UK 2022 Consensus on Normal Tissue Dose-Volume Constraints for Oligometastatic, Primary Lung and Hepatocellular Carcinoma Stereotactic Ablative Radiotherapy.

The use of stereotactic ablative radiotherapy (SABR) in the UK has expanded over the past decade, in part as the result of several UK clinical trials and a recent NHS England Commissioning through Evaluation programme. A UK SABR Consortium consensus for normal tissue constraints for SABR was published in 2017, based on the existing literature at the time. The published literature regarding SABR has increased in volume over the past 5 years and multiple UK centres are currently working to develop new SABR services. A review and update of the previous consensus is therefore appropriate and timely. It is hoped that this document will provide a useful resource to facilitate safe and consistent SABR practice.

UK Consensus on Normal Tissue Dose Constraints for Stereotactic Radiotherapy.

Six UK studies investigating stereotactic ablative radiotherapy (SABR) are currently open. Many of these involve the treatment of oligometastatic disease at different locations in the body. Members of all the trial management groups collaborated to generate a consensus document on appropriate organ at risk dose constraints. Values from existing but older reviews were updated using data from current studies. It is hoped that this unified approach will facilitate standardised implementation of SABR across the UK and will allow meaningful toxicity comparisons between SABR studies and internationally.

Electronic brachytherapy--current status and future directions.

In the past decade, electronic brachytherapy (EB) has emerged as an attractive modality for the treatment of skin lesions and intraoperative partial breast irradiation, as well as finding wider applications in intracavitary and interstitial sites. These miniature X-ray sources, which operate at low kilovoltage energies (<100 kV), have reduced shielding requirements and inherent portability, therefore can be used outside the traditional realms of the radiotherapy department. However, steep dose gradients and increased sensitivity to inhomogeneities challenge accurate dosimetry. Secondly, ease of use does not mitigate the need for close involvement by medical physics experts and consultant oncologists. Finally, further studies are needed to relate the more heterogeneous dose distributions to clinical outcomes. With these provisos, the practical convenience of EB strongly suggests that it will become an established option for selected patients, not only in radiotherapy departments but also in a range of operating theatres and clinics around the world.

A national dosimetry audit of intraoperative radiotherapy.

OBJECTIVE: National dosimetry audits are a fundamental part of quality assurance in radiotherapy, especially for new techniques. Intraoperative radiotherapy with a compact mobile kilovoltage X-ray source is a novel approach for the treatment of breast and other cancers. All seven current clinical sites in the UK were audited by a single visiting group and set of measurement equipment. METHODS: Measurements of output, isotropy and depth doses were performed using an ion chamber in solid water, thermoluminescent dosemeters and radiochromic film, respectively. RESULTS: The mean difference between measured and planned dose across all centres was -3.2±2.7%. Measured isotropy was within ±3% around the lateral plane of the X-ray source and +11±4% in the forward direction compared with the lateral plane. Measured depth doses were agreed within 5±2% of manufacturer-provided calibration values or a mean gamma index of 97% at a tolerance of 7%/0.5 mm. CONCLUSION: Agreement within measurement uncertainties was found for all three parameters except forward anisotropy, which is unlikely to be clinically significant. Steep dose gradients increase the sensitivity to small variations in positioning, but these tests are practical for use in interdepartmental audits and local baseline comparison. ADVANCES IN KNOWLEDGE: The first UK interdepartmental audit of intraoperative radiotherapy builds confidence in the delivery of this treatment.

Quality assurance and independent dosimetry for an intraoperative x-ray device.

PURPOSE: Quality assurance is an essential component of accurate and safe radiotherapy delivery, and should include measurements which are independent of manufacturer-provided calibration. However, the physical and dosimetric properties of the INTRABEAM compact mobile 50 kV x-ray source are different from conventional kilovoltage therapy units and few reports describe methods for independent checks, frequencies, or tolerances for quality assurance tests. METHODS: Based on the available evidence and local experience, methods are described for determination of the key dosimetric parameters: beam quality, output, isotropy, and depth doses. Internal system checks are also described, along with measurements of long-term stability. RESULTS: A small volume parallel plate ionization chamber in a liquid water tank is the gold standard for measurements with this unit, but solid water-equivalent materials, thermoluminescent dosimeters and radiochromic film can all be used as practical alternatives with an accuracy of 5%-10%. The main cause of measurement uncertainty is positioning of the detector in the steep dose gradient, but energy dependence should also be considered. CONCLUSIONS: A quality assurance schedule with suggested tolerances is proposed, which includes both internal tests, before each treatment and on a monthly basis, and independent tests every year or after servicing or recalibration.

Radiation protection for an intra-operative X-ray device.

OBJECTIVES: Therapeutic partial breast irradiation can be delivered intra-operatively using the Intrabeam 50 kVp compact X-ray device. Spherical applicators are added to the source to give an isotropic radiation dose. The low energy of this unit leads to rapid attenuation with distance, but dose rates are much greater than for diagnostic procedures. METHODS: To investigate the shielding requirements for this unit, attenuation measurements were carried out with manufacturer-provided tungsten-rubber sheets, lead, plasterboard and bricks. A prospective environmental dose rate survey was also conducted in the designated theatre. RESULTS: As a result of isotropic geometry, the scattered dose around shielding can be 1% of primary and thus often dominates measured dose rates compared with transmission. The absorbed dose rate of the unshielded source at 1 m was 11.6 mGy h(-1) but this was reduced by 95% with the shielding sheets. Measured values for the common shielding materials were similar to reference data for the attenuation of a 50 kVp diagnostic X-ray beam. Two lead screens were constructed to shield operators remaining in the theatre and an air vent into a service corridor. A lead apron would also provide suitable attenuation, although a screen allows greater flexibility for treatment operators. With these measures, staff doses were reduced to negligible quantities. Survey measurements taken during patient treatments confirmed no additional measures were required, but the theatre should be a controlled area and access restricted. CONCLUSION: Results from this study and reference data can be used for planning other facilities.

Dosimetry measurements with an intra-operative x-ray device.

The INTRABEAM 50 kV x-ray device can be used for intra-operative partial breast irradiation. Spherical applicators are added to the source probe to deliver a radially symmetric radiation dose. Dosimetric data for calculation of absorbed dose were measured for this unit and a superficial unit with a similar beam quality, as defined by half value layer (HVL). Chamber calibration factors, N(K), and chamber correction factors, k(ch), were determined based on HVL, according to the IPEMB code of practice and addendum. Depth doses were also measured using an ionization chamber and GafChromic EBT film. HVL was measured as 0.85-1.30 mm Al across the range of applicator sizes. Values for N(K) and k(ch) were found to be similar for the two units and all INTRABEAM applicator sizes. Therefore, calibration of ionization chambers, radiochromic film and other relative dosimeters could be performed on the superficial unit. This has the advantage of higher dose rates and lower dependence on small variations in detector positioning. Depth dose measurements performed using film also agreed with chamber values, published and manufacturer data, giving a simple and robust method for commissioning and regular quality assurance.

Equivalent squares for small field dosimetry.

The use of equivalent squares is of value when determining output and depth dose data for rectangular fields. We have looked at the variation with field shape of head scatter factors (S(c)), phantom scatter factors (S(p)) and tissue phantom ratios (TPRs) using measurements on a 6 MV linac with a Moduleaf mini-multileaf collimator. Measurements were made for fields with dimensions down to 1 cm. A different approach to calculating equivalent squares needs to be made depending on the quantity of interest. For TPRs, good agreement for rectangular fields can be obtained using the well established E = 2XY/(X+Y) formula where E is the equivalent square field size and X and Y are the field dimensions. For S(c) measurements, where a collimator exchange effect is observed, better agreement is obtained using E = (1+A)XY/(AX+Y), where A is an empirically determined constant. For S(p) measurements, E = 2XY/(X+Y) only gives agreement with measurements when the minimum field dimension is at least 2.5 cm. For smaller fields, the equivalent square overestimates S(p), with the difference being strongly related to the value of the smaller dimension. We propose an empirical formula, based on the size of the smaller dimension.

Exercise during decompression reduces the amount of venous gas emboli.

To determine the effects of moderate, intermittent exercise during decompression on the Doppler detectable amount of venous gas emboli (VGE), 29 healthy male volunteers performed 44 wet (8 degrees +/- 2 degrees C) dives to 45 msw (450 kPa) for 30 min with standard air decompression. During compression and the bottom period, all subjects were inactive; during decompression, 28 remained inactive, 11 performed leg exercise, and 5 did arm exercise. Intermittent exercise was controlled at approximately 50% of each subject's arm or leg aerobic capacity. At 30-min intervals after surfacing, subjects were monitored with a Doppler ultrasonic bubble detector. The Doppler scores were used to calculate the Kisman Integrated Severity Score (KISS). The KISS were log transformed (with zeroes being equivalent to log 0.01) and analyzed with a one-way analysis of variance. No significant differences (P < or = .05) between mean KISS scores after arm or leg exercise were observed, thus these data were pooled and compared to those of the inactive controls. The mean pooled KISS after exercising during decompression were significantly lower than those of the inactive controls. Moderate, intermittent exercise during decompression apparently reduces the amount of Doppler-detectable VGE after diving. The incidence rate of decompression sickness in both groups was not significantly different (P < 0.05).

On-orbit performance of the Hubble Space Telescope fine guidance sensors.

The observed and measured on-orbit performance of various aspects of the fine guidance sensors is presented and discussed in the light of the original requirements and predictions. The fine guidance sensors are shown to meet or exceed the original requirements concerning dynamic pointing errors, photometric repeatability, and moving-target tracking capability. Calibration accuracy has been sufficient for observations to date, and fine-lock acquisitions are approaching a 100% success rate. Improvements to the fine-guidance-sensor tolerance of telescope spherical aberration, the South Atlantic anomaly, and solar-panel vibrations have been made, and further improvements are expected.