Megavoltage Imaging Research Articles

The efficacy of megavoltage imaging in the radical radiotherapy of non-small cell lung cancer

The efficacy of megavoltage imaging in the radical radiotherapy of non-small cell lung cancer

Monte Carlo studies of x-ray energy absorption and quantum noise in megavoltage transmission radiography.

The subject contrast of bony anatomy in megavoltage medical radiographs is very low, making detection of bony landmarks difficult if additional noise sources are introduced into the images. One source of noise, which is inherent to the x-ray detection process, is x-ray energy absorption noise. X-ray energy absorption noise results from variations in the amount of energy deposited in the imaging detector per interacting x ray. These variations increase the noise content of the image. In this study, EGS4 Monte Carlo simulations of x-ray interactions in metal plate phosphor screen detectors have been performed to determine the distribution of energy absorption events within the phosphor screen. From these "absorbed energy distributions (AEDs)", the x-ray energy absorption noise and the quantum absorption efficiency of the detector are determined. These calculations are performed for a range of detector thicknesses (0.1-4 mm) and x-ray energies (0.1-10 MeV). A number of conclusions can be drawn from these investigations. (i) The x-ray absorption noise reduces the detective quantum efficiency (DQE) of metal plate/phosphor screen detectors by as much as 50% at energies used in megavoltage imaging (1-10 MeV). (ii) It is important to include secondary particle (electron) transport in estimating the quantum absorption efficiency of these detectors. For instance, the quantum efficiency of a typical portal detector is approximately 2%, even though 4%-5% of the incident photons are attenuated. (iii) The metal "conversion" plate commonly used in megavoltage imaging enhances the DQE of the phosphor screen by increasing the quantum absorption efficiency and reducing the magnitude of the x-ray absorption noise.

A real-time, flat-panel, amorphous silicon, digital x-ray imager.

As part of the development necessary for implementing a fully digital radiology department, the authors have investigated thin-film photodiodes and transistors for use in new photoelectronic imaging devices. One such device, a large-area, flat-panel, amorphous silicon imaging array, has been developed and is currently being tested. The array has a format of 512 x 560 pixels, a pixel-to-pixel pitch of 450 microns, and an area of 230 x 252 mm2, making it the largest self-scanning, solid-state imaging array developed to date. The array is used in conjunction with an overlying x-ray converter. Although specifically designed for megavoltage imaging, the device can produce high-quality, low spatial resolution, diagnostic x-ray images. Qualitative comparisons of array images of anthropomorphic head, chest, and pelvic phantoms and a spatial resolution pattern suggest that much of the information content of the film images at low spatial resolution is present in the corresponding array images. Current trends in the development of large-area, flat-panel imaging technology hold the promise of higher resolution arrays in the near future.

Technical note: the efficacy of megavoltage imaging in the radical radiotherapy of non-small cell lung cancer.

Megavoltage imaging (MVI) has been used to obtain digital images of treatment fields during therapy for non-small cell lung cancer. Tumours were seen in all 33 cases studied and in three cases MVI was used to improve the set-up. It is concluded that in the setting of radical radiotherapy for non-small cell lung cancer this is an appropriate technique for the verification and correction of field position and is an aid to quality assurance.

Reproducibility of patient positioning during routine radiotherapy, as assessed by an integrated megavoltage imaging system

A portal imaging system has been used, in conjunction with a movie measurement technique to measure set-up errors for 15 patients treated with radiotherapy of the pelvis and for 12 patients treated with radiotherapy of the brain. The pelvic patients were treated without fixation devices and the brain patients were treated with individually-moulded plastic shells. As would be expected the brain treatments were found to be more accurate than the pelvic treatments. Results are presented in terms of five error types: random error from treatment to treatment, error between mean treatment position and simulation position, random simulation error, systematic simulator-to-treatment errors and total treatment error. For the brain patients the simulation-to-treatment error predominates and random treatment errors were small (95% ≤ 3 mm, 77% ≤ 1.5 mm). Vector components of the systematic simulation-to-treatment errors were 1–2 mm with maximal random simulation error of ±5 mm (2 S.D.). There is much interest in the number of verification films necessary to evaluate treatment accuracy. These results indicate that one check film performed at the first treatment is likely to be sufficient for set-up evaluation. For the pelvis the random treatment error is larger (95% ≤ 4.5 mm, 87% ≤ 3 mm). The systematic simulation-to-treatment error is up to 3 mm and the maximal random simulation error is ±6 mm (2 S.D.). Thus corrections made solely on the basis of a first day check film may not be sufficient for adequate set-up evaluation.

Clinical applications of composite and realtime megavoltage imaging

The versatility of electronic portal imaging devices (EPIDs) is best demonstrated by their ability to perform novel megavoltage imaging protocols, which are still pertinent to good radiotherapy practice. This paper examines two such techniques: composite and realtime imaging. Our EPID can be programmed to acquire and manipulate images very easily, allowing images from segmented treatment protocols to be mixed and displayed, giving a composite image of the effective treatment result. Its use for verifying the efficacy of spinal shielding using a segmented, offset collimator technique is described. By acquiring images very quickly, realtime imaging sequences can be obtained and used to analyse anatomical movement within a single treatment field. The technique is employed here to investigate movement in radical lung, breast, abdomen, pelvis and thyroid treatments. Our results show that the protocol is vital for treatment sites involving the lungs; changes up to 5 mm have been observed in the maximum lung depth for breast treatments, and displacements up to 16 mm for radical lung treatments. It is also useful in other anatomical sites for ensuring that no movement occurs.

Thick phosphor screens for on-line portal imaging.

Gd2O2S phosphor screens between 250 and 1000 mg/cm2 thick were evaluated for use in megavoltage imaging systems. The phosphor layers were placed on brass plates ranging from 1 to 5 mm thick, each with and without an optical back reflector (white paint). Light output and spatial resolution were measured at 6- and 23-MV x-ray energies. Light output was found to increase linearly with phosphor thickness up to 500 mg/cm2, reaching a plateau at 1000 mg/cm2. Spatial resolution [modulation transfer function (MTF)] decreased exponentially with phosphor thickness up to 750 mg/cm2, where a minimum was reached. The variation of MTF with phosphor thickness was found to obey a simple empirical relation.

A randomised trial of patient repositioning during radiotherapy using a megavoltage imaging system

Effectiveness of radiotherapy is dependent on the accuracy of beam alignment. Recent developments in megavoltage imaging allow real-time monitoring of beam placement. Maximum gains from this new technology can only be made if the information is utilised to correct patient positioning prospectively before the majority of a treatment fraction is delivered. We have developed and utilised an integrated megavoltage imaging system to perform a randomised trial demonstrating significant improvements in accuracy using treatment intervention techniques for pelvic radiotherapy. The mean field-placement accuracy improved from 4.3 mm to 2 mm and the proportion of treatments given with a field-placement error of ≥ 5 mm decreased from 69% to 7%. This improvement in accuracy may enable smaller margins around the target volume to be chosen whilst ensuring complete target coverage at each treatment fraction.

Quality assurance of the simultaneous boost technique for prostatic cancer: dosimetric aspects

Quality assurance (QA) in radiotherapy is of particular importance if a new irradiation technique is introduced. The dosimetric aspects of such a QA program concern the check of the dose calculation procedure, i.e the prediction of the relative dose distribution, as well as the verification of the absolute value of the target absorbed dose specified at a particular point. In our institution a QA program has been developed for a new conformal irradiation technique of prostatic cancer: the simultaneous boost technique. With this technique the dose of the boost field and the large field are given simultaneously, using customized 10 mm thick Roses-metal plates in which the boost field has been cut out. The computation of the dose distribution, using a procedure adapted from a commercially available 2D treatment planning system, has been compared with isodose distributions measured in a water phantom. Good agreement, better than 3% or 3 mm, was observed for both open and wedged 8 MV X-ray beams. In vivo dose measurements have been performed on individual patients to check the dose delivery at the specification point. An agreement better than ±2% with the calculated dose value was required. The average ratio for 18 patients of the actual and expected dose value amounted to 1.005 ± 0.017 (1 SD) after a correction of the number of monitor units for 2 patients during the treatment. Quality control of the dose transmission factor of the Roses-metal plates has been performed. On average, this factor was 63.4 ± 0.6% (I SD), which is in good agreement with the value of 64% accepted in the simultaneous boost technique. The uniformity of the transmission factor over the whole plate inside the irradiation field was required to be better than 2%, and has been checked using megavoltage imaging. In about 10% of the cases a replacement of the transmission plate was required due to enclosed air cavities. The results of our QA program demonstrated that with regard to the dosimetric aspects the simultaneous boost technique can be considered to be a reliable treatment technique.

Preliminary clinical performance of a scanning detector for rapid portal imaging

A scanning megavoltage imaging detector, with associated image storage and analysis facilities has been developed. This produces images of the treatment portals in under 10 seconds, in a digital format, facilitating rapid, quantitative image analysis. Image quality is comparable to, and at some sites improves upon, that available from film. Clinical problems in the use of megavoltage imaging include limited field of view, loss of information at the field edge due to penumbra effects, degradation of the image by bowel gas, and difficulties in the detection of soft tissue-air interfaces. Possible solutions to these problems are discussed. The imaging system has been used to assess the random errors occurring during routine para-aortic nodal irradiation. The errors detected are small, with over 95% of set-ups lying within ±4.5 mm of the mean daily position. No differences were detected in the magnitude of random errors between anterior and posterior treatment fields.

Proceedings of the British Institute of Radiology Megavoltage portal imaging. 5 April 1993

SRI-100 is a megavoltage imaging (MVI) system designed as an accessory to the Philips SL series of linear accelerators. Many centres consider MVI to be essential for accurate radiotherapy. X-ray energies used in radiotherapy give poor images, as they present a lower contrast ratio per unit waterdepth than kilovoltage diagnostic X-rays. Furthermore, the finite radiation source size does not allow high spatial resolution. For maximal linearity and sensitivity, SRI-100 uses a non-intensified chargecoupled device (CCD) camera; high sensitivity and low image noise are achieved by the use of long exposure and sensor cooling. Image processing is performed on a 486 based personal computer, equipped with frame grabber and frame processor boards.

Technical note: the implementation of patient position correction using a megavoltage imaging device on a linear accelerator.

The problem of using information from the analysis of megavoltage images to adjust patient set-up has been addressed. In the case of rotational corrections it has been assumed that the treatment head is to be adjusted, although for gantry angles of 0 degree and 180 degrees couch rotation may be used. In the case of translational shifts adjustment of the collimator jaws or of the couch have both been considered for arbitrary combinations of couch and gantry angle. For couch movement the case has been considered where it is desirable to minimize both the number of parameters to be adjusted and also the magnitude of the change in the patient's position. Values obtained for frequently used set-up parameters have been presented. Adjustment of the treatment couch positioning is the most desirable option, as this should bring the patient closer to the correct position for subsequent treatment fields. However, rotational errors are not correctable for all gantry angles and furthermore the collimator settings may be set more accurately than those of the treatment couch. Hence, in some cases, adjustment of the collimation system may be desirable or necessary. The formulae given in Equations (13) to (18) are currently being used in an intervention study to correct patient set-up during the course of a treatment fraction.

Black and white in accuracy assessment of megavoltage images: the medical decision is often grey

Using different criteria for acceptance of the portal film taken at the first treatment session, a comparison was made of the relevance of the information obtained from such a single check for the subsequent irradiations. A total number of 234 verification films have been taken on 29 fields for 27 head and neck patients. Patients were immobilised with individual plastic masks fixed to the couch and treated on a 6-MV linac fitted with an automatic verification system. Field alignment was checked with a measurement in the anteroposterior (AP) and craniocaudal (CC) direction on each film. Referring to the simulated field, this group of patients was treated with excellent average precision (mean, −0.7 mm) and reasonable spread (s = 5 mm). The percentage of ‘large’ deviations (≥6 mm) occurring during the whole treatment course is proportional to the upper limit of deviation accepted in the first assessed field (for an upper limit <6 mm): it goes progressively up from 5% (AP-CC direction) to 17% (AP) and 13% (CC) for accepted magnitudes of deviation going from 2 mm to 6 mm in the first film. As the reproducibility of the different treatment series (s = 2 mm) is independent of the upper level of error accepted on the first film, this means that errors are mainly systematic errors coming from the transfer of the simulation unit to the treatment unit. Precision in a series of set-ups is always expressed by a Gaussian curve. A control port film at the first session is most frequently representative for the median value of the whole series and subsequent set-ups always show a spread of values. It is therefore important to choose an upper limit of the acceptable deviation on the first measurement which is small enough, as compared with the ‘unacceptable deviation’, to be sure that most of the subsequent set-ups can also be considered as correct. The acceptance limit of a single first image should therefore be directly linked to the known precision of a technique (determined by the standard deviation of the reproducibility distribution) and to the clinical situation involved. It should not be limited to a ‘black’ or ‘white’ decision with a single numerical value for different treatment modalities.

Image comparison techniques for use with megavoltage imaging systems.

In this paper we describe software facilities for enabling patient positioning studies using the megavoltage imaging system developed at the Royal Marsden Hospital and Institute of Cancer Research. The study focuses on the use of the system for three purposes: patient position verification (by comparing images taken at treatment simulation with megavoltage images taken at treatment time); reproducibility studies (by analysing a set of megavoltage images); and set-up correction (by adjusting the set-up until the megavoltage image obtained at treatment registers with the simulation image). The need is discussed for suitably presented simulator images, a method of determining field boundaries and the possibility of delineating soft-tissue interfaces. Several algorithms of different types, developed specifically for the purpose of intercomparison of planar projection images, are presented. The techniques employed and their usefulness, in both the qualitative and the quantitative sense, are discussed. The results are presented of a phantom and clinical study, to evaluate the rigour and reproducibility of the algorithms. These results indicate that measurements can be made to an accuracy of about 1-2 mm, with a similar value for interobserver reproducibility for the best image comparison techniques available.

Quality assurance in daily treatment procedure: patient movement during tangential fields treatment

Fifteen women undergoing breast radiotherapy following wide local excision of an early stage breast cancer were submitted to repeated measurements of surface landmarks to check the reproducibility of patient positioning, and to portal imaging using a megavoltage imaging device. When the patient is being set-up the mean rise and fall of a lateral skin mark (tattoo) was within 4 mm in 95 observations of 15 patients. At the end of the lateral field exposure, the mean displacement of the lateral tattoo was close to zero, with only 15/95 (16%) observations falling outside the range ± 2 mm. The daily measurements of lung thickness fell above and below the simulated lung thickness, consistent with random fluctuations. Eighty-eight percent of lung thickness measurements were within ± 5 mm of the simulator position. A tentative conclusion is made that more sophisticated immobilisation and imaging devices may be unnecessary for breast irradiation with a high degree of reproducibility.

Portal imaging for the verification of breast treatments

Increasing demands are being made for more accurate versatile and precise methods of quality control for radiation therapy, in general. To this end, we have used a digital, on-line, megavoltage imaging system for the verification of breast treatments, in particular. Quick, high-quality images are produced from which the depth of lung irradiated during treatment is very easily determined using the system software. We describe how this quantitative clinical information has been used to assess critical aspects of the treatment technique, and also patient orientated criteria, throughout the course of treatment.

Improved dose homogeneity in the breast using tissue compensators

The improvements achievable by introducing individual tissue compensators in photon therapy of the breast were assessed. In 37 patients the dose ranged from + 15% to − 10% of the mid target dose using combinations of wedge filters and beam weights alone. With a tissue compensator the dose ranged from + 4% to − 11% provided that allowance was made for lung attenuation. A megavoltage imaging system is a potential source of the X-ray transmission data which can provide a basis for the calculation of thickness of the compensator.

The lens coupling efficiency in megavoltage imaging

In TV-based megavoltage imaging systems it is useful to be able to estimate the geometrical efficiency g2 of the camera lens. It is shown that the appropriate expression is g2 = (16n2)-1 X [F(1 + 1/m)]-2, where n is the refractive index of the scintillator, F is the F-number of the lens and m is the optical magnification. This expression yields estimates for g2 that are 5 to 10 times smaller than the estimates that have appeared previously in the literature.

The simultaneous boost technique: the concept of relative normalized total dose

The simultaneous boost technique in radiotherapy consists of delivering the boost treatment (additional doses to reduced volumes) simultaneously with the basic (large-field) treatment for all treatment sessions. Both the dose per fraction delivered by the basic-treatment fields and by the boost-treatment fields have to be reduced to end up with the same total dose in the boost volume as in the original schedule, where the basic treatment preceded the boost treatment. These dose reductions and corresponding weighting factors have been calculated using the linear-quadratic (LQ) model and the concept of Normalized Total Dose (NTD). Relative Normalized Total Dose (RNTD) distributions were computed to evaluate the dose distributions resulting for the simultaneous boost technique with respect to acute and late normal tissue damage and tumor control. For the example of the treatment of prostatic cancer the weighting factors were calculated on the basis of the NTD for late normal tissue damage. For the treatment of oropharyngeal cancer the NTD for acute normal tissue damage was used to determine the weighting factors. In this last example a theoretical sparing of late normal tissue damage can be demonstrated. A second advantage of the simultaneous boost technique is that the megavoltage images of the large basic-treatment fields facilitates the determination of the position of the patient with respect to the small boost-treatment fields.

Progress in radiotherapy by treatment tailoring in head and neck cancer

Radiotherapy plays a key role with surgery in the treatment of head and neck cancer. Encouraging results have recently been published indicating new ways to improve locoregional control by tailoring the radiation treatment to individual tumors. This has been reached by the following means: adapting fractionation schemes to the potential doubling time of tumors, technical developments resulting in higher doses to the primary tumor and at the same time reducing normal tissue doses, and the concomitant use of radiotherapy and chemotherapy. Fractionation studies have shown that changing the standard fractionation schedule can increase the therapeutic margin of radiotherapy. Predictive assays are now becoming available to provide guidelines for the optimal fractionation schedules in individual patients. Combined treatment with radiotherapy and chemotherapy has been widely tested and although the results have been generally disappointing, one approach remains promising, ie, the concomitant use of these two modalities, which seems to result in a modest improvement of survival. Increasing the radiation dose to the tumor together with a better shielding of normal tissues has become possible by technical developments in radiotherapy such as three-dimensional treatment planning, multileaf collimators, and quality control procedures using megavoltage imaging devices.