Guidelines on Measurement of UV Levels in UV Phototherapy
Most UV phototherapy treatments are delivered in hospital or clinic settings. As many patients have large proportions of their skin surface needing treatment, whole-body phototherapy cabins where the patient is surrounded on all sides by banks of UV lamps are the equipment of choice for most treatments. The last few decades have seen an increase in the use of NB-UVB phototherapy, with an accompanying decline in PUVA and BB-UVB treatments. The use of whole-body UVA1 phototherapy has also become more common recently.
For areas with more remote populations without easy access to phototherapy centres, the use of home phototherapy units has proved popular and cost-effective in enabling self-administration of treatment. Whole-body home phototherapy units are more usually an open single bank of lamps rather than an enclosed whole-body cabin. Any one treatment session therefore consists of four separate exposures: to the front and back, and to the left and right sides.
Partial-body irradiation equipment is also widely used for both self-administered and clinic-based phototherapy. A small bank of lamps in either a flat or curved array is suitable for treating hands, feet or lower limbs. Smaller, hand-held devices are often used for less accessible treatment sites such as the scalp. All such units should be operated in areas where access can be controlled to avoid unnecessary exposure to the beam. In the case of the patient, untreated skin and eyes should be protected as necessary by means of clothes, drapes, goggles or face shields.
Equipment management issues associated with whole-body treatment cabins are considered in the following subsections. Whole-body cabins are typically more sophisticated in operation than partial-body irradiation units, with many having inbuilt automated dosimetry systems. They also present a greater potential hazard because of the higher UV irradiances they generate and the fact that a larger area of the patient's skin is typically irradiated. However, many of the UV dosimetry principles discussed will apply equally to the smaller partial-body units.
Different models of whole-body phototherapy cabins contain differing numbers of lamps – 24, 26, 40 or 48 lamps being the more common. The UV irradiance comprises UVR emitted directly from the lamps and UVR reflected from polished surfaces to the side and rear of the lamps. The angles of these reflectors have a significant influence on the overall cabin efficiency. The reflectivity coefficients of different materials used for the reflectors by the various cabin manufacturers can also vary significantly. Increasing the number of lamps within a cabin beyond a certain point does not necessarily increase the irradiance proportionately. A recent study compared the outputs from two sets of cabins, with similar dimensions, differing only in the numbers of lamps, from the same manufacturer. Cabins with 24 lamps gave irradiances that were only 11% less than those with 40 lamps. It was concluded that the smaller reflector angle in the 40-lamp cabins reduced the useful output per lamp by a third. The uniformity of illumination, which is the most important factor for treatment delivery, was found to be similar in the cabins with 24 and 40 lamps. Cabins that only employed a simple, flat reflector behind the lamps had lower efficiencies. Irradiances in the range of 6–8 mW cm are generally required for NB-UVB therapy and in the range of 10–14 mW cm for UVA therapy. Therefore, cabins with > 24–30 lamps may offer little advantage in terms of treatment times.
To allow treatment with either spectrum, some cabins have a combination of UVA and UVB fluorescent lamps that are operated using separate controllers. These cabins hold either 16 UVB and 32 UVA lamps or 13 UVB and 27 UVA lamps. Although these cabins save space, the overall cabin irradiance of each modality is proportionally reduced and, consequently, treatment times are increased. There is also the risk of selecting the wrong treatment mode, although internal dosimetry systems, if these are used, will typically include exposure limits that prevent the longer UVA exposure times being given by the UVB lamps. Therefore, this type of cabin is not generally recommended. If it is necessary to have a dual cabin because of space limitations, extreme care must be exercised when entering the lamp type and treatment dose.
The number of lamps in a cabin may affect installation and running costs. Cabins with fewer lamps require less complex electrical supply arrangements, whereas those with 40 or more fluorescent lamps typically require a three-phase electricity supply that may not be readily available on some sites. They also produce more heat and therefore efficient air conditioning systems are required to maintain patient comfort.
Exposure control in whole-body phototherapy cabins can be either time-mode or dose-mode. Some cabin designs can be operated in only one of these modes, while others offer a choice of either mode of operation. Depending on user preferences, the choice of control modes may be an important consideration when acquiring a new treatment cabin.
Time-mode may be the only control method available on older cabins. The user sets an exposure time corresponding to the prescribed treatment dose, and based on prior irradiance measurements. The cabin's inbuilt electronic timer then controls the exposure; there is no automatic allowance for differences in patient size or variations in the cabin irradiance. To maintain accuracy, a programme of regular irradiance calibration tests is necessary, typically after 50 h of use and repeated at least every 4 months.
Most currently available cabins are capable of dose-mode control. The cabin is fitted with internal detectors that measure the internal irradiance in real time during treatment. The operator sets the required dose on the controls and starts the treatment. The control system electronically integrates the continuous irradiance reading, and the exposure is automatically terminated when the set dose is reached. Dose-mode operation can also compensate automatically for fluctuations in irradiance arising both during individual treatments and over a full clinic session. More consistent doses may then result.
The effectiveness and accuracy of inbuilt sensor systems is dependent on detector position and cabin geometry. Some early designs have been prone to give misleading readings (Moseley, personal communication). Especially problematic are types reliant on monitoring a small number of lamps. More recent types compensate reasonably well for differences in the amount of shielding of the fluorescent tubes by patients of differing sizes. However, it should be recognized that internal dosimeters monitor the UV that is reflected from a relatively small area of skin and do not measure the average irradiance to the whole patient.
Internal detectors may also be sensitive to the patient's relative position within the cabin. If a patient moves off centre, the detected cabin irradiance level will alter as the different banks of lamps within the cabin will contribute more or less to the total irradiance. This may cause some variation in the patient's actual received dose, leading to either under- or overdosing.
It has been shown that cabins fitted with a pair of detectors are less susceptible to this type of dose error than cabins with single detectors.
When inbuilt, dose-mode sensors are fitted, users should not assume that the dose displayed on the cabin's control panel is correct. A programme of regular calibration checking of any inbuilt metering system should be in place to ensure accuracy and to guard against malfunctions. To avoid confusion, this should be done even if the cabin is usually operated in time mode.
Should a patient fall against unprotected lamps inside a phototherapy cabin, there is a high risk of laceration. Many older phototherapy cabins either had no protection at all against this or had relatively open metal grilles. Now, full acrylic guards over the lamps are generally fitted as standard. Users of phototherapy cabins without guards in place were required to consider retrofitting them following the publication of the medical device alert (MDA/2003/006) issued by the Medical Devices Agency (now the Medicines and Healthcare products Regulatory Agency) in 2003 and the associated Scottish Safety Action Notice in 2003 [(SAN(SC)03/14].
Improved ventilation within cabins has also enhanced safety by increasing patient comfort and making it less likely that they will become faint and stumble.
Through better-fitting doors and UV-opaque viewing windows, newer cabin designs generally have lower UV leakage. Moreover, most cabin doors are now interlocked so that the exposure will stop immediately if a patient pushes against the door. Interlocked patient-actuated pull cords fitted in some cabin designs have a similar safety function. It may be acceptable to continue to use older cabins without such safety features provided an assessment is made of their safety in the light of current regulatory requirements and best practice guidance.
Regular cleaning of cabins is imperative for infection control. Accumulated skin flakes and dust on lamps can also degrade the cabin output and internal dosimetry systems. Thorough cleaning of cabins – screens taken out and cleaned, reflectors and lamps wiped, and accumulated dust removed – can increase the output of cabins by up to 20% (Amatiello, personal communication).
Although concerns have been raised about the safety of patients with artificial implanted devices, a recent investigation in two phototherapy cubicles demonstrated that the cabinets were safe for patients fitted with electrical implanted devices, such as pacemakers.
The absolute output declines as lamps age. For Philips type TL-01 100 W fluorescent tubes, this decline is rapid over the first 200 operating hours, dropping to 60–70% of the initial intensity, before maintaining a relatively constant output until lamp failure. There is a large variation in operating life depending upon local circumstances: in one study, mean ± SD lamp lifetime was observed to be 470 ± 170 h.
When lamps fail, 'cold spots', or areas of lower localized irradiance, are formed within the overall irradiance distribution, thereby underdosing an area of the patient. New tubes have higher irradiances and so create 'hot spots' or areas of higher localized irradiance. For cabins of the size supplied by most manufacturers, single-lamp failures give cold spots with 7–12% lower irradiances, and replacement with a new lamp gives hot spots of 3–6%. If failed lamps are replaced promptly, localized patient erythema is unlikely. However, in cabins with fewer lamps, where each lamp contributes more to the overall irradiance, and in smaller cabins where the contribution to irradiance from individual lamps is more localized, irradiance may be some 30% lower in cold spots from single-lamp failures. This effect is particularly important in dual UVA/NB-UVB cabins as these have fewer lamps of each type, meaning the impact of a failed lamp is greater. An added complication is that failed lamps are more difficult to identify among lamps of the other type that are not illuminated. A robust system to identify and replace failed lamps is therefore required.
Replacement of lamps should be carried out in accordance with an agreed policy that is known and understood by the end-users. One option is to replace all lamps when treatment times become unacceptably long; an alternative strategy is to replace those lamps showing a low output so that irradiance in the cabin is kept constant, for example within 10–20% of a desired figure.
To avoid accidental treatment with the wrong UV spectrum, it is critical that the correct fluorescent tubes are fitted in the cabin. Some suppliers label NB-UVB tubes with blue and red stickers for easy identification but this helpful practice is not a requirement. This means that there remains a risk of an unlabelled NB-UVB tube being fitted in to a UVA cabin, or vice versa, with potentially serious clinical consequences. Recommendations concerning identification have been made in the 2012 Estates and Facilities Alert (EFA/2012/002).
Consider uniformity of dose distribution, treatment times, control mode options and installation implications when selecting whole-body cabins. Cabins fitted with tubes providing identical spectral output are recommended over cabins that can be switched to operate two (or more) different spectral outputs. The use of cabins with dosimetry systems providing a biologically weighted dose are not recommended. Regular measurements using a calibrated UV radiometer should be made in order to assess the irradiance to which patients are exposed by phototherapy equipment and to check the accuracy of any dosimetry systems that are incorporated within the equipment. An infection control and hygiene policy should be in place to ensure adequate cleaning of equipment and other surfaces in phototherapy areas. A lamp replacement policy should be in place to ensure that failed or low-output lamps are replaced with lamps of the correct type, and that localized areas of low or high irradiance are avoided.
Phototherapy Equipment
Most UV phototherapy treatments are delivered in hospital or clinic settings. As many patients have large proportions of their skin surface needing treatment, whole-body phototherapy cabins where the patient is surrounded on all sides by banks of UV lamps are the equipment of choice for most treatments. The last few decades have seen an increase in the use of NB-UVB phototherapy, with an accompanying decline in PUVA and BB-UVB treatments. The use of whole-body UVA1 phototherapy has also become more common recently.
For areas with more remote populations without easy access to phototherapy centres, the use of home phototherapy units has proved popular and cost-effective in enabling self-administration of treatment. Whole-body home phototherapy units are more usually an open single bank of lamps rather than an enclosed whole-body cabin. Any one treatment session therefore consists of four separate exposures: to the front and back, and to the left and right sides.
Partial-body irradiation equipment is also widely used for both self-administered and clinic-based phototherapy. A small bank of lamps in either a flat or curved array is suitable for treating hands, feet or lower limbs. Smaller, hand-held devices are often used for less accessible treatment sites such as the scalp. All such units should be operated in areas where access can be controlled to avoid unnecessary exposure to the beam. In the case of the patient, untreated skin and eyes should be protected as necessary by means of clothes, drapes, goggles or face shields.
Equipment management issues associated with whole-body treatment cabins are considered in the following subsections. Whole-body cabins are typically more sophisticated in operation than partial-body irradiation units, with many having inbuilt automated dosimetry systems. They also present a greater potential hazard because of the higher UV irradiances they generate and the fact that a larger area of the patient's skin is typically irradiated. However, many of the UV dosimetry principles discussed will apply equally to the smaller partial-body units.
Numbers of Fluorescent Lamps
Different models of whole-body phototherapy cabins contain differing numbers of lamps – 24, 26, 40 or 48 lamps being the more common. The UV irradiance comprises UVR emitted directly from the lamps and UVR reflected from polished surfaces to the side and rear of the lamps. The angles of these reflectors have a significant influence on the overall cabin efficiency. The reflectivity coefficients of different materials used for the reflectors by the various cabin manufacturers can also vary significantly. Increasing the number of lamps within a cabin beyond a certain point does not necessarily increase the irradiance proportionately. A recent study compared the outputs from two sets of cabins, with similar dimensions, differing only in the numbers of lamps, from the same manufacturer. Cabins with 24 lamps gave irradiances that were only 11% less than those with 40 lamps. It was concluded that the smaller reflector angle in the 40-lamp cabins reduced the useful output per lamp by a third. The uniformity of illumination, which is the most important factor for treatment delivery, was found to be similar in the cabins with 24 and 40 lamps. Cabins that only employed a simple, flat reflector behind the lamps had lower efficiencies. Irradiances in the range of 6–8 mW cm are generally required for NB-UVB therapy and in the range of 10–14 mW cm for UVA therapy. Therefore, cabins with > 24–30 lamps may offer little advantage in terms of treatment times.
To allow treatment with either spectrum, some cabins have a combination of UVA and UVB fluorescent lamps that are operated using separate controllers. These cabins hold either 16 UVB and 32 UVA lamps or 13 UVB and 27 UVA lamps. Although these cabins save space, the overall cabin irradiance of each modality is proportionally reduced and, consequently, treatment times are increased. There is also the risk of selecting the wrong treatment mode, although internal dosimetry systems, if these are used, will typically include exposure limits that prevent the longer UVA exposure times being given by the UVB lamps. Therefore, this type of cabin is not generally recommended. If it is necessary to have a dual cabin because of space limitations, extreme care must be exercised when entering the lamp type and treatment dose.
The number of lamps in a cabin may affect installation and running costs. Cabins with fewer lamps require less complex electrical supply arrangements, whereas those with 40 or more fluorescent lamps typically require a three-phase electricity supply that may not be readily available on some sites. They also produce more heat and therefore efficient air conditioning systems are required to maintain patient comfort.
Exposure Control Systems
Exposure control in whole-body phototherapy cabins can be either time-mode or dose-mode. Some cabin designs can be operated in only one of these modes, while others offer a choice of either mode of operation. Depending on user preferences, the choice of control modes may be an important consideration when acquiring a new treatment cabin.
Time-mode may be the only control method available on older cabins. The user sets an exposure time corresponding to the prescribed treatment dose, and based on prior irradiance measurements. The cabin's inbuilt electronic timer then controls the exposure; there is no automatic allowance for differences in patient size or variations in the cabin irradiance. To maintain accuracy, a programme of regular irradiance calibration tests is necessary, typically after 50 h of use and repeated at least every 4 months.
Most currently available cabins are capable of dose-mode control. The cabin is fitted with internal detectors that measure the internal irradiance in real time during treatment. The operator sets the required dose on the controls and starts the treatment. The control system electronically integrates the continuous irradiance reading, and the exposure is automatically terminated when the set dose is reached. Dose-mode operation can also compensate automatically for fluctuations in irradiance arising both during individual treatments and over a full clinic session. More consistent doses may then result.
The effectiveness and accuracy of inbuilt sensor systems is dependent on detector position and cabin geometry. Some early designs have been prone to give misleading readings (Moseley, personal communication). Especially problematic are types reliant on monitoring a small number of lamps. More recent types compensate reasonably well for differences in the amount of shielding of the fluorescent tubes by patients of differing sizes. However, it should be recognized that internal dosimeters monitor the UV that is reflected from a relatively small area of skin and do not measure the average irradiance to the whole patient.
Internal detectors may also be sensitive to the patient's relative position within the cabin. If a patient moves off centre, the detected cabin irradiance level will alter as the different banks of lamps within the cabin will contribute more or less to the total irradiance. This may cause some variation in the patient's actual received dose, leading to either under- or overdosing.
It has been shown that cabins fitted with a pair of detectors are less susceptible to this type of dose error than cabins with single detectors.
When inbuilt, dose-mode sensors are fitted, users should not assume that the dose displayed on the cabin's control panel is correct. A programme of regular calibration checking of any inbuilt metering system should be in place to ensure accuracy and to guard against malfunctions. To avoid confusion, this should be done even if the cabin is usually operated in time mode.
Safety Features
Should a patient fall against unprotected lamps inside a phototherapy cabin, there is a high risk of laceration. Many older phototherapy cabins either had no protection at all against this or had relatively open metal grilles. Now, full acrylic guards over the lamps are generally fitted as standard. Users of phototherapy cabins without guards in place were required to consider retrofitting them following the publication of the medical device alert (MDA/2003/006) issued by the Medical Devices Agency (now the Medicines and Healthcare products Regulatory Agency) in 2003 and the associated Scottish Safety Action Notice in 2003 [(SAN(SC)03/14].
Improved ventilation within cabins has also enhanced safety by increasing patient comfort and making it less likely that they will become faint and stumble.
Through better-fitting doors and UV-opaque viewing windows, newer cabin designs generally have lower UV leakage. Moreover, most cabin doors are now interlocked so that the exposure will stop immediately if a patient pushes against the door. Interlocked patient-actuated pull cords fitted in some cabin designs have a similar safety function. It may be acceptable to continue to use older cabins without such safety features provided an assessment is made of their safety in the light of current regulatory requirements and best practice guidance.
Regular cleaning of cabins is imperative for infection control. Accumulated skin flakes and dust on lamps can also degrade the cabin output and internal dosimetry systems. Thorough cleaning of cabins – screens taken out and cleaned, reflectors and lamps wiped, and accumulated dust removed – can increase the output of cabins by up to 20% (Amatiello, personal communication).
Although concerns have been raised about the safety of patients with artificial implanted devices, a recent investigation in two phototherapy cubicles demonstrated that the cabinets were safe for patients fitted with electrical implanted devices, such as pacemakers.
Fluorescent Tube Replacement
The absolute output declines as lamps age. For Philips type TL-01 100 W fluorescent tubes, this decline is rapid over the first 200 operating hours, dropping to 60–70% of the initial intensity, before maintaining a relatively constant output until lamp failure. There is a large variation in operating life depending upon local circumstances: in one study, mean ± SD lamp lifetime was observed to be 470 ± 170 h.
When lamps fail, 'cold spots', or areas of lower localized irradiance, are formed within the overall irradiance distribution, thereby underdosing an area of the patient. New tubes have higher irradiances and so create 'hot spots' or areas of higher localized irradiance. For cabins of the size supplied by most manufacturers, single-lamp failures give cold spots with 7–12% lower irradiances, and replacement with a new lamp gives hot spots of 3–6%. If failed lamps are replaced promptly, localized patient erythema is unlikely. However, in cabins with fewer lamps, where each lamp contributes more to the overall irradiance, and in smaller cabins where the contribution to irradiance from individual lamps is more localized, irradiance may be some 30% lower in cold spots from single-lamp failures. This effect is particularly important in dual UVA/NB-UVB cabins as these have fewer lamps of each type, meaning the impact of a failed lamp is greater. An added complication is that failed lamps are more difficult to identify among lamps of the other type that are not illuminated. A robust system to identify and replace failed lamps is therefore required.
Replacement of lamps should be carried out in accordance with an agreed policy that is known and understood by the end-users. One option is to replace all lamps when treatment times become unacceptably long; an alternative strategy is to replace those lamps showing a low output so that irradiance in the cabin is kept constant, for example within 10–20% of a desired figure.
To avoid accidental treatment with the wrong UV spectrum, it is critical that the correct fluorescent tubes are fitted in the cabin. Some suppliers label NB-UVB tubes with blue and red stickers for easy identification but this helpful practice is not a requirement. This means that there remains a risk of an unlabelled NB-UVB tube being fitted in to a UVA cabin, or vice versa, with potentially serious clinical consequences. Recommendations concerning identification have been made in the 2012 Estates and Facilities Alert (EFA/2012/002).
Phototherapy Equipment Recommendations (Strength of Recommendation D; Level of Evidence 4)
Consider uniformity of dose distribution, treatment times, control mode options and installation implications when selecting whole-body cabins. Cabins fitted with tubes providing identical spectral output are recommended over cabins that can be switched to operate two (or more) different spectral outputs. The use of cabins with dosimetry systems providing a biologically weighted dose are not recommended. Regular measurements using a calibrated UV radiometer should be made in order to assess the irradiance to which patients are exposed by phototherapy equipment and to check the accuracy of any dosimetry systems that are incorporated within the equipment. An infection control and hygiene policy should be in place to ensure adequate cleaning of equipment and other surfaces in phototherapy areas. A lamp replacement policy should be in place to ensure that failed or low-output lamps are replaced with lamps of the correct type, and that localized areas of low or high irradiance are avoided.
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