Dennis D. Leavitt

Dynamic field shaping to optimize stereotactic radiosurgery

A dynamic field shaping collimation system is evaluated for use in stereotactic radiosurgery of non-spherical lesions. The concept is as follows: (a) use the existing circular collimators to define a cone which encompasses the maximum dimensions of the target volume; (b) position two sets of independent rectangular photon collimators immediately upstream from the circular aperture and allow each collimator to have independent translational and rotational motion in order to define, for each increment of arc, a polygonal field shape having up to four straight and four curved edges which enscribe the beams eye projection of the target; (c) modify the translational and rotational position of each independent collimator with each change in arc angle to continuously shape the instantaneous field to the target shape. A prototype device has been constructed and uses vane control technology developed in a related research project in electron arc therapy. The efficacy of this device is illustrated by dose calculations and measurement based on actual clinical data. Dose volume histograms are used to compare the dose received by three techniques: single isocenter treatment using a single circular aperture, dual isocenter treatment, and single isocenter treatment using dynamically shaped fields. Doses were calculated throughout the brain using a volume grid of 3 mm spacing. Dose volume histograms comparing dose within the target volume and brain volume excluding target volume, as well as computed isodose distributions, demonstrate the possible reduction in normal tissue dose burden while simultaneously preserving dose uniformity throughout the prescribed target volume. This simple four-vane collimation system may provide a viable alternate treatment technique for non-spherical lesions.

Dynamic wedge field techniques through computer-controlled collimator motion and dose delivery

Clinical treatment planning situations arise which require different wedge angles within segments of a single therapeutic x-ray field. Idealized wedge-shaped dose distributions, including combination of several wedge segments of different angle within a single field, are generated and delivered through computer control of asymmetric collimator motion and dose per field segment. A dual-pass technique is introduced to provide improved adherence to the prescribed isodose distribution. Dynamic wedge distributions are verified by film densitometry and ionization chamber measurement. These results suggest the potential importance of this technique as an added clinical radiotherapy tool.

Electron arc therapy: Physical measurement and treatment planning techniques

An electron beam arc therapy technique has been developed for the treatment of the post-mastectomy chest wall using a clinical linear accelerator modified for arc therapy. The effects on the dose distribution of primary X ray collimators, secondary cerrobend blocks attached to the accelerator accessory tray, and tertiary cerrobend casting of the treatment area on the patients thorax have been investigated. Multiple electron energies within the same arc, variable rad per degree, and variable shaped secondary and tertiary applicators have been employed to optimize dose uniformity across the treated surface. A computerized treatment planning program has been developed to aid in visualization and optimization of dose distributions. A simple technique to estimate the width variation in the secondary collimator necessary to compensate for radial patient thickness changes in the cephalocaudad direction is described. Electron beam energies of 6 MeV, 9 MeV, 12 MeV, 15 MeV, and 18 MeV have been studied. The physical measurements needed to implement this technique are described, and a comparison of electron arc therapy dose distributions with other standard treatment techniques is presented.

Field edge smoothing for multileaf collimators

PURPOSE To smooth the scalloped dose pattern that occurs for stepped leaves at a treatment field edge defined by a multileaf collimator. METHODS AND MATERIALS Fields with centers shifted slightly in space were superimposed to blur the staggered dose distribution at the field edge. Film dosimetry was used to monitor changes. The dose distribution for a single field position was compared to the distribution for one and three shifts. Three depths were examined and divergent alloy blocks were included in the comparison. RESULTS The structure that appears at an edge for a single field when leaves are staggered was nearly eliminated when the field was shifted three times to give a total of four different positions. However, shifting the field one time so that two fields were superimposed gave an intermediate result with only slight improvement in the undulating dose distribution. For the four superimposed fields, the 50% isodose pattern converged to a smoothed line running along the center of the original undulating pattern. The 80 and 20% isodoses did not converge to the center of their scalloped patterns. Instead, these isodose lines were spread leaving a larger penumbra width than a divergent alloy block. CONCLUSIONS Shifting and adding fields is an effective method for smoothing the staggered dose distribution that results when the leaves of a multileaf collimator are stepped to form an irregular field pattern. However, the width of the penumbra for the combined fields is wider than the penumbra for a cerrobend block.

Electron arc therapy: chest wall irradiation of breast cancer patients

From 1980 to October 1985 we treated 45 breast cancer patients with electron arc therapy. This technique was used in situations where optimal treatment with fixed photon or electron beams was technically difficult: long scars, recurrent tumor extending across midline or to the posterior thorax, or marked variation in depth of target tissue. Forty-four patients were treated following mastectomy: 35 electively because of high risk of local failure, and 9 following local recurrence. One patient with advanced local regional disease was treated primarily. The target volume boundaries on the chest wall were defined by a foam lined cerrobend cast which rested on the patient during treatment, functioning as a tertiary collimator. A variable width secondary collimator was used to account for changes in the radius of the thorax from superior to inferior border. All patients had computerized tomography performed to determine Internal Mammary Chain depth and chest wall thickness. Electron energies were selected based on these thicknesses and often variable energies over different segments of the arc were used. The chest wall and regional node areas were irradiated to 45 Gy-50 Gy in 5-6 weeks by this technique. The supraclavicular and upper axillary nodes were treated by a direct anterior photon field abutted to the superior edge of the electron arc field. Follow-up is from 10-73 months with a median of 50 months. No major complications were observed. Acute and late effects and local control are comparable to standard chest wall irradiation. The disadvantages of this technique are that the preparation of the tertiary field defining cast and CT treatment planning are labor intensive and expensive. The advantage is that for specific clinical situations large areas of chest wall with marked topographical variation can be optimally, homogeneously irradiated while sparing normal uninvolved tissues.

Electron arc irradiation of the postmastectomy chest wall: clinical results

Abstract Background and purpose : Since 1980 electron arc irradiation of the postmastectomy chest wall has been the preferred technique for patients with advanced breast cancer at our institution. Here we report the results of this technique in 140 consecutive patients treated from 1980 to 1993. Materials and Methods : Thoracic computerized tomography was used to determine internal mammary lymph node depth and chest wall thickness, and for computerized dosimetry calculations. Total doses of 45–50 Gy in 5 to 5 12 weeks were delivered to the chest wall and internal mammary lymph nodes via electron arc and, in most cases, supraclavicular and axillary nodes were treated with a matching photon field. Patients were assessed for acute and late radiation changes, local and distant control of disease, and survival. Patients had a minimum follow-up of 1 year after completion of radiation treatment, and a mean follow up interval of 49 months and a median of 33 months. All patients had advanced disease: T stages 1, 2, 3, and 4 represented 21%, 39%, 21% and 19% of the study population, with a mean number of positive axillary lymph nodes of 6.5 (range, 0–29). Analysis was performed according to adjuvant status (no residual disease, n = 90), residual disease (positive margin, n = 15, and primary radiation, n = 2), or recurrent disease ( n = 33). Results : Acute radiation reactions were generally mild and self limiting. A total of 26% of patients developed moist desquamation, and 32% had brisk erythema. Actuarial 5 year local-regional control, freedom from distant failure, and cause-specific survival was 91%, 64%, and 75% in the adjuvant group; 84%, 50%, and 53% in the residual disease group; and 63%, 34%, and 32% in the recurrent disease group, respectively. In univariate Cox regressions, the number of positive lymph nodes was predictive for local failure in the adjuvant group ( P = 0.037). Chronic complications were minimal with 11% of patients having arm edema, 17% hyperpigmentation, and 13% telangectasia formation. Conclusion : These data demonstrate that local-regional control with electron arc therapy of the postmastectomy chest wall is comparable to photon techniques. Acute radiation reactions are well tolerated and mostly of minor extent. A previous report demonstrated a significant reduction in the dose-volume relationship of the lung using the electron arc compared with two photon techniques. Consequently, with careful attention to treatment planning and dosimetry, electron arc therapy of the postmastectomy chest wall is safe and effective. The radiation dose to heart and lung is minimized without compromise on local control.

Dose to the contralateral breast: A comparison of two techniques using the enhanced dynamic wedge versus a standard wedge

The dose to the contralateral breast has been associated with an increased risk of developing a second breast malignancy. Varying techniques have been devised and described in the literature to minimize this dose. Metal beam modifiers such as standard wedges are used to improve the dose distribution in the treated breast, but unfortunately introduce an increased scatter dose outside the treatment field, in particular to the contralateral breast. The enhanced dynamic wedge is a means of remote wedging created by independently moving one collimator jaw through the treatment field during dose delivery. This study is an analysis of differing doses to the contralateral breast using two common clinical set-up techniques with the enhanced dynamic wedge versus the standard metal wedge. A tissue equivalent block (solid water), modeled to represent a typical breast outline, was designed as an insert in a Rando phantom to simulate a standard patient being treated for breast conservation. Tissue equivalent material was then used to complete the natural contour of the breast and to reproduce appropriate build-up and internal scatter. Thermoluminescent dosimeter (TLD) rods were placed at predetermined distances from the geometric beams edge to measure the dose to the contralateral breast. A total of 35 locations were used with five TLDs in each location to verify the accuracy of the measured dose. The radiation techniques used were an isocentric set-up with co-planar, non divergent posterior borders and an isocentric set-up with a half beam block technique utilizing the asymmetric collimator jaw. Each technique used compensating wedges to optimize the dose distribution. A comparison of the dose to the contralateral breast was then made with the enhanced dynamic wedge vs. the standard metal wedge. The measurements revealed a significant reduction in the contralateral breast dose with the enhanced dynamic wedge compared to the standard metal wedge in both set-up techniques. The dose was measured at varying distances from the geometric field edge, ranging from 2 to 8 cm. The average dose with the enhanced dynamic wedge was 2.7-2.8%. The average dose with the standard wedge was 4.0-4.7%. Thermoluminescent dosimeter measurements suggest an increase in both scattered electrons and photons with metal wedges. The enhanced dynamic wedge is a practical clinical advance which improves the dose distribution in patients undergoing breast conservation while at the same time minimizing dose to the contralateral breast, thereby reducing the potential carcinogenic effects.

Electron arc therapy: design, implementation and evaluation of a dynamic multi-vane collimator system.

Innovative techniques in motion control technology have been applied to the design and implementation of a portable computer-controlled multi-vane collimator for use in electron arc therapy. The collimator, consisting of 18 independently controlled vanes, is inserted into the standard accessory mount assembly of a linear accelerator, in the same fashion as standard field shaping blocks. Power is supplied to the collimator vane motors via a self-contained battery system. The range of motion of the vanes, symmetrically mounted nine on each side, provides a variable aperture width projected to isocenter of 2 cm minimum to 8 cm maximum. The projected length of the aperture at isocenter is 38 cm. The transition time between vane positions is less than 1 second, corresponding to gantry movement of less than 1 degree. The movement of each of the 18 vanes is monitored and controlled by six individually addressed three axis processors that are shielded from the electron beam. A table of collimator vane positions versus gantry angle, as determined by dose optimization calculations, is stored in a data file. The desired collimator vane position corresponding to the current arc segment is conveyed from the control console to each vane controller via packets within a token passing network. Communication between the computer in the console area and the vane controllers is accomplished through encoded infra-red pulse transmission, eliminating the need for additional communication lines between the console and the accelerator. This dynamic collimator offers improved dose uniformity while simplifying the delivery of electron arc therapy.

Electron arc therapy: Clinical experience with chest and irradiation

In order to overcome the technical problems commonly encountered in fixed field photon and electron beam chest wall irradiation, we have treated the chest wall in 22 breast cancer patients with a moving electron beam that rotates about the patients thorax. This paper discusses the clinical results of chest wall treatment by electron arc therapy. Twenty-one patients were treated following mastectomy, 16 electively because of high risk for local failure, and 5 because of local recurrence. One patient with advanced local-regional disease was treated primarily. During a median follow-up period of 24 months there has been one chest failure in one of the patients treated for local recurrence. No major complications were observed in skin, lung, soft tissue or esophagus, even in the 7 patients recently or concomitantly treated with multiagent cytotoxic chemotherapy. In 3 patients, small areas of telangiectasia developed in the region of abutment of the electron arc field to a photon field used to irradiate the supraclavicular nodal area. We conclude that the technique as applied to these patients is safe and efficacious. In certain clinical settings it has advantage over standard, fixed field approaches to treat the chest wall.

Intensity-modulated radiosurgery/radiotherapy using a micromultileaf collimator

Intensity modulation with inverse treatment planning for 3 clinical stereotactic radiotherapy cases were directly compared against forward planning techniques using beam modification by enhanced dynamic wedge. Dose-volume histogram (DVH) analysis demonstrated that a significant reduction in dose to neighboring critical structures can-be achieved through intensity modulation patterns determined from inverse planning, while a marginal change is achieved in the target volume dose uniformity. This study also demonstrates that the intensity modulated dose patterns generated from inverse planning may differ significantly from the intuitive beam modified patterns developed in the forward planning model. These results suggest that one advantage of intensity modulated radiosurgery/radiotherapy with inverse planning is the significant reduction in dose to normal tissue and critical structures, with its coincident implications for dose escalation studies.