Optimal clinical implementation of the Siemens virtual wedge.

Installation of a modern high-energy Siemens Primus linear accelerator at the Northern Centre for Cancer Treatment (NCCT) provided the opportunity to investigate the optimal clinical implementation of the Siemens virtual wedge filter. Previously published work has concentrated on the production of virtual wedge angles at 15 degrees, 30 degrees, 45 degrees, and 60 degrees as replacements for the Siemens hard wedges of the same nominal angles. However, treatment plan optimization of the dose distribution can be achieved with the Primus, as its control software permits the selection of any virtual wedge angle from 15 degrees to 60 degrees in increments of 1 degrees. The same result can also be produced from a combination of open and 60 degrees wedged fields. Helax-TMS models both of these modes of virtual wedge delivery by the wedge angle and the wedge fraction methods respectively. This paper describes results of timing studies in the planning of optimized patient dose distributions by both methods and in the subsequent treatment delivery procedures. Employment of the wedge fraction method results in the delivery of small numbers of monitor units to the beam's central axis; therefore, wedge profile stability and delivered dose with low numbers of monitor units were also investigated. The wedge fraction was proven to be the most efficient method when the time taken for both planning and treatment delivery were taken into consideration, and is now used exclusively for virtual wedge treatment delivery in Newcastle. It has also been shown that there are no unfavorable dosimetric consequences from its practical implementation.

Validation of treatment planning system data: variation in the modelling of three 6 MV beams for extreme rectangular fields.

The validation of photon beam data on the Helax-TMS treatment planning system at the Northern Centre for Cancer Treatment takes the form of profile and depth dose comparisons against measured data for square fields. With a view to using Helax-TMS to plan fixed source-to-surface distance spine fields, validation of extreme rectangular fields was undertaken, based on guidelines published by the American Association of Physicists in Medicine Radiation Therapy Committee (Task Group report 53). The accuracy with which the Helax-TMS-generated profiles matched those measured varied between treatment machines. For three 6 MV beams in the department, two beams showed agreement to within 2% in the central portion of the field, while the third beam demonstrated differences of up to 4.4% in this region. The disparities were attributed to the modelling of the primary dose for each beam by Helax-TMS.

The use of high density metal foils to increase surface dose in low-energy clinical electron beams.

BACKGROUND AND PURPOSE: This paper describes a practical method of elevating the surface dose of clinical electron beams in the energy range 3-12 MeV using thin high density metal foils (tin and lead) as an alternative to tissue equivalent bolus. Because, relative to water, these materials exhibit a high scattering power to stopping power ratio, the desired dose elevation may be achieved with less energy loss than conventional bolus and consequently a gain in therapeutic interval. METHODS: The foil thickness required to raise the surface dose to 90% off peak, for a given electron energy, was calculated using published scattering and stopping power data. An empirical expression is derived to facilitate calculation of foil thickness (tin or lead) to produce a given surface dose. RESULTS AND CONCLUSIONS: Measurements were made to confirm the predictions of the derived expression and were found to be in good agreement.

Tumour dose estimation using automated TLD techniques.

Lithium fluoride (TLD-700) dosimeters were used to measure exit surface absorbed doses in external beam radiotherapy using an automated TLD reader. Delivered tumour absorbed doses were derived from these measurements for head and neck, pelvis and breast treatments. For the head and neck treatments (first fraction only), the mean percentage difference between prescribed and delivered tumour absorbed doses was -0.15 +/- 3.0% (+/- 1 SD), for the pelvic treatments -0.83 +/- 2.8% and for the breast treatments +0.26 +/- 2.9%. The spread of results is approximately +/- 3% (+/- 1 SD). This is comparable with the estimated uncertainty in a single TLD absorbed dose measurement in phantom (+/- 2%; +/- 1 SD). Thus, ICRU recommended tolerances for absorbed dose delivery of +/- 5% may not be unequivocally detectable using this method. An action level of +/- 10% is suggested, allowing investigation of possible gross errors in treatment delivery at an early stage, before the course of treatment has progressed to a point at which absorbed dose compensation is impossible.

Identification of nociceptin in human cerebrospinal fluid: comparison of levels in pain and non-pain states.

We have measured plasma and cerebrospinal fluid (CSF) concentrations of nociceptin, the endogenous agonist of the orphan opioid receptor-like receptor (ORL-1). We studied two groups of ten patients presenting for elective Caesarean section (Group E) or in established labour and requiring combined spinal epidural anaesthesia for pain relief (Group L). Nociceptin was identified in all CSF samples with mean +/- SD concentrations of 52.49 +/- 34.25 and 63.39 +/- 33.26 pg/ml in groups E and L, respectively. Nociceptin was identified in 16/20 plasma samples with mean +/- SD concentrations of 7.59 +/- 21.58 and 13 73 +/- 23.79 pg/ml in groups E and L, respectively. CSF concentrations were significantly higher than plasma concentrations and there were no differences between groups E and L. These data report the first measurements of CSF nociceptin in man and show no association with the acute pain of labour.

Automated TLD system for tumor dose estimation from exit dose measurements in external beam radiotherapy.

PURPOSE: An automated TLD facility has been commissioned and calibrated, and techniques have been developed for the measurement of exit doses in external beam radiotherapy, to enable the routine estimation of delivered tumor doses. METHODS AND MATERIALS: An automated TLD system, originally intended for use in diagnostic radiology and radiation protection, has been evaluated and configured for the measurement of exit doses in radiotherapy. Linearity, optimum heating cycles and calibration procedures have been determined. At the photon energies used, encapsulated lithium fluoride chips provide insufficient buildup to ensure electronic equilibrium, necessitating calibration to allow for oblique exit surfaces. Expressions are derived to allow the calculation of delivered tumor doses. RESULTS: Under the calibration conditions described, the uncertainty in a single TLD measurement is approximately +/-2% (+/-1 standard deviation). Over the dose range 0.4-1.5 Gy, TL response is linear. The total heating cycle time, including annealing, is 75 s. Measurements of R(exit) (the ratio of exit dose with and without full backscatter), used in the estimation of tumor doses, decreases with field size for small fields and varies only slightly for field sizes greater than 7 x 7 cm. Lack of electronic equilibrium leads to a decrease in R(exit) with increasing exit surface obliquity for all energies considered. Application of the technique to a simulated treatment showed good agreement between estimated and applied tumor doses, when surface obliquity was taken into account. CONCLUSION: This work describes the commissioning and calibration of an automated TLD facility and demonstrates that exit surface measurements using TLD chips used under conditions where electronic equilibrium was not established, have the potential for identifying discrepancies in delivered tumor doses.

Matching wedge field characteristics on different treatment machines.

A method is presented for matching wedge field dose distributions on different treatment units, simplifying the transfer of patients between machines during machine failure or scheduled downtime and avoiding the need for a full re-plan in most cases. Differences in wedge field characteristics between machines are accounted for and differences in energy are easily accommodated.

A method and contrast-detail phantom for the quantitative assessment of radiotherapy portal imaging systems.

Several designs of electronic portal imaging systems for radiotherapy have been reported or are now commercially available, as well as the familiar radiographic screen-film combinations. In order to evaluate imaging performance of these, and future, systems a prototype contrast-detail test object has been developed in conjunction with a method for determining contrast from a model of the X-ray spectra produced by linear accelerators. Several existing test objects rely on qualitative or semi-quantitative estimates of contrast. In this technique, quantitative estimates of contrast may be determined for each detail size from realistic estimates of the X-ray spectrum by using a spectral model either in conjunction with narrow-beam attenuation measurements on the linear accelerator used, or by using the nominal value of maximum photon energy. This technique should facilitate the comparison of imaging systems used with linear accelerators of different energies as well as providing a quantitative quality control tool for regular measurements of imaging performance. Examples of the use of the test object in the evaluation of several commercial screen-film combinations are given.

The effect of secondary electron spread on the penumbra in high energy photon beam therapy.

In photon beam therapy, the geometric penumbra width is determined by the source-size and the collimator design. The width of the physical (i.e. dosimetric) penumbra involves an additional contribution from secondary electron spread. Using a suitably defined measure of penumbra width, the separate widths due to photons and secondary electrons are additive in quadrature. Secondary electron spread functions were measured using photographic dosimetry for 60Co gamma rays and for 4, 8 and 16 MV X rays. The results suggest that, under typical treatment conditions, secondary electron spread may be the predominant contributor to the penumbra at effective generating voltages exceeding 10 MV. The implications for selection of beam energy in precision small-field radiotherapy are discussed briefly.

Results of treatment of carcinoma of the uterine cervix using the Newcastle afterloading technique.

Eighty-seven patients with carcinoma of the uterine cervix were seen between 1 August 1977 and 31 December 1985. Seventy-one patients were treated radically. A combination of intracavitary radiotherapy, using the Newcastle manual afterloading technique, and external-beam radiotherapy was used. The actuarial 4-year survival for all Stage 1 cervical carcinomas was 84.9%. For Stage 2 cancers the actuarial local control rate was 71.1% and the survival 49.2% and for Stage 3 cancers the 4-year actuarial local control rate was 24.3% and the survival 14.8%. Complications of treatment were confined to the bowel and bladder and occurred in 11.5% of cases. The results of treatment of Stage 1 cancer suggest that the intracavitary technique gives a satisfactory distribution of radiation. The results of treatment of more advanced cases are disappointing and suggest that improvements are possible for some patients (i.e. wedge malalignment patients). There is sufficient flexibility in the technique for it to be applied satisfactorily for most anatomical situations.

Gynaecological treatments using the Selectron remote afterloading system.

The Manchester method of gynaecological treatment using radium 226, ovoids and a flexible intrauterine tube has been used in many centres. In Newcastle, the Manchester technique was used until 1971, when a manually afterloaded system (MAS) using caesium 137 and two source trains was introduced. In 1985, the MAS was replaced by the Selectron remote afterloading system (RAS), which also uses caesium-137 sources. The dosimetry of this remote system has been designed to produce the same shape of isodose distributions as the MAS, but shorter treatment times. The applicator design and the dosimetry of the system used in Newcastle are discussed. Dose rates achieved by the RAS are more than double those currently produced by the MAS, and a reduction in prescribed dose of 10-20% is being made. The percentage of ward staff receiving monthly doses of more than 0.2 mSv has fallen from 78% to 18% since the introduction of the remote afterloading system. The almost-cylindrical isodose distributions facilitate matching of external beam treatments using a central lead wedge to the intracavitary treatments, and work is in progress to develop this technique.

Improved external beam radiotherapy for the treatment of retinoblastoma.

An applicator attached to a 6 MV linear accelerator has been adapted to produce a well defined beam for the treatment of retinoblastoma. By using a contact lens as the reference point on the patient's cornea, the beam edge is aligned to just behind the posterior margin of the crystalline lens, thus sparing the anterior chamber and lens from the radiation field. In addition to basic dosimetric measurements made using an ionisation chamber and film densitometry to quantify the output and dose distribution from the applicator, extensive use was made of thermoluminescent dosemeters and a tissue-equivalent anthropomorphic head phantom. This was designed for detailed dosimetry on the eye and was used to confirm the correct positioning and reproducibility of the clinical set-up. We present data that corroborate and extend the dosimetric analyses documented in an earlier Dutch publication. In addition to the improved dosimetric analysis, other advantages include patient immobilisation, anaesthetic procedure and finer couch control as well as a recommendation on the dose prescription.

Improved total-body irradiation dosimetry.

The total-body irradiation (TBI) technique at St Bartholomew's Hospital has been developed to improve dose homogeneity within the patient. Using a standard 6 MV linear accelerator in an orthodox-sized treatment room, the midpoint doses in head, neck, shoulders, mid-mediastinum, pelvis, knees and ankles are +/- 5% of that of the umbilicus in our current technique. This homogeneity has been achieved by a four-field technique, a reproducible patient set-up, careful use of a new bolus material and an additional beam-flattening filter mounted near the machine head. In addition, thermoluminescent dosimetric data collected at a test irradiation before TBI are used to influence field weightings and further improve dosimetry. This individualised and empirical TBI technique has dosimetric advantages over theoretical TBI dosimetric considerations in reducing dose gradients within the patient. These advantages are discussed.

Combining backscattered electrons and low energy photons to improve the dose distribution to an eyelid.

In electron beam therapy, backscattered electrons from metal shields result in a high dose to tissue in the vicinity of the shield. Data are now available which enable both the magnitude of the dose enhancement at the interface and the fall off in dose 'up stream' to be determined. With the aid of these data, a combined modality therapy (5 MeV electrons, 1.1 mm A1 HVL X rays) was given to an eyelid during which the lens was protected with a superficial X ray lead eye shield. The eyelid was treated to a mid lid dose of 45 Gy in 28 days, and the dose gradient on the beam central axis was more uniform (+/- 3%) than that obtainable with superficial X rays alone. The lens dose from the combined therapy was estimated to be less than 0.25 Gy.

Exit dose studies in megavoltage photon therapy.

A coin-shaped ionisation chamber, orientated with its thin window facing away from the radiation source, was used to investigate the dose perturbations caused by the absence of back-scattering material near the exit surface of solid phantoms. Cobalt 60 and 4, 8 and 16 MV X-ray beams were used in the study. With no scattering material beyond the chamber window the ionisation was found to be as much as 17% less than the full scatter value. This was attributed to the absence of both back-scattered electrons and back-scattered photons. Full electron back-scattering could be restored by placing between 1.0 and 2.7 mm of unit density material beyond the chamber, depending on the primary beam energy. Under these circumstances the reduction in dose, now due to the absence of back-scattered photons only, was found to be small.

Backscattering in electron beam therapy for energies between 3 and 35 MeV.

Whenever a heterogeneity is present in an electron beam treatment field during radiotherapy, there is the possibility of tissue overdosage at the tissue-heterogeneity interface due to electrons backscattered from the heterogeneity. Measurements of this effect were made in a polystyrene phantom using a purpose-built thin-window parallel-plane ionisation chamber. Materials of various atomic numbers were used as scatterers and the investigations were made over a wide range of electron energies. Electron backscatter factor (EBF), defined as the ratio fo dose at the interface surface with and without the scatterer present, was found to increase with increasing atomic number and decrease with increasing beam energy. Both of these relationships were found to be non-linear. The EBF dependence on the scatterer thickness was also investigated. All data in this work were expressed in relation to the beam energy incident on the scatterer in preference to the nominal beam energy set on the accelerator. This approach enables the dose enhancement at an interface to be predicted from a knowledge of the heterogeneity (atomic number and thickness,), its depth in tissue and the beam energy being used for treatment. The results of this work were compared with the published data and an explanation is offered to account for the difference.