Loco-regional differences in pulmonary function and density after partial rat lung irradiation

PURPOSE

The purpose of this study was to explore regional differences in radiosensitivity of rat lung using lung function and computed tomography (CT) density as endpoints.

METHODS

At first, CT scans were used to determine rat lung volumes. The data obtained enabled the design of accurate collimators to irradiate 50% of the total lung volume for the apex, base, left, right, mediastinal and lateral part of the lung. Male Wistar rats were irradiated with a single dose of 18 Gy of orthovoltage X-rays. Further rat thorax CT scans were made before and 4, 16, 26, and 52 weeks after irradiation to measure in vivo lung density changes indicative of lung damage. To evaluate overall lung function, breathing frequencies were measured biweekly starting 1 week before irradiation.

RESULTS

Qualitative analysis of the CT scans showed clear density changes for all irradiated lung volumes, with the most prominent changes present in the mediastinal and left group at 26 weeks after radiation. Quantitative analysis using average density changes of whole lungs did not adequately describe the differences in radiation response between the treated groups. However, analysis of the density changes of the irradiated and non-irradiated regions of interest (ROI) more closely matched with the qualitative observations. Breathing frequencies (BF) were only increased after 50% left lung irradiation, indicating that the hypersensitivity of the mediastinal part as assessed by CT analysis, does not result in functional changes.

CONCLUSIONS

For both BF and CT (best described by ROI analysis), differences in regional lung radiosensitivity were observed. The presentation of lung damage either as function loss or density changes do not necessarily coincide, meaning that for each endpoint the regional sensitivity may be different.

Three-dimensional dose distribution for partial irradiation of rat parotid glands with 200kV X-rays

PURPOSE

To investigate dose distributions in partial-volume irradiation experiments in small experimental animals, in particular the parotid gland of rat.

MATERIALS AND METHODS

High-resolution magnetic resonance imaging images were made that provided the outlines of the parotid glands, which were used to design collimators with conformal radiation ports for 100 and 50% cranial/caudal partial-volume irradiation. A protocol for absolute dosimetry was designed and relative dose measurements were performed. From the three-dimensional topographical data and the three-dimensional dose distribution, dose-volume histograms were determined.

RESULTS

The standard uncertainty of absorbed entrance dose was within 3%. Radiochromic film, thermoluminescence dosemeters and ionization chamber dose measurements revealed that the relative doses measured were in good agreement. The 20-80% penumbra of the beam across the 50% field edge was only 0.4 mm at a 6 mm depth. The gradient of the percentage depth dose from the skin of the rat to a depth of 12 mm was 1.5% mm(-1). The absorbed doses in the cranial 50% and the caudal 50% partial volumes were comparable. This finding was reflected in the calculated dose-volume histograms of the different regions, which were similar. The dose in the shielded area between the left and right ports was about 14% of the dose near the centres of the beams.

CONCLUSION

The designed set-up showed that irradiation of small volumes could be performed with high accuracy allowing the study of differences in radiation damage. Similar doses were given to the 50% cranial and 50% caudal gland volumes and, therefore, a possible difference in radiosensitivity in these volumes was not a dose effect. The approach used was also applicable for the irradiation of small volumes of other tissues.

Parotid and submandibular/sublingual salivary flow during high dose radiotherapy

It was studied whether differences in acute radiosensitivity exist between parotid and submandibular/sublingual glands. The results revealed that salivary flow rates decreased dramatically during the first 2 weeks of radiotherapy. Neither recovery nor significant differences were observed between the production of saliva from the parotid and submandibular/sublingual glands during the 13 weeks observation period.

Calculation of the uncertainty in complication probability for various dose-response models, applied to the parotid gland

PURPOSE

Usually, models that predict normal tissue complication probability (NTCP) are fitted to clinical data with the maximum likelihood (ML) method. This method inevitably causes a loss of information contained in the data. In this study, an alternative method is investigated that calculates the parameter probability distribution (PD), and, thus, conserves all information. The PD method also allows the calculation of the uncertainty in the NTCP, which is an (often-neglected) prerequisite for the intercomparison of both treatment plans and NTCP models. The PD and ML methods are applied to parotid gland data, and the results are compared.

METHODS AND MATERIALS

The drop in salivary flow due to radiotherapy was measured in 25 parotid glands of 15 patients. Together with the parotid gland dose-volume histograms (DVH), this enabled the calculation of the parameter PDs for three different NTCP models (Lyman, relative seriality, and critical volume). From these PDs, the NTCP and its uncertainty could be calculated for arbitrary parotid gland DVHs. ML parameters and resulting NTCP values were calculated also.

RESULTS

All models fitted equally well. The parameter PDs turned out to have nonnormal shapes and long tails. The NTCP predictions of the ML and PD method usually differed considerably, depending on the NTCP model and the nature of irradiation. NTCP curves and ML parameters suggested a highly parallel organization of the parotid gland.

CONCLUSIONS

Considering the substantial differences between the NTCP predictions of the ML and PD method, the use of the PD method is preferred, because this is the only method that takes all information contained in the clinical data into account. Furthermore, PD method gives a true measure of the uncertainty in the NTCP.

Fast 2D phantom dosimetry for scanning proton beams

A quality control system especially designed for dosimetry in scanning proton beams has been designed and tested. The system consists of a scintillating screen (Gd2O2S:Tb), mounted at the beam-exit side of a phantom, and observed by a low noise CCD camera with a long integration time. The purpose of the instrument is to make a fast and accurate two-dimensional image of the dose distribution at the screen position in the phantom. The linearity of the signal with the dose, the noise in the signal, the influence of the ionization density on the signal, and the influence of the field size on the signal have been investigated. The spatial resolution is 1.3 mm (1 s.d.), which is sufficiently smaller than typical penumbras in dose distributions. The measured yield depends linearly on the dose and agrees within 5% with the calculations. In the images a signal to noise ration (signal/1 s.d.) of 10(2) has been found, which is in the same order of magnitude as expected from the calculations. At locations in the dose distribution possessing a strong contribution of high ionization densities (i.e., in the Bragg peak), we found some quenching of the light output, which can be described well by existing models if the beam characteristics are known. For clinically used beam characteristics such as a Spread Out Bragg peak, there is at most 8% deviation from the NACP ionization chamber measurements. The conclusion is that this instrument is a useful tool for quick and reliable quality control of proton beams. The long integration-time capabilities of the system make it worthwhile to investigate its applicability in scanning proton beams and other dynamic treatment modalities.

The use of a transverse CT image for the estimation of the dose given to the rectum in intracavitary brachytherapy for carcinoma of the cervix

BACKGROUND AND PURPOSE

The three-dimensional (3D) dose distribution in combination with 3D anatomy of 13 patients treated for cervical carcinoma with intracavitary brachytherapy was analyzed. The aim of this study was to determine the correlation between a dose value obtained from the integral dose volume histogram (DVH) of the rectum and (a) the Nederlands Kanker Instituut (NKI) point of reference for the rectum dose (R) and (b) the highest dose to the frontal rectum wall in the transverse CT slice near the top of the vagina through point R.

RESULTS

The correlation between the DVH rectum dose value for 2 cm3 in the highest dose region and the rectum dose at point R was poor (regression coefficient 0.50). On the contrary, however, the correlation between the DVH rectum dose value for 2 cm3 in the highest dose region and the maximum rectum dose value in a transverse CT slice through point R was good (regression coefficient 0.90).

CONCLUSIONS

The maximal rectum dose value obtained from a transverse CT slice near the top of the vagina through point R was found to be a more representative point for the rectal dose burden and might therefore show a good correlation with complications. The point of reference for the rectal dose (R) was found not to be a reliable estimation of the maximal dose in the rectum.

Radiological and functional assessment of radiation-induced lung injury in the rat

The purpose of this study is to develop an experimental model to measure localized radiation-induced lung injury using multiple end-points including breathing frequency, high-resolution computed tomography (CT), and radionuclide perfusion. The rats were anesthetized and the right lung irradiated with a single dose of 18 Gy using 200-kVp x-rays. The lung function of the animals was measured every 2 weeks after irradiation with the breathing rate assay. CT scanning and radionuclide lung perfusion assay were performed prior to and 2, 4, 10, 16, and 34 weeks after irradiation. Significant elevation in breathing rate occurred after 16 weeks, with a maximal increase between 22 and 28 weeks. An increase in the right lung density started 4 weeks after irradiation. Regional measurements indicated a relatively uniform increase in density at 4 and 10 weeks, while foci of high-density areas were observed at the later time points. Changes in rat lung volume indicated shrinkage of the irradiated right lung and accompanying compensatory hypertrophy of the shielded left lung. Radionuclide perfusion assay showed significant decrease in relative blood flow in the irradiated right lung 4 weeks after hemithoracic irradiation. Changes in breathing rate provide an index of overall lung function while changes in lung density, volume, and perfusion are of particular importance for evaluating loco-regional differences in lung sensitivity. This study is the first demonstration that CT can be used to measure volume changes after thoracic irradiation in rats.

11C-tyrosine position emission tomography and 1H magnetic resonance spectroscopy of the response of brain gliomas to radiotherapy

We monitored 10 patients with unresected (9) or partially resected (1) supratentorial gliomas with 11C-tyrosine position emission tomography (TYR-PET) before and after radiotherapy. TYR-PET tumour volumes were measured using a threshold technique. In seven patients the tumour volume decreased after radiotherapy, although all gliomas persisted on TYR-PET images. In eight patients the tumour protein synthesis rate (PSR) was calculated using a dynamic study protocol in combination with a PATLAK analysis. There were no changes in PSR after radiotherapy, but the PSR was calculated on the remaining tumour volume using the same threshold technique as before therapy, i.e. the decrease in tumour volume was not taken into account. In eight patients the PET data were compared with magnetic resonance spectroscopic imaging (1H-MRSI) performed simultaneously. Although there was no statistically significant correlation between TYR-PET volume changes and 1H-MRSI choline level we observed a simultaneous decrease in volume and choline in four patients.

Sampling methods for a matrix ionization chamber system

To achieve increased image acquisition speed or better image quality, several read-out methods for a matrix ionization chamber system have been investigated. In this device, which is applied for portal imaging in radiation therapy, 256 x 256 small liquid-filled ionization chambers are scanned by switching the polarizing voltage applied to rows of chambers. The ionization current of each column is measured by a separate amplifier. In this approach, instead of measuring row by row, more complex switching schemes can be applied for the polarizing voltage. These schemes are useful either for varying speed and spatial resolution of the imaging device or for coded sampling of the ionization signal. The former option allows for doubling or quadrupling the acquisition speed with a small loss in image quality, or for obtaining a large improvement in signal-to-noise ratio at the cost of image resolution. In the latter option, coded sampling, the image is reconstructed mathematically from the measured signals. It is shown that in this case the application of Hadamard or derived matrices for sampling leads, under certain circumstances, to a noise reduction in the reconstructed image.

A review of electronic portal imaging devices (EPIDs)

On-line electronic portal imaging devices are beginning to come into clinical service in support of radiotherapy. A variety of technologies are being explored to provide real-time or near real-time images of patient anatomy within x-ray fields during treatment on linear accelerators. The availability of these devices makes it feasible to verify treatment portals with much greater frequency and clarity than with film. This article reviews the physics of high-energy imaging and describes the operation principles of the electronic portal imaging devices that are under development or are beginning to be used clinically.

First clinical experience with a newly developed electronic portal imaging device

In our institute an electronic portal imaging device (PID) has been developed and it recently became available for routine clinical practice. Images are available within 3 to 6 seconds after the start of irradiation; they are displayed on a video monitor next to the control console of the accelerator. The image quality is similar to the quality of images obtained with films. Because of its cassette-like shape and its low weight, the PID can easily be handled by technicians. An important advantage of the PID over conventional films is its pseudo-real time viewing facility. Typically, 5 to 10 images of each field can be made during one treatment session. In case a high accuracy in setup is demanded, the field edges of the first image, obtained with about 10% of the fraction dose, can be studied for acceptability before the rest of the dose is delivered. Using two prototype PID's first clinical experience has been obtained with patients treated for malignant tumors at various sites. Intra-treatment motion as a result of breathing, swallowing, or patient motion in a cast was seen. Motion of high contrast objects, for example, a field edge during irradiation, can be followed. This feature is important for future applications in computer controlled radiotherapy. Another advantage of the PID over film is that the image is digitally available. Therefore it can be further processed for quality improvement and quantitative analysis. Simple processing is done within seconds on the PID unit. A local network for the transfer of images from the accelerators to the evaluation room, where a detailed analysis of the field placement is performed, is under installation. Simulator film images are digitized in this room and are sent to the PID at the accelerator for a quick comparison with portal images during irradiation. We conclude that our device can replace the conventional film detector for portal imaging, that useful images are obtained within seconds during irradiation, and that the position of the field outline relative to the patient anatomy can be followed during dose delivery.

In-phantom calibration of Selectron-LDR sources

Source strength measurements were performed for cesium-137 spherical sources of nine Selectron-LDR remote afterloading systems in The Netherlands. The mean reference air kerma rate of a set of sources was obtained from air kerma rate measurements in a phantom at a distance of about 5.5 cm from a large number (24 to 30) of sources. The results were compared with the source strengths specified by the manufacturer. Discrepancies between measured and manufacturers data ranging from -2.3% to +3.9% were observed. The frequency distribution of the strength of the sources within a set was measured by the use of a well-type ionisation chamber. The root mean square deviations of the nine source sets ranged from 1.0 to 3.2%. Application of the in-phantom calibration method would improve the precision of source strength measurements and therefore reduce the differences in dose delivery between institutes. The uncertainty of the in-phantom method, which could be estimated by statistical methods, was 0.3% (one standard deviation). That part of the uncertainty, which could not be evaluated by statistical methods, was estimated to be about 1.2% (one effective standard deviation). This latter part in the uncertainty needs further investigation in order to reduce the overall uncertainty.

A method for the measurement of field placement errors in digital portal images

Correct placement of radiation fields relative to patient anatomy is essential in radiotherapy in order to minimise serious side effects to reduce the probability of recurrence of the tumour. One way to determine patient setup accuracy is to analyse portal images obtained in the therapy beam distal to the patient. A field placement analysis (FPA) method has been developed for detailed evaluation of patient setup by comparing positions of corresponding radiation field edges in digitised simulator and portal images. A simulator image is matched to a portal image using similar anatomical landmarks in both images and mapping these landmarks against each other applying a least squares minimisation approach. Discrepancies between the simulator field edge (reference) and a portal field edge are determined by comparing the distances between the central axis of the beam and corresponding edge segments and the angles of these segments with a reference line. Uncertainties in these distances and angles are to a large extent determined by the magnification, rotation and translation procedure. Uncertainties due to the FPA method itself are of about 1.0 mm and 0.5 degrees in portal images of head and neck fields. These FPA uncertainties are in general smaller than the variations due to patient setups. Matching of simulator and portal images of lateral pelvic fields revealed larger uncertainties: 1.7 mm and 1.1 degrees. Setup variations in this kind of pelvic radiation field are usually also larger, and therefore meaningful results can be obtained with the new FPA method.

A comparison of dose calculations at points around an intracavitary cervix applicator

An intercomparison was made between dose-rate calculations performed in 10 institute in The Netherlands for intracavitary applications with the Selectron-LDR remote afterloading machine. The results of 11 computer planning systems of five different manufacturers were compared with reference dose-rate calculations. The difference in clinically relevant dose-points like rectum and bladder, were less than 2% if the values of the coordinates of source and dose-points were given, except for one type of planning system, that showed differences up to 10%. The errors observed for the latter system were probably due to a coarse calculation grid, which becomes important in regions with a high dose gradient. If the reconstruction process of source and dose-point positions from radiographs was included as it usually is in practice, then the accuracy became worse and the planning system that showed large errors before, gave unacceptable errors of 30% to 40% in the rectal area. This is due to additional errors introduced in the reconstruction. The intercomparison showed that it is very important that computer treatment planning systems for brachytherapy calculations are tested before clinical use. The reconstruction technique for source and dose-point positions should be part of this test. Radiographs should be made of a phantom that simulates the cervix application, and the radiography technique should be similar to the one that is clinically employed.

An inverse filter for digital restoration of portal images

Portal images obtained of patients during irradiation with high energy photon beams have a poor sharpness and contrast due to the physical limitations of the equipment forming these images. Consequently, improvement of the images is desirable. An inverse filter method was investigated for this purpose and values of the parameters of the filter have been determined for various clinical irradiation conditions. Data on the line spread function (LSF) were obtained from the image of a 0.2 mm wide slit between tungsten blocks that were positioned at the isocentre in front of a polystyrene phantom. Slit images were made in a 60Co beam and in a number of 8 and 16 MV x-ray beams with a copper screen-film detector. These images were made for various isocentre to detector distances, field sizes and phantom thicknesses. All slit images and a number of clinical images were digitised both with a TV camera system and a densitometer which were connected to a computer. The main factors that affect the LSF are the radiation source size and the isocentre to film distance. The photon energy had only a minor influence. The thickness of the phantom and the field size, however, did influence the contrast in the image and therefore they determined, together with the digitising system, the noise to signal ratio parameter of the inverse filter to a great extent. Application of the inverse filter improved significantly the visual appearance of anatomical details in the portal images. In addition to image restoration, the contrast was enhanced by the application of a low frequency cut-off filter.

A matrix ionisation chamber imaging device for on-line patient setup verification during radiotherapy

It is very important to have a daily verification of patient setup during radiotherapy. Therefore, we have developed an on-line imaging device for high energy photons. It consists of a matrix of 128 x 128 liquid filled ionisation chambers and has a field of view of 320 mm x 320 mm. This device has an extremely flat cassette-like housing for easy handling and for application with existing radiotherapy equipment. A dedicated microcomputer is used to measure the currents of the 16384 ionisation chambers at high speed. The same computer is used to restore and process the images. With an imaging time of 3.1 s, an image quality comparable to film is obtained. Images of high and low contrast phantoms and of patients are presented. With this device, high quality portal images will be available within only a few seconds after the start of the treatment. This allows an almost instantaneous decision on the approval of patient setup. In addition, it enables observation of organ or patient motion during a single treatment. Analysis of these images at high speed will be an interesting new area of research.

First clinical experience with a remote afterloading system for low dose rate interstitial breast implants

A new remote afterloading system, developed for low or medium dose rate brachytherapy has been clinically evaluated. With this apparatus, the Micro-Selectron, up to 15 192Ir wires or seeds encapsulated in a plastic tubing can be loaded simultaneously into implants. Improved radiological protection for the medical staff has now been achieved with this apparatus. One hundred and sixty patients have been treated so far with breast implants using remote afterloading. The mean error frequency over the last year was about one error every 10 treatments of which about half could be considered as machine failures. All treatments could, however, be finished by using spare channels of the afterloader. We have found that a set of 45 source assemblies is adequate for the loading of the breast-needle implants performed in our institute. Every 6 to 8 weeks a new set of 192Ir sources is prepared.

A comparison of build-up and depth dose characteristics of different photon beams for the treatment of Hodgkin's disease

Measurements have been performed of build-up and depth-dose characteristics of photon beams under Hodgkin's disease treatment conditions as applied in two hospitals (AVL, Amsterdam and IGR, Villejuif). Although different types of accelerators, photon energies, field sizes and SSD are employed, similar dose distributions along the beam axis have been obtained in both centers. In order to explain this unexpected good agreement, the influence of the geometrical conditions of irradiation on the build-up and depth-dose distribution has been studied in detail for five photon beams (8 MV-25 MV) of three types of accelerators.

A liquid ionisation detector for digital radiography of therapeutic megavoltage photon beams

Experiments with an ionisation detector were performed in order to determine whether it was possible to obtain high energy photon beam images for radiotherapy treatment verification. A small prototype detector with a field of view of 78 mm X 78 mm and constructed from printed circuit boards was used. The imaging area was a matrix ionisation chamber, filled with air or liquid (2,2,4-trimethylpentane). A minicomputer was used to control the data acquisition electronics and to reconstruct and restore the images. The images were displayed on a viewing console for computed tomography images. The liquid filled detector with a front-rear board separation of 1.0 mm gave the best results. The spatial resolution was about 3.8 mm with a density resolution of 0.5% for a data acquisition time of 120 s. Comparison of the liquid detector images with corresponding metal screen-film detector images showed that the image qualities were the same. An important advantage of the ionisation detector image is that grey scale modification, sharpening and smoothing by digital processing can easily be performed.

Digital processing of high-energy photon beam images

The potentialities of digital megavoltage radiation field images for image quality improvement and evaluation of patient treatment setup errors are investigated. A treatment verification film is digitized with a densitometer and a treatment planning computer. The 256 X 256 picture matrix is displayed on a viewing console of a computer tomograph. The results of some processed pictures show an improved visibility of anatomical structures, which is of importance for comparison of the treatment beam images with the corresponding localization radiograph made on the simulator. In addition, automated techniques for comparison of irradiation and simulation setup are now possible.

Screens in ovoids of a Selectron cervix applicator

The addition of screens in the vaginal source holders of a cervix applicator for intracavitary brachytherapy reduces the dose to rectum and bladder and therefore diminishes the number of rectal and vesical complications. Shielding properties of tungsten rectal and bladder screens of a Selectron cervix applicator, loaded with spherical cesium sources, were determined for verification of dose calculations. Transmission characteristics of half-disk shaped tungsten screen segments in a single ovoid were measured in a water phantom. The minimum transmission ratios are 60, 70 and 80% for segment thickness of 5.0, 3.5 and 2.0 mm, respectively. The accuracy of the new screen correction algorithm of the Selectron Planning System was assessed by comparing measured and calculated dose rates and was found to be better than +/- 4%. The correction algorithm provides a method to analyse the efficacy of screens in the ovoids for various segment geometries and orientations without extensive phantom measurements. Isotransmission and isodose calculations were made for a typical clinical applicator set-up and source distribution. The dose reduction to rectum and bladder, near the bottom and top of the ovoids was analysed in detail. A 3.5 mm thick rectum and bladder screen in each ovoid reduces the dose approximately by 20% to the rectum and by 15% to the bladder. A distance enlargement of about 5 mm between ovoid and rectum or bladder, e.g. by packing, results in a comparable dose reduction. Shielding properties of a Selectron cervix applicator, provided with screens, were compared with those of some Fletcher-type applicators. Significant differences between the transmission ratios and shielded areas of the screens of both systems near rectum and bladder were observed.