Megavoltage cone beam digital tomosynthesis (MV-CBDT) for image-guided radiotherapy: a clinical investigational system.

Cone beam digital tomosynthesis (CBDT) is a new imaging technique proposed recently as a rapid approach for creating tomographic images of a patient in the radiotherapy treatment room. The purpose of this work is to investigate the feasibility of performing megavoltage (MV) CBDT clinically. A clinical investigational MV-CBDT system was installed on an existing LINAC. After the installation, the treatment machine can be operated in two distinct modes: (1) normal clinical treatment mode; (2) CBDT mode, in which tomographic images of the patient can be obtained using MV-CBDT. Various calibration and phantom measurements were performed on the system, followed by a patient study. Our phantom measurements have shown that: (1) for the same imaging dose, MV-CBDT has the same signal-difference-to-noise ratio as megavoltage cone beam computed tomography (MV-CBCT); (2) MV-CBDT has a better spatial resolution than MV-CBCT in the planes of reconstruction but a worse spatial resolution in the direction perpendicular to the planes of reconstruction. MV-CBDT patient images were also obtained and compared to that of MV-CBCT. We have demonstrated that it is clinically feasible to perform MV-CBDT in the treatment room for image-guided radiotherapy.

Imaging of 1.0-mm-diameter radiopaque markers with megavoltage X-rays: an improved online imaging system.

PURPOSE: To improve an online portal imaging system such that implanted cylindrical gold markers of small diameter (no more than 1.0 mm) can be visualized. These small markers would make the implantation procedure much less traumatic for the patient than the large markers (1.6 mm in diameter), which are usually used today to monitor prostate interfraction motion during radiation therapy. METHODS AND MATERIALS: Several changes have been made to a mirror-video based online imaging system to improve image quality. First, the conventional camera tube was replaced by an avalanche-multiplication-based video tube. This new camera tube has very high gain at the target such that the camera noise, which is one of the main causes of image degradation of online portal imaging systems, was overcome and effectively eliminated. Second, the conventional linear-accelerator (linac) target was replaced with a low atomic number (low-Z) target such that more diagnostic X-rays are present in the megavoltage X-ray beam. Third, the copper plate buildup layer for the phosphor screen was replaced by a thin plastic layer for detection of the diagnostic X-ray components in the beam generated by the low-Z target. RESULTS: Radiopaque fiducial gold markers of different sizes, i.e., 1.0 mm (diameter) x 5 mm (length) and 0.8 mm (diameter) x 3 mm (length), embedded in an Alderson Rando phantom, can be clearly seen on the images acquired with our improved system. These markers could not be seen on images obtained with any commercial system available in our clinic. CONCLUSION: This work demonstrates the visibility of small-diameter radiopaque markers with an improved online portal imaging system. These markers can be easily implanted into the prostate and used to monitor the interfraction motion of the prostate.

Rapid quantitation of plasma heparin and antithrombin III levels for cardiopulmonary bypass monitoring, using fluorometric substrate assays.

Plasma heparin and ATIII levels were determined using recently developed fluorometric substrate techniques on serial samples taken from 9 unselected patients undergoing open-heart surgery on cardiopulmonary bypass. The fluorometric assay for ATIII and established clotting and immunological methods showed a highly significant correlation (p less than 0.001). Heparin recovery was reduced in all cases, and plasma levels (fluorometric assay) demonstrated poor correlation to Whole Blood Activated Clotting Time (Hemochron) estimations. ATIII levels were dramatically reduced during bypass, and in 3 cases reached levels below 0.08 iu/ml. Heparin reversal schedules based upon empirical dosage led to excessive protamine administration by a mean factor of 3.3, as assessed by in vitro neutralization of standardized heparin concentrations.

Shielding effects of Selectron applicator and pellets on isodose distributions.

The Selectron is a mobile remote afterloading device which was developed specifically to reduce the radiation exposure to hospital personnel during intracavitary brachytherapy. Applicators for these devices contain source trains of 48 spherical pellets per channel, composed of both active and inactive pellets which have an external diameter of 2.5 mm. The configuration of pellets is chosen to optimize the dose distribution in accordance with the patient's geometry. A new dose calculation model has been proposed based on the separation of radiation into primary and secondary components. This model takes into account the attenuation of the primary radiation by the pellets. Isodose distributions around a Selectron standard applicator containing a typical patient loading of active pellets were measured and compared with those calculated by program SEEDS of the Atomic Energy of Canada Ltd. (AECL) Theraplan computer planning system. The shielding effects of the stainless steel applicator tubes, the ovoids, screws, inactive and active pellets will be considered. The effects of scatter and attenuation by the pellets will be illustrated by measured isodose distributions around a single applicator tube.

Megavoltage imaging with low Z targets: implementation and characterization of an investigational system.

The poor quality of stereotactic radiotherapy portal images is a limiting factor in precise image registration. To alleviate this problem, a low atomic number (Z) target was implemented on our Siemens MXE linear accelerator. This investigational system was used to assess the performance of various target materials by filming an aluminum contrast object. Beryllium, carbon and conventional target materials were studied. The bremsstrahlung spectra of these materials were simulated using Monte Carlo techniques. These spectra were used to calculate the dependence of narrow beam contrast on phantom thickness for verification of the data measured from film. A Monte Carlo simulation of the beryllium spectrum in a wide beam geometry was used to evaluate the effect of phantom-to-film distance on contrast. Although the same degree of contrast improvement with distance was not realized in practice, the improvement in image quality rivaled that achieved using a scatter reduction grid. A comparison of conventional localization images of the head and neck of an anthropomorphic phantom with images produced with a beryllium or carbon target and a mammography film and screen system supports earlier suggestions that the technique is clinically useful.

Absorbed dose perturbation caused by diodes for small field photon dosimetry.

Uncertainties in dose measurements which may be considered acceptable for standard radiotherapy techniques may not be acceptable for radiosurgery or other precision radiotherapy techniques. One example of this type of uncertainty are the small perturbations to measured dose distributions which are caused by the use of diode detectors. In this article the effect of using diodes to measure small field penumbra is assessed and comments are made about diode orientation and diode construction.

Measurements in high-intensity beams from medical linear accelerators.

There is a potential to produce very high doses and dose rates on dual mode (electron and x-ray capability) medical electron accelerators [C. J. Karzmark, Br. J. Radiol. 40, 697 (1967)]. Measurements were taken on two accelerators to determine the size of the doses which were possible. The experiments demonstrate the limitations of many of the commonly used dosimeters when they are used outside of the dose and dose rate ranges normally encountered in a radiotherapy department. Dose rates from 110 to 165 cGy/pulse were measured using ionization chamber, ferrous sulphate and thermoluminescent dosimetry, and water calorimetry.

Characteristics of the photon beam from a new 25-MV linear accelerator.

The Therac 25 is a relatively compact therapy machine, the heart of which is a double-pass electron linear accelerator. The electron beam is injected into the accelerator at the treatment head end of the machine and is accelerated back down the arm to an energy of 13 MeV. At this end of the machine a magnet system reflects the beam back into the structure where it gains up to an additional 12 MeV of energy. After leaving the linear accelerator the beam is bent by an achromatic head magnet through 270 degrees to the treatment head. The machine produces eight electron beams and a 25-MV photon beam. In this work only the parameters of the photon beam are addressed based on measurements at the first two clinical sites. Percentage depth doses, tissue phantom ratios, and beam symmetry and stability are presented and discussed.

A QA phantom for dynamic stereotactic radiosurgery: quantitative measurements.

A spherical acrylic phantom was designed for quality assurance measurements of dynamic radiosurgery. The phantom consists of two mating hemispheres mounted on a base plate. The interhemispheric plane may be oriented at any angle to the base, the angle being identified by visible marks on the base plate of the phantom. The phantom has a set of replaceable, radiologically identifiable markers, suitable for Computed Tomography (CT), Magnetic Resonance (MR), and Digital Subtraction Angiography (DSA) imaging. The frame coordinates of each marker are calculated from its known positions with respect to the center of the sphere. The measured errors of these positions using CT and MR images, were within the voxel size of the displayed image, while for DSA images the error was greater than 2.5 mm at the periphery of the image. The calculated depths from the planning software, for various beam intersection points to the isocenter, agreed within 0.6 mm with the known depths. A variation of 3.6 +/- 2.6 mm in the calculated depths was observed between using MR and CT image data. This difference results in a 1% variation in Tissue Maximum ratio (TMR) calculations. Comparisons of measured and known volumes resulted in differences of 8%-10%.

The use of radiochromic film in treatment verification of dynamic stereotactic radiosurgery.

Standard silver-based films are usually too sensitive to be used as direct indicators of dose in dynamic radiosurgery because of optical saturation. This paper describes the use of a new radiochromic film to measure 6-MV radiosurgery doses and dose distributions in a head phantom. Dose calibration of the radiochromic film was performed in the range of 2.3-50.2 Gy using light of 632- and 530-nm wavelengths. Radiosurgery dose distributions were measured using the radiochromic film in a head phantom undergoing the same treatment as a patient, and were compared with the planned distributions. For an example case (nominal 2.0-cm-diam cone), film measurement verified the calculated dose distribution in one plane. The simple measurement technique described led to experimental uncertainties of +/- 0.1 cm for the 90% and 50% isodose lines, +/- 0.3 cm for the 20% line, and +/- 0.5 cm for the 10% line. Isocenter dose was measured with an uncertainty of +/- 3%. Refinements to the technique should allow more precise measurements. It is concluded that the radiochromic film, with some limitations, is a convenient and useful tool for dynamic radiosurgery quality assurance.

Radiosurgery with unflattened 6-MV photon beams.

One of the major drawbacks to doing stereotactic radiosurgery with a linear accelerator is the long time required to deliver the target dose. Single fractions of 25 Gy delivered at the isocenter and at depth in the skull may require beam times in excess of 15 min for a typical linear accelerator with a maximum dose rate of 250 cGy/min in tissue. In an effort to decrease the treatment time for this technique, the flattening filter has been removed from an AECL Therac-6 linear accelerator and the characteristics of the resulting beam have been measured. Flatness is acceptable for the field sizes used with this technique and the dose rate is increased by a factor of 2.75.

Engineering modifications for dynamic stereotactically assisted radiotherapy.

To perform dynamic stereotactic radiotherapy, a number of modifications to the commercial linear accelerator were required and these are described in this report. A couch drive system was added to rotate the couch in synchrony with the gantry. A high dose rate beam for this technique is acquired through a head modification, which allows easy removal of the radiation field flattener. Because of these modifications, a number of changes and additions were required to the linac interlock system, including a complete new chain of interlocks for use with the stereotactic technique.

Fluorescent screen video imaging for checking linac beams in dynamic stereotactic radiosurgery.

Precise beam targeting is crucial to stereotactic radiosurgery. A monitoring system is described consisting of a fluorescent screen, video camera, and computer interface. Approximately ten frames are analyzed each second, verifying the beam intensity, uniformity, position, and diameter. When mounted on the gantry, the system can indicate the dynamic isocenter position using the "ball test" technique. The fluorescent screen video indicates that the 6-MV beam used for radiosurgery at our facility is acceptably stable; moreover, the small isocenter shift versus gantry and couch angles is reasonably reproducible. At our facility, quality assurance tests with this apparatus are performed every month.

Frameless stereotaxy for pre-treatment planning and post-treatment evaluation of radiosurgery.

In our centre, 111 patients have been treated with linear accelerator stereotactic radiosurgery. Angiographic, CT and MRI images are generated and the target coordinates calculated in 3 dimensions. For CT scanning, cross sections of perpendicular and oblique fiducial markers are seen. For follow-up CT scans done without the frame, a virtual frame is generated by means of a computer program that places fiducial markers on each CT scan cut, as if the patient had been wearing the OBT frame and the scan produced with the gantry parallel to the base of the frame. The position of the oblique marker may be calculated by knowing the thickness and position of each CT cut. Various natural fiducial markers (bony landmarks) are identified by coordinates in the scan with the patient wearing the real frame and in the scan with the virtual frame applied. A transformation matrix is utilized to establish the equivalence between the original CT scan with the real frame applied and subsequent scans without the real frame but with the virtual frame applied. In effect, the virtual frame is re-applied in exactly the same position as the real frame. Lesion measurements may then be duplicated and growth or regression accurately established. The uncertainty in this system of re-application residues in possible patient movement, CT scan slice thickness and inter-observer error in the identification of natural fiducial markers.

Implementation of multiple isocentre treatment for dynamic radiosurgery.

Radiosurgery using the dynamic rotation technique with a single isocentre was introduced at the Toronto-Bayview Regional Cancer Centre (T-BRCC) in 1988. Since then, over 100 patients have been treated. It was soon recognized that 25-30% of patients were referred with either non-spherical lesions or multiple lesions located sufficiently close together that consideration had to be given to the overlapping dose distributions throughout the treated volume. To treat these more complex targets a multiple isocentre technique was developed which also took account of these effects and the resulting normalization problem. This technique was implemented in September 1992. Comparisons between calculated doses and actual doses delivered have been undertaken using a spherical phantom containing radiochromic film. Measured dose distributions agreed with the planned distributions to within +/- 1 mm. The effect of multiple isocentres on the penumbra of dose distributions has been examined. The methods adopted for the normalization of treatment plans and clinical examples illustrating the application of the multiple isocentre technique are presented.

Frameless stereotaxy for radiosurgical planning and follow-up.

In our centre, 111 patients have been treated with linear accelerator stereotactic radiosurgery. Angiographic, CT and MRI images are generated and the target coordinates calculated in 3 dimensions. For CT scanning, cross sections of perpendicular and oblique fiducial markers are seen. For follow-up CT scans done without the frame, a virtual frame is generated by means of a computer program that places fiducial markers on each CT scan cut, as if the patient had been wearing the OBT frame and the scan produced with the gantry parallel to the base of the frame. The position of the oblique marker may be calculated by knowing the thickness and position of each CT cut. Various natural fiducial markers (bony landmarks) are identified by coordinates in the scan with the patient wearing the real frame and in the scan with the virtual frame applied. A transformation matrix is utilized to establish the equivalence between the original CT scan with the real frame applied and subsequent scans without the real frame but with the virtual frame applied. In effect, the virtual frame is re-applied in exactly the same position as the real frame. Lesion measurements may then be duplicated and growth or regression accurately established. The uncertainty in this system of re-application resides in possible patient movement, CT scan slice thickness and inter-observer error in the identification of natural fiducial markers.

Laparoscopic ovarian transposition.

Preservation of ovarian function is both safe and feasible in many young women with pelvic malignancies. Techniques utilized to transpose the ovaries to date have uniformly required a laparotomy either at the time of surgical treatment or as a separate operation in patients about to undergo pelvic radiotherapy. We report our preliminary results in 3 patients who underwent laparoscopic ovarian transposition and pelvic lymphadenectomy as part of an experimental protocol using intracavitary radiation alone in patients with small node negative stage 1B cervical carcinoma desiring preservation of fertility. Dose calculations were performed to estimate the amount of radiation each transposed ovary received from the intracavitary radiation, as well as the dosage that would have been received had external pelvic (4500 cGy) with or without para-aortic nodal irradiation (4500 cGy) been required. The mean estimated distance each ovary was transposed was 14.4 cm for the right ovary and 14.3 cm for the left ovary. Operative times ranged from 2.75-4.0 hours, and the blood loss 100-300 mls. Post-operative hospital stays ranged from 1-2 days, and no complications were encountered. Two of the 3 patients are menstruating regularly 25-32 months after completion of treatment with serum FSH in the normal premenopausal range. Based on the above distances, the mean dose of radiation each transposed ovary received was estimated to be 126 cGy, whereas the range in dosage of radiation each ovary would have received had external pelvic +/- para-aortic nodal irradiation been required was 135-190 cGy, and 230-310 cGy respectively. One patient has become menopausal after her transposed ovaries slipped back into the pelvis. Laparoscopic ovarian transpositions can be performed. This procedure is technically easy to perform for those surgeons skilled in laparoscopic surgery and its preliminary morbidity appears to be low. More experience, longer followup, and refinement in the methods of ovarian transfixation are required.

A head swivel technique for breast tangents.

Two of the major problems with treating breast tangents are the difficulties in simulating the treatment and the reproducibility of the setup on a day to day basis. This paper describes a simple technique which takes advantage of the head swivel capability of certain cobalt-60 units to allow simulation and treatment of the breast and chest wall based upon a setup at the field edges which are normally not on mobile breast tissue. The technique offers some of the advantages of half block techniques but does not require the use of half blocks or bridges.