Peter K. Kijewski

Analysis of prostate and seminal vesicle motion: Implications for treatment planning

PURPOSE To quantify prostate and seminal vesicle positional changes (target motion) between treatment planning and delivery, and to identify the factors contributing to target motion. METHODS AND MATERIALS Thirty patients with adenocarcinoma of the prostate were prospectively evaluated by analyzing two sequential planning computerized tomography (CT) scans (S1, obtained prior to treatment, and S2, obtained during the fourth week of treatment) for each patient. All anatomical volumes of interest (soft tissue and bony) were reconstructed from transverse CT images and projected onto anterior and lateral beams-eye view projections. Positional changes between S1 and S2 were eliminated by applying a rigid body translation and rotation. Target motion was then measured by recording the positional change between S1 and S2 at the edges (right, left, superior, inferior). Potential correlation of target motion with bladder volume, rectal volume, and rectal diameter changes were evaluated by linear regression analysis. RESULTS Neither the prostate nor seminal vesicles remained fixed with respect to bony anatomy between S1 and S2. The distribution of positional changes were generally small (< 0.5 cm), but maximum displacements of 1.5-2.2 cm did occur, particularly in the lateral view. In this study, bladder volume changes between the scans were small and did not correlate with target motion (P = 0.67). Both rectal volume and rectal diameter changes correlated with target motion for both the prostate (p = 0.004 and 0.005, respectively) and seminal vesicles (p < 0.001 and < 0.001, respectively). However, neither the initial rectal volume nor the initial rectal diameter could be used to predict subsequent target motion when evaluated either singly or as part of a multiple regression model. CONCLUSIONS Target motion occurs during the course of treatment planning and delivery and should be considered when designing conformal radiation fields. Although the target position at the time of planning CT may differ substantially from the mean treatment position, target motion cannot be predicted by evaluating simply measured parameters from a single scan, or double scan sequence.

Acetabular Dysplasia in the Adolescent and Young Adult

Hip dysplasia is a major cause of osteoarthrosis in adults. Early aggressive osteotomy has the potential of preventing the development of arthritis, but carries with it significant risks. The problem is further complicated because the surgeon has no means of quantifying the dysplastic deformity or of predicting what a particular combination of osteotomies would do to correct the deformity. This study describes methods of quantifying hip-joint geometry in three dimensions based on computed tomography and magnetic resonance studies, and of simulating pelvic osteotomy to correct the deformities. The study analyzes 49 normal hip joints and 20 dysplastic hip joints. The results show that the normal acetabulum is nearly a full hemisphere, which is anteverted 20 degrees and abducted 53 degrees. The normal lateral center-edge angle is 37 degrees. The dysplastic acetabulum is not anterolaterally maldirected, as has been assumed, but is globally dysplastic. Analysis of the individual dysplastic hip joints showed a wide variability. Some patients were deficient globally, some anterolaterally, and some posterolaterally. Methods of analyzing a patients hip joint, quantifying abnormalities, simulating surgery, and predicting results are demonstrated in a case example.

Wedge-shaped dose distributions by computer-controlled collimator motion.

We have recently installed a linear accelerator, modified to allow computer control of several machine parameters during irradiation of the patient. As an initial feasibility study of computer-controlled radiation therapy, its application to produce wedge-shaped dose distributions by moving the collimator jaws has been evaluated. The required collimator motions have been calculated with an iterative technique. When these routines were used during irradiations of phantoms containing radiographic film, a good correspondence between calculated and measured dose distributions was observed. It is concluded that computer-controlled motion of the collimator jaws to shape the dose distribution is technically feasible. Additionally, this technique has the advantage that the wedge angle can be continuously adjusted and the isodose curves optimized for a particular depth and field size.

Computer-Controlled Radiation Therapy

Radiation therapy is often hampered in important body regions by the need to transit sensitive normal tissues which act as dose-limiting barriers. Computer-controlled radiation therapy permits the simultaneous variation of multiple treatment parameters during irradiation of the patient, producing improved dose distributions with the potential for improved local control. Equipment used for this purpose includes a Mevatron XII linear accelerator, redesigned for automatic control, and a PDP 11/45 minicomputer. Dose distributions are shown and potential clinical gains discussed.

Dose optimization with computer-controlled gantry rotation, collimator motion and dose-rate variation

The applications of a computer-controlled radiation therapy system to optimize dose distributions in two dimensions are explored. This study is limited to a target volume with constant cross-section along an axis parallel to the long axis of the patient. The machine components that are continuously varied during treatment are the dose rate, the gantry angle, and the four independent collimator jaws, two of which can cross the beam centerline. Basic control strategies, treatment planning and delivery techniques are illustrated with clinical examples. We conclude that the computer-controlled radiation therapy system can easily and reliably deliver dose distributions which are significantly better than those produced by conventional multiple-field techniques.

Radiosurgery for arteriovenous malformations of the brain using a standard linear accelerator: Rationale and technique

We have recently initiated a program for irradiating small, unresectable arteriovenous malformations (AVMs) in the brain. The treatments are delivered using a modified and carefully calibrated 6 MV linac. We are using high, single doses (15 to 25 Gy) with a goal of sclerosing the vessels and preventing hemorrhages. This technique, radiosurgery, is somewhat controversial in the radiotherapy community. Since the treatment is given in a single sitting, rather than in the more conventional pattern of multiple small daily fractions, there is some concern about late radiation damage to the normal brain tissue. However an extensive review of the literature leads us to the conclusion that if a technique is used that keeps the volume irradiated to high dose small, radiosurgery is a safe and efficacious treatment for small (less than 2.5 cm) AVMs. To decrease the risk of necrosis of normal brain tissue, it is important to confine the high dose region as tightly as possible to the target volume. Precise target localization and patient immobilization is achieved using a stereotactic head frame which is used during angiography, CT scanning, and during the radiation treatment. This minimizes the margin of safety that must be added to the target volume for errors in localization and set-up. The treatment is delivered using multiple noncoplanar arcs, with small, sharp edged X ray beams, and with the center of the AVM at isocenter. This produces a rapid dropoff of dose beyond the target volume. Early results in our first few patients are encouraging.

Automated quantification of body fat distribution on volumetric computed tomography.

Objective: To develop a computerized method to automatically quantify visceral and subcutaneous fat distribution within the abdomen and pelvis on volumetric computed tomographic (CT) images. Methods: Given the slices of interest, the algorithm automatically delineates a contour that separates the visceral fat from the subcutaneous fat on each slice. Explicitly, starting with extraction of the body perimeter, radii at a fixed angle increment are drawn from the perimeter to the center of the body. Along each radius, intensity profile is analyzed to determine the point on the subcutaneous fat layer that is closest to the body center (inner point). All inner points are then connected to form an inner contour, and a specific smoothing algorithm is subsequently applied to correct suboptimal results. Pixels having HU values between −190 and −30 are considered fat pixels. This procedure is repeated on each of the slices of interest. The visceral and subcutaneous fat volumes computed automatically were compared with those after the radiologists adjustments. Ratios of volumetric visceral fat-to-total fat and visceral fat-to-subcutaneous fat were compared on average and with single-slice measurements obtained at L4 and L5 vertebral body levels. Results: Subcutaneous and visceral fat were automatically segmented using this algorithm on 419 axial CT slices in 9 CT scans (patients) within the abdomen and pelvis. The overall average percentage difference between the automated segmentation and the segmentation edited by the radiologist were 1.54% for the visceral fat and 0.65% for the subcutaneous fat. Conclusions: Preliminary results have shown that total compartmental fat, including visceral and subcutaneous fat, can be automatically and accurately segmented on volumetric CT.

A computer-controlled radiation therapy machine for pelvic and para-aortic nodal areas

Abstract A computer-controlled radiation therapy technique has been developed to treat cancer of the uterine cervix that has extended to the pelvic and pare-aortic lymph nodes. During five longitudinal scans with a 4 cm wide 8 MV X ray beam, conformation of the high dose region to the target volume was achieved primarily by varying the other field dimensions. Treatment planning and dose calculation were performed in three dimensions. Conventional two-dimensional planning in a series of transverse planes through the patient was combined with a mathematical normalization of dose in the longitudinal direction to attain uniform dose throughout the target volume and to fulfill criteria for protection of critical organs. The resulting dose distributions were compared, in detail, with those resulting from conventional treatment techniques. It was concluded that the computer-controlled therapy scheme offered the potential of reducing the probability of complications, for the same target dose, by lowering the doses to small bowel, duodenum, kidneys, liver and spinal cord.

Correction for beam hardening in computed tomography

Corrections for beam‐hardening artifacts in computed tomography can be made by using a model which assumes that water and bone mineral are the only constituents of tissue. With this model, a correction factor for the measured transmission values can be calculated such that the reconstructed attenuation coefficients have values corresponding to a monoenergetic source of known energy. Systematic errors in the uncorrected attenuation coefficients, which may be 5%, can be reduced to less than 1% if corrected transmission values are used.

The planning of orthopaedic reconstructive surgery using computer-aided simulation and design

Three-dimensional reconstructions from computed tomographic (CT) images are currently being used clinically in a wide variety of orthopaedic surgical applications. The computer may be used to select the optimum standard artificial joint replacement or to design a custom artificial joint replacement for a particular patient. Where large bony defects exist, the computer may be used to design bone allografts for joint reconstruction and to manufacture models of the bones for use in planning the surgery. In cases where osteotomies are performed to improve the mechanics of the joint, each proposed osteotomy may be simulated on the computer to identify the surgical plan that will optimally normalize the diseased joint.