Albert Y. C. Fung

Postimplantation dosimetric analysis of permanent transperineal prostate implantation: improved dose distributions with an intraoperative computer-optimized conformal planning technique.

PURPOSE To compare the target coverage and dose to normal tissues after I-125 transperineal permanent implantation (TPI) of the prostate in 90 patients treated with one of three different transperineal techniques. METHODS AND MATERIALS Detailed postimplant dosimetric evaluations of permanent I-125 implantation procedures were performed on 30 consecutive patients treated between 1995-1996 who underwent TPI using a preplanning CT-based technique, on 30 consecutive patients treated in 1997-1998 who underwent an ultrasound-guided approach with intraoperative determination of seed distribution based on an I-125 nomogram, and on 30 consecutive patients in 1998-1999 who underwent TPI with intraoperative computer-based 3-dimensional conformal optimization. For all three techniques, postimplant CT scans were obtained 4-6 hours after TPI. Dosimetric parameters included V(100), V(90), V(150), D(100), D(90), D(80), as well as maximal and average doses to the urethra and rectal wall. These parameter outcomes are reported as a percentage of the prescription dose. RESULTS The intraoperative 3D-optimized technique (I-3D) provided superior target coverage with the prescription dose for all dosimetric variables evaluated compared to the other treatment techniques. The median V(100), V(90), and D(90) values for the I-3D technique were 96%, 98%, and 116%, respectively. In contrast, the V(100), V(90), and D(90) values for the CT preplan and ultrasound manual optimization approaches were 86%, 89%, and 88%, respectively and 88%, 92%, and 94%, respectively (I-3D versus other techniques: p < 0.001). The superior target coverage with the I-3D technique was also associated with a higher cumulative implant activity required by the optimization program. A multivariate analysis determined that the treatment technique (I-3D versus other approaches) was an independent predictor of improved target coverage for each parameter analyzed (p < 0.001). In addition, higher cumulative implant activities and smaller prostate target volumes were independent predictors of improved target coverage. The maximum and average urethral doses were significantly lower with the I-3D technique compared to the other techniques; a modest increase in the average rectal dose was also observed with this approach. CONCLUSION Three-dimensional intraoperative computer optimized TPI consistently provided superior target coverage with the prescription dose and significantly lower urethral doses compared to two other techniques used. These data provide proof-of-principle that improved therapeutic ratios can be achieved with the integration of more sophisticated intraoperative planning for TPI and may potentially have a profound impact on the outcome of patients treated with this modality.

Commissioning and clinical implementation of a sliding gantry CT scanner installed in an existing treatment room and early clinical experience for precise tumor localization.

The primary objective of the present study is to demonstrate that a unique computed tomography (CT)-linear accelerator combination can be used to reduce uncertainties caused by organ motion and setup inaccuracy. The acceptance, commissioning, and clinical implementation of a sliding gantry CT scanner installed in an existing linear accelerator room are reported in this paper. A Siemens CT scanner was installed directly opposite to an existing accelerator. The scanner is movable on a pair of horizontal rails mounted parallel to the longitudinal axis of the treatment couch replaced with a carbon fiber tabletop. Acceptance and commissioning of the CT scanner were verified with phantom studies. For clinical implementation, quality assurance (QA) procedures have been instituted to ensure the integrity of the CT gantry axis alignment and the accuracy of its movement using a phantom designed in house. A clinical example employing the CT-Linac combination to correct the isocenter positioning caused by organ motion and setup inaccuracy was presented for a prostate irradiation. Dose calculations were performed to study the effects on tumor coverage without the adjustments of the isocenter. A summary of the isocenter adjustments for the first 30 patients is also presented. The geometric accuracy of the CT scanner is ≤1 mm. An isocenter deviation of ≥2 mm from the original plan can be detected. For the clinical example of a prostate patient, the average movement of the prostate gland was found to be ∼3mm in the anterior-posterior (AP/PA) direction and 5 mm in the cephalic-caudal direction. Variations in the isocenter position may result in underdosage of the PTV if correction is not made for the change in the isocenter position. Our experience with the first 30 patients indicates that while the left-right adjustment of the isocenter is minimal, in the AP/PA direction, about 33% of treatments required an adjustment of 3–5 mm, and about 18% required a 5.1-mm to 10-mm adjustment. In the caudal-cephalic direction, about 26% required an adjustment of 3–5 mm, and 8% required a 5.1-mm to 10-mm adjustment. Retrofitting a CT scanner in an existing linear accelerator room requires careful planning and well-coordinated efforts from all personnel involved. Special QA procedures are needed to ensure the mechanical integrity and imaging accuracy of the CT scanner. A CT scan of the patient prior to irradiation provides valuable information on organ motion. Any deviations from treatment plan can be corrected before dose delivery. Significant deviation from the planning isocenter may occur due to daily variations in the rectal filling. The CT-Linac combination has significant implications for the treatment of prostate cancer.

The measurement of three dimensional dose distribution of a ruthenium-106 ophthalmological applicator using magnetic resonance imaging of BANG polymer gels.

The BANG (MGS Research Inc., Guilford, CT) polymer gel has been used as a dosimeter to determine the three‐dimensional (3D) dose distribution of a ruthenium‐106 (Ru‐106) ophthalmologic applicator. An eye phantom made of the BANG gel was irradiated with the Ru‐106 source for up to 1 h. The phantom and a set of calibration vials were scanned simultaneously in a GE 1.5 T MR imager using the Hahn spin‐echo pulse sequence with a TR of 2000 ms and two TEs of 20 ms and 100 ms. The T2 values were evaluated on a pixel‐by‐pixel basis using custom‐built software on a DEC alpha workstation and converted to dose using calibration data. Depth doses and isodose lines of the Ru‐106 eye‐plaque were generated. It is concluded that the BANG gel dosimetry offers the potential for measuring the 3D dose distributions of an ophthalmologic applicator, with high spatial resolution and relatively good accuracy. PACS number(s): 87.66.–a, 87.90.+y

Computed tomography localization of radiation treatment delivery versus conventional localization with bony landmarks

A computed tomography (CT) scanner was installed in the linear accelerator room (Primatom) at Morristown. Since June 2000, we have been providing prostate, lung, and liver cancer patients with fusion of CT and linac radiation treatment. This paper describes our registration methods between planning and treatment CT images, and compares treatment localization by CT versus conventional localization by bony landmarks such as portal imaging. For image registration, we printed out beforehand the beams eye view of the treatment fields. Prostate tumor volume from each Primatom CT slice was mapped on the printouts, and the necessary isocenter shift relative to the skin marks was deduced. No port film was necessary for our Primatom patients. For ten patients we generated digitally‐reconstructed radiographs (DRRs) with bone contrast from the CT scans, and deduced the required shift as the difference between the DRRs of the Primatom CT versus the planning CT This represented the best observable shift should portal imaging be employed. Shift from bony landmark significantly correlated with the Primatom CT shift. Positioning adjustment based on bony anatomy was generally in the same direction as the CT shift for individual patient, but frequently did not go far enough. Our study confirmed that prostate organ motion relative to the bones has an average length of 4.7 mm (with standard deviation of 2.7 mm), and indicated the superiority of CT versus conventional bony structure (such as portal imaging) localization. PACS number(s): 87.53.Kn, 87.53.–j

Automated planning volume definition in soft-tissue sarcoma adjuvant brachytherapy

In current practice, the planning volume for adjuvant brachytherapy treatment for soft-tissue sarcoma is either not determined a priori (in this case, seed locations are selected based on isodose curves conforming to a visual estimate of the planning volume), or it is derived via a tedious manual process. In either case, the process is subjective and time consuming, and is highly dependent on the human planner. The focus of the work described herein involves the development of an automated contouring algorithm to outline the planning volume. Such an automatic procedure will save time and provide a consistent and objective method for determining planning volumes. In addition, a definitive representation of the planning volume will allow for sophisticated brachytherapy treatment planning approaches to be applied when designing treatment plans, so as to maximize local tumour control and minimize normal tissue complications. An automated tumour volume contouring algorithm is developed utilizing computational geometry and numerical interpolation techniques in conjunction with an artificial intelligence method. The target volume is defined to be the slab of tissue r cm perpendicularly away from the curvilinear plane defined by the mesh of catheters. We assume that if adjacent catheters are over 2r cm apart, the tissue between the two catheters is part of the tumour bed. Input data consist of the digitized coordinates of the catheter positions in each of several cross-sectional slices of the tumour bed, and the estimated distance r from the catheters to the tumour surface. Mathematically, one can view the planning volume as the volume enclosed within a minimal smoothly-connected surface which contains a set of circles, each circle centred at a given catheter position in a given cross-sectional slice. The algorithm performs local interpolation on consecutive triplets of circles. The effectiveness of the algorithm is evaluated based on its performance on a collection of soft-tissue sarcoma tumour beds within various anatomical structures. For each of 15 patient cases considered, the algorithm takes approximately 2 min to generate the planning volume. Although the tumour shapes are rather different, the algorithm consistently generates planning volumes that visually demonstrate smooth curves compactly encapsulating the circles. This general-purpose contouring algorithm works well whether the catheters are all close together, spread far apart in the plane or arranged in a convoluted way. The automatic contouring algorithm significantly reduces labour time and provides a consistent and objective method for determining planning volumes for soft-tissue sarcoma. Further studies are needed to validate the significance of the resulting planning volumes in designing treatment plans and the role that sophisticated brachytherapy treatment planning optimization may have in producing good plans.

From track structure to stochastic chemistry and DNA damage: Microdosimetric perspective

The effect of all types of ionizing radiations on higher organisms is nonspecific in the sense that all interactions occur through the agency of ionization and excitation processes. This, and the relative constancy of the amount of energy required to induce such processes, has led to the concept of absorbed dose as a quantifier for the amount of radiation delivered. However, equal doses of different radiations have different effects depending on the stopping power of the charged particles and on the temporal pattern of irradiation. Because individual energy transfers depend on neither one of these factors, it follows that the biological effectiveness of ionizing radiation depends on their spatial and temporal configuration. Microdosimetry is the study of the distribution in space and time of elementary energy deposits and their relation to subsequent damage. We discuss physico-chemical events that occur within the first microsecond following the interaction of charged particles with deoxyribonucleic acid (DNA) and argue that this particular time interval is uniquely important for understanding the biological effectiveness of radiation. Radiation biologists distinguish between direct hits and damage induced indirectly by radicals produced in the condensed medium surrounding the DNA target. The interaction and diffusion of these radicals (primarily OH) are described with the techniques of stochastic chemistry because—unlike “regular” chemistry—their initial spatial distribution is highly nonuniform. The information thus obtained is usually summarized in terms of proximity functions or microdosimetric distributions. The ultimate object of such studies is to obtain information on specific DNA alterations (e.g., strand breaks) or chromosomal damage and correlate them to such events as mutagenesis and carcinogenesis.

TREATMENT-PLAN OPTIMIZATION FOR SOFT-TISSUE SARCOMA BRACHYTHERAPY USING A GENETIC ALGORITHM

PURPOSE To describe a treatment-plan optimization system for temporary implant of soft-tissue sarcomas using a genetic algorithm, and evaluate its potential advantages over manual planning. METHODS AND MATERIALS A planning system that optimizes the distribution of radioactive seeds needed for adequate coverage of the target in the treatment of soft-tissue sarcomas has been designed and implemented. The treatment-planning procedures include simulation, film digitization, target-volume definition, optimized planning, and plan evaluation. The input to the optimization program consists of seed coordinates reconstructed from isocentric films, prescription points, and a list of available seed activities. The optimization is performed using a genetic algorithm. RESULTS Case studies are presented, which compare plans generated by computer optimization or by trial and error (manually). As expected, computer-optimized plans are often (but not always) superior to manual plans. This is particularly evident for situations where (unavoidably) catheters are far apart or irregularly spaced, in which case the advantages of optimized planning in terms of tumor coverage can be quite dramatic. When the target volume is well contained, the optimized plan minimizes the dose to normal tissue. Computer-based optimization has the additional advantage of being much faster than manual planning; this is valuable because it often reduces the total time the patient will spend in the hospital before implantation. CONCLUSION Optimized planning with a genetic algorithm and seeds of different activities significantly improves planning efficiency and generally results in improved plan quality. The utility of this optimization system is not limited to sarcoma implants.

Optimal needle arrangement for intraoperative planning in permanent I-125 prostate implants.

One limitation of intraoperative planning of permanent prostate implants is that needles must already be in the gland before planning images are acquired. Improperly placed needles often restrict the capability of generating optimal seed placement. We developed guiding principles for the proper layout of needles within the treatment volume. The Memorial Sloan-Kettering Cancer Center planning system employs a genetic algorithm to find the optimal seed implantation pattern consistent with pre-assigned constraints (needle geometry, uniformity, conformity and the avoidance of high doses to urethra and rectum). Ultrasound volumes for twelve patients with 1-125 implants were used to generate six plans per patient (total 72 plans) with different needle arrangements. The plans were evaluated in terms of V100 (percentage prostate volume receiving at least the prescription dose), U135 (percentage urethra volume receiving at least 135% of prescription dose), and CI (conformity index, the ratio of treatment volume to prescription dose volume.) The method termed POSTCTR, in which needles were placed on the periphery of the largest ultrasound slice and posterior central needles were placed as needed, consistently gave superior results for all prostate sizes. Another arrangement, labelled POSTLAT, where the needles were placed peripherally with additional needles in the posterior lateral lobes, also gave satisfactory results. We advocate two needle arrangements, POSTCTR and POSTLAT, with the former giving better results.

Accuracy in catheter reconstruction in computed tomography planning of high dose rate prostate brachytherapy.

In high dose rate prostate brachytherapy, inadequate reconstruction of catheter geometry in treatment planning may result in erroneous dose delivery. Catheters may be digitized with: (1) Parallel reconstruction: digitized at only one point and assumed parallel and horizontal: (2) Straight reconstruction: digitized at both ends and assumed straight while at an angle: (3) Slice-by-slice reconstruction: digitized on all slices to obtain exact geometry. Our results show that individual catheters are often not parallel to each other, but fairly straight. Parallel reconstruction is the least accurate for dosimetric planning, while slice-by-slice reconstruction is time-consuming. Straight (two-point) reconstruction represents a balance between accuracy and efficiency.

The Syed temporary interstitial iridium gynaecological implant: an inverse planning system

Patients with advanced gynaecological cancer are often treated with a temporary interstitial implant using the Syed template and Ir- 192 ribbons at the Memorial Sloan-Kettering Cancer Center. Urgency in planning is great. We created a computerized inverse planning system for the Syed temporary gynaecological implant, which optimized the ribbon strengths a few seconds after catheter digitization. Inverse planning was achieved with simulated annealing. We discovered that hand-drawn target volumes had drawbacks; hence instead of producing a grid of points based on target volume, the optimization points were generated directly from the catheter positions without requiring an explicit target volume. Since all seeds in the same ribbon had the same strength, the minimum doses were located at both ends of the implant. Optimization points generated at both ends ensured coverage of the whole implant. Inverse planning took only a few seconds, and generated plans that provide a good starting point for manual improvement.