Purpose: The heart and lungs can be exposed to potentially significant doses during external beam, and interstitial or balloon based brachytherapy. In brachytherapy, where radiation fields drop rapidly with distance, there is a strong dependence on the location of the cavity with respect to the heart. Furthermore there is not unanimity nor a great deal of data on the effects of heart dose. However there is evidence from patients treated for Hodgkin's disease that there are significant long term (10 years or longer) effects. Thus it is of interest to attempt to quantify heart dose and explore any differences among the various options available for breast brachytherapy. While each case has its own details, a generalized model can be constructed to study global differences in the various treatment modalities available.

Purpose: To describe a new collimator design and methodology for controlling the effective intensity of radiation produced by a microminiature x-ray source which allows the modulation of dose distributions for electronic brachytherapy applications. Method and Materials: A close fitting, two-section adjustable lead collimator consisting of a lower cylindrical band and an upper cap was constructed for a 50kV electronic brachytherapy source (Xoft Inc., Fremont, CA) to provide controllable beam hardening and attenuation. Micro-indexing the source through an indwelling catheter using variable step sizes, in combination with adjustment of the collimator gap width, modifies the resulting depth-doses. Results: Depth dose increases with wider collimator aperture settings and for decreasing step sizes. For a multi-stepped plan using step sizes of 1mm, the relative dose at 1cm depth was 0.50 for a gap of 0.5mm and 0.74 for a gap of 2.0mm. For a multi-stepped plan with a constant 2mm aperture size, the relative dose at 1cm depth increased from 0.70 for a step size of 2.5mm to 0.74 for a step size of 1.0mm. Conclusion: Appropriate depth dose adjustment by microindexing and variable collimation of an electronic brachytherapy source facilitates the development of IMEB plans that can be made to conform to irregularly shaped tumor cavities or those closely bounded by critical structures. Conflict of Interest: Partial financial support was provided by Xoft, Inc.

Purpose: Conformal and intensity-modulated radiotherapy (IMRT) plans for 9 patients were compared based on characterization of plan quality and effects on the oncology department.Methods and Materials: These clinical cases, treated originally with conformal radiotherapy (CRT), required extraordinary effort to produce conformal treatment plans using nonmodulated, shaped noncoplanar fields with multileaf collimators (MLCs). IMRT plans created for comparison included rotational treatments with slit collimator, and fixed-field MLC treatments using equispaced coplanar, and noncoplanar fields. Plans were compared based upon target coverage, target conformality, dose homogeneity, monitor units (MU), user-interactive planning time, and treatment delivery time. The results were subjected to a statistical analysis.Results: IMRT increased target coverage an average of 36% and conformality by 10%. Where dose escalation was a goal, IMRT increased mean dose by 4-6 Gy and target coverage by 19% with the same degree of conformality. Rotational IMRT was slightly superior to fixed-field IMRT. All IMRT techniques increased integral dose and target dose heterogeneity. IMRT planning times were significantly less, whereas MU increased significantly; estimated delivery times were similar.Conclusion: IMRT techniques increase dose and target coverage while continuing to spare organs-at-risk, and can be delivered in a time frame comparable to other sophisticated techniques. (C) 2000 Elsevier Science Inc.

PURPOSE:The goal of modulated-beam conformal therapy is to reduce the dose to healthy tissue and sensitive structures around a uniformly irradiated target volume. Multiple intensity-modulated fields offer improved tissue-sparing dose distributions. New computer-based systems for planning and delivering such treatments may soon be available from different commercial sources that will make the formulation of an intensity-modulated treatment plan and its execution widely available at any treatment facility that has the resources to acquire the necessary equipment. This work reports on a study of the integration of two such systems. METHODS AND MATERIALS:Treatment planning was done using a commercially available inverse planning algorithm based on simulated annealing. The plans arbitrarily assumed nine coplanar x-ray beams at nonopposed gantry angles. Intensity modulation was computed for each beam. The modulated field at each gantry angle was broken down into a series of uniform (nonmodulated) subfields, which could be delivered as a sequence to produce the desired dose distribution. Because a large number of subfields was delivered, a multileaf collimator (MLC) was used for field shaping. This allowed rapid and accurate field shaping for treatments made up of several hundred subfields. Computer control of the MLC and linear accelerator allowed delivery of doses less than .01 Gy per subfield. Treatment was delivered on a prototype, computer-controlled accelerator and MLC system. Resulting dose distributions were analyzed using film and an anatomically specific, homogeneous phantom. RESULTS:The treatment plans were evaluated using dose-volume histogram analysis. The plans provided acceptably uniform irradiation of the target volume without exceeding dose tolerances for nearby critical structures. The plans were successfully delivered by a prototype dynamic MLC. The time needed to deliver a sequence of subfields at one gantry angle ranged from 0.7 to 2.0 min. Isodoses from film agreed reasonably well with planned isodose distributions. CONCLUSIONS:It is feasible to plan and deliver fixed gantry, modulated-beam conformal therapy for head and neck tumors with systems being developed commercially. The planned dose distributions exhibit significant potential for sparing closely spaced normal tissue structures in the head and neck.

Peacock is a 3-D conformal treatment planning and delivery system for conformal radiation therapy which delivers intensity-modulated fields. A group of 13 patients were treated between March and February, 1995. Patient age ranged from 10 to 74. Six of the patients had previously received radiation therapy. Target volume was from 2.5 to 70 cm3: all treatments were fractionated, using single table angle plan with 270 degrees of rotation. A removable invasive cranial screw fixation device was used in all cases. Treated isodose line averaged 85%; conformality index was better than 2, with 0-5% of critical structure volume exceeding dose limits. Setup time averaged 8 min per fraction: treatment time ranged from 10 to 80 min. There were two equipment failures in 200 fractions/1,000 gantry rotations. Patient position averaged about 1 mm within initial setup. Follow-up has revealed no complications from the radiation.

Purpose: To compare the stereotactic radiosurgery treatment plans generated by a conventional radiosurgery treatment system with the plan generated by a system using intensity modulated beams.Methods and Materials: Optimized conformal radiation treatment plans were generated for both single and multiple intracranial lesions using a conventional radiosurgery treatment-planning system computer and the Peacock treatment-planning computer, The Peacock system is a conformal therapy system that uses intensity modulated beams, back projection, and the simulated annealing optimization technique, The dose delivered to critical structures and the target volume were compared by means of dose volume histograms between plans generated by the two different systems, The Radiation Therapy Oncology Group (RTOG) stereotactic radiosurgery criteria were also used to evaluate each plan.Results: (a) For a single small target, radiosurgery plans generated by the conventional radiosurgery system and the Peacock system were comparable, (b) For two separate small targets, where nonoverlapping arcs could be used, plans generated by the two systems were also comparable, (c) For a single large (>4 cm) irregular-shaped target, the Peacock system appeared to be able to generate a treatment plan superior to that of the conventional radiosurgery system.Conclusions: A treatment plan generated using intensity modulated beams appears to be superior to a multiple isocenter plan using a conventional radiosurgery system, for the treatment of a large irregular shaped intracranial target.

PURPOSE:Intensity modulated beam systems have been developed as a means of creating a high-dose region that closely conforms to the prescribed target volume while also providing specific sparing of organs at risk within complex treatment geometries. The slice-by-slice treatment paradigm used by one such system for delivering intensity modulated fields introduces regions of dose nonuniformity where each pair of treatment slices abut. A study was designed to evaluate whether or not the magnitude of the nonuniformity that results from this segmental delivery paradigm is significant relative to the overall dose nonuniformity present in the intensity modulation technique itself. An assessment was also made as to the increase in nonuniformity that would result if errors were made in indexing during treatment delivery.METHODS AND MATERIALS:Treatment plans were generated to simulate correctly indexed and incorrectly indexed treatments of 4, 10, and 18 cm diameter targets. Indexing errors of from 0.1 to 2.0 mm were studied. Treatment plans were also generated for targets of the same diameter but of lengths that did not require indexing of the treatment couch.RESULTS:The nonuniformity that results from the intensity modulation delivery paradigm is 11-16% for targets where indexing is not required. Correct indexing of the couch adds an additional 1-2% in nonuniformity. However, a couch indexing error of as little as 1 mm can increase the total nonuniformity to as much as 25%. All increases in nonuniformity from indexing are essentially independent of target diameter.CONCLUSIONS:The dose nonuniformity introduced by the segmental strip delivery paradigm is small relative to the nonuniformity present in the intensity modulation paradigm itself. A positioning accuracy of better than 0.5 mm appears to be required when implementing segmental intensity modulated treatment plans.

A stereotactic system has been designed based upon a series of interlocking discs secured to the skull with self-tapping screws. Unlike previous skull-mounted systems, this system is a true, advanced imaging based stereotactic device with the capabilities and accuracy of more traditional, frame based devices. It has been used in a range of applications, from simple biopsies to interstitial radiation implant procedures. Well tolerated by the patient, it allows reaccess to the intracranial target without rescanning. It is convenient for the physician to utilize, both mechanically and timewise, is adaptable to MRI, DSA, and conventional X-ray techniques without modification, and is affordable.