within this waveband. The final low pressure is achieved using a barium getter. The evacuated envelope is mounted with the receiving element centrally positioned behind the aperture and enclosed within an aluminium housing with a 5-pin socket at therear for electrical connection to a voltmeter. a maximum angle of acceptance of 150' with a typical Thedetectorhas
Fig. 1. The complete assembly of the detector.
1126
B. L. Diffey and A . V . J . Challoner
sensitivity of 15.0 mVmW-l and a typical approximate response time better than 0.1 S for 63% full scale deflection. A photograph of the complete assembly is shown in fig. 1.
3. Angularresponse of the detector
The detector was mounted on a turntableon an optical bench and irradiated with a high intensity, point source mercury arc lamp which had an arc size of 0.25 mm x 0.25 mm. The output from this lamp was 'cept constant by using a stabilised power supply. The response of the detector was noted as it was rotated on its turntable about the normal a t 10" intervals up to 70". The resultsare shown in fig. 2, where it may be seen that the response of the
io
20
30
LO
50
60
70
Incident angle,8 Ideg)
Fig. 2. The angular response of the detector.
detector is within the experimental error of 2% of a cosine response a t angles up to 50". At angles greater than 50" the response falls more rapidly than that predicted for a cosine response due to increased reflection of radiation from the quartz window. However, it should be borne in mind that the detector will be used to measure the radiance a t a distance of, say, 20 cm from the midpoint of an extended source of 2 m in lengthwhich results in a subtended angle of 78". The intensity falling on the detector from angles at which there is a deviation from a cosine response (50" to 78") is 16% of the total intensity, and even a t these angles the detector still approximates closely to a cosine response. Consequently, it is estimated that the reading will be approximately 3% smaller than fora true cosine detector; an error which canreadilybe tolerated in this application.
Absolute Radiation Dosimetry in Photochemotherapy
1127
Measurement of absoluteirradiance If the detector is placed in front of an extended source, it will measure radiation with uniform response in the range 250-2200 nm by virtue of its wavelength andangular response. Lettheirradianceincidentuponthe detector be R mW cm-2. If the relativespectral emission of the source is wavelength of the known,such that S(h) is the relative intensity per unit emission spectrum a t wavelength h, thentheabsolutespectralirradiance, I(X),is given by 4.
I@)
=
R S ( h ) / fS(h)dX (mWcm-2nm-1)
(1)
where the integration is over all wavelengths present in the emission spectrum, not just in the UVA region. As an example of the way in which the present detector may be used to give absolute dosimetry, the following measurements were carried out.The detector was placed in a Waldmann PUVA 4000 Photochemotherapy Unit in a position approximating that of a patient’s skin. The unit hadbeen in clinical use for about 500 h.Thetotalirradiance measuredby the detector was 9.62 mW cm-2. A Beckman DBG Spectrophotometer with a l nm bandwidth was then used to determine the relativespectral emission from the lamps. The analogue signal from the spectrophotometer was digitised by means of a voltage-to-frequencyconverter andtheresultant pulses accumulatedina scalar. Every ten seconds the contents of the scalar were recorded on paper tape and the cycle repeated.Duringthe 10 S data integrationperiod, the wavelength change on the spectrophotometerwas 1.70 nm. It was necessary to correct the recordedspectrum for the variationwithwavelength of the sensitivity of the spectrophotometer.This was achievedbyrecording the spectrum of a 200 W quartz-iodine lamp of known spectral irradiance and deriving a correction function which could be applied to the PUVA spectrum. The lamp had been calibrated by the suppliers against a standard obtained from the National Bureau of Standards, Washington, using a method described by Stair, Schneider and Jackson (1963). Thephotomultipliertube employed intheBeckmann DBGSpectrophotometer was such that the efficiency of detection fell rapidly a t wavelengthsgreater thanabout 650 nm. Since the thermoelectricdetector will respond to wavelengths up to 2500 nm, it was necessary to make an estimate of the fraction of the measured irradiance in the near infrared region. This was achieved by using a Schott red glass filter (type RG63O), 3 mm thick and 50 mm in diameter. This filter transmits uniformly from 650 nm to beyond 2500 nm with a 50% internal transmittance a t 630 nm and negligible transmittance a t wavelengths below 600 nm. Measurements with this filter indicated was due to wavethat 27 f 3% of the irradiance measured by the detector lengths greater than 600 nm. This means that the value used for R in eqn (1) should in fact be 7-02 mWcm-2 (i.e. 9.62 x 0.73). Finally, the nature of the measurements necessitated modifying eqn (1) from its continuous form to the discrete form. The spectral irradiance in the wavelength interval h, - AX12 $0
B. L.Diffey and A . V . J . Challoner
1128
hi
+ Ah12 is now given as
I
I(&,Ah)= RX(i) C S ( i ) A h(mWcm-2nm-1) where
i=l
h, = the central wavelength in the ith timing period, AA = the wavelength increment during each data integration period (Ah = 1.70 nm), R = the irradiance incident upon the detector for wavelengths up to 600 nm ( R = 7.02 mW cm-2), S ( i )= the number of pulses collected during the ith timing period corrected for nonlinearity of spectrophotometer response, N = the number of timing periods ( X - 200). This procedure enables the spectral distribution of radiation at thepatient’s skin to be determined in absolute units as shown in fig. 3.
JLl l
300
I
350
Fig. 3. Absolute spectralirradiancefrom Unit.
L,
l I
LOO 450 500 Wavelength lnmi
a Waldman
11 550 PUVA
\
600
4000 Photochemotherapy
5. Discussion
Knowledge of I(X),the absolute spectral irradiance a t each wavelength in the emission spectrum, means that it is possible to compound I ( h ) with any chosen action spectrum for the regression of lesions in psoriasis and arrive a t a biologically effective dose. It follows that measurement of I(X) will enable retrospective calculations of the ‘biological dose’ to be carried out when more information is available on the action spectrum in photochemotherapy. At thepresent time it is customary to use wavelength dependent photometers, or ‘black-ray’ meters, as a means of quantifying the irradiance from photochemotherapyunit’s.These devices can be calibrated bytheQuantum Metrology Division at the NationalPhysicalLaboratory, where afiltered medium-pressuremercury arc source is used asthecalibrationspectrum.
Absolute Radiation Dosimetry
in Photochemotherapy
1129
This results in effectively monochromatic radiation a t 365 nm. However, the spectral sources used in practice for PUVA treatment are a continuum from about 320 t o 400 nm accompanied by several lines in the visible (see fig. 3). This means that the reading on the black-ray meter in mW cm-2 represents an effective irradiance a t 365 nm. Calculation of the true irradiance will demand knowledge of both the spectral emission of the lamp and the spectral response of the detector. It is also important that theaction spectrum forthe regression of psoriatic lesions falls to zero whilst the spectral response of the detector is still finite, Since this action spectrum is uncertain at the present time, it is suggested that the technique outlined in this paper, using a detector with a wavelength response which uniformly covers the whole emission spectrum, is to be preferred. At the present time there is nopressurefromdermatologists engaged in PUVA treatment to pursue the problem of basic uv dosimetry. This may be because they are unaware of the limitations of black-ray meters or because they do not regard more elaborate measurement as being necessary. We are not in a positionto sayhow beneficial a fundamental approach to uv dosimetry will be in the long term, but experience from radiotherapy has shown that carefulmeasurementleads to a betterunderstanding of the biological mechanisms a t work and ultimately t o improved treatment for the patient. Acknowledgements are due to M r . J. R. Corfield for his critical evaluation of the manuscript and Mr. D. S. Day for his helpful interest in the development of the wide-angle detector. We are also grateful to Mi. C. P. Wells for assisting with the instrumentation.
RBsunlE DosimBtrie de radiation absolue en photochimioth6rapie Description de la construction et des propri6tAs d’un d6tecteurqui peut servir a mesurer l’irradiance absolue de sources d’ultraviolets non ponctuelles. Les auteurs montrent que l’usage de ce dbtecteur, joint21 une connaissance de 1’6missionspectrale relative d’une unit6de photochimiothbrapie, peut mener a une Bvaluation pr6cise de l’irradiance spectrale absolue qui frappe la peau d’un malade.
ZUSAMMEXFASSCRG Dosimetrie der absoluten Bestrahlung in der Photochemotherapie Konstruktion und Eigenschaften eines Detektors werden beschrieben, der zur Messmg der absoluten Beleuchtung von ausgedehnten Ultraviolettquellen verwendet werden kann. Es wird gezeigt, wie die Verwendung dieses Detektors werbunden mit der Kenntnis der relativen Spektral. aussaat einer Photochemotherapieeinheit eine genaue Bestimmungder auf die Haut eines Patienten einfallenden absoluten Spektralbeleuchtung ergeben kann.
REFERENCES CHALLONER,A. V. J., and DIFFEY,B. L., 1977, Br. J . Dermatol., 97, 643. LAKSHMIPATHI, T., GOULD,P. W., MACKENZIE,L. A., JOHNSON, B. E., and FRAINBELL,W., 1977, Br. J . Dermatol., 96, 587. SCHWARZ, E., 1952, Research, 5, 407. STAIR,R.,SCHNEIDER, W. E., and JACKSON, J . K., 1963, Applied Optics, 2, 1151.
