Paul Geis

THE CYBERKNIFE : A FRAMELESS ROBOTIC SYSTEM FOR RADIOSURGERY

The Cyberknife is a unique instrument for performing frameless stereotactic radiosurgery. Rather than using rigid immobilization, the Cyberknife relies on an image-to-image correlation algorithm for target localization. Furthermore, the system utilizes a novel, light-weight, high-energy radiation source. The authors describe the technical specifications of the Cyberknife and summarize the initial clinical experience.

Estimates of whole-body dose equivalent produced by beam intensity modulated conformal therapy

PURPOSE To estimate the dose delivered to patients by photons and neutrons outside the radiation fields when beam intensity modulation conformal radiotherapy is given. These estimates are then used to compute the risk of secondary cancers as a sequela of the radiation therapy. MATERIALS AND METHODS The x-ray and neutron leakage accompanying two beam-intensity modulation techniques delivered by currently available linear accelerators were estimated for 6-MV, 18-MV, and 25-MV x-ray energies. Estimates of whole-body dose equivalents were determined using leakage measurements reported in the literature and treatment parameters derived for two modulated beam-intensity conformal therapy techniques. Risk values recommended by the National Council on Radiation Protection and Measurements (NCRP) were used to estimate the resulting risk of fatal radiation-induced cancer for 70.00 Gy prescribed tumor doses. RESULTS The computed worst-case risks of secondary cancers increased in the range from 1.00% for 6-MV x-rays to 24.4% for 25-MV x-rays. CONCLUSIONS Careful consideration should be made of the risks associated with secondary whole-body radiation before implementation of beam intensity modulated conformal therapy at x-ray energies greater than 10 MV.

Modulated beam conformal therapy for head and neck tumors

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.

Physical characteristics of a miniature multileaf collimator

A preliminary study of the physical characteristics of a miniature multileaf collimator (mMLC) used with 4 MV x rays is reported. The mMLC attached to the accessory mount of a class C or D Varian linear accelerator (Varian Oncology Systems, Palo Alto, CA) with a source to aperture distance of 65 cm. The field penumbra using the small leaves was found to be consistent with the anticipated field penumbra using photon jaws at the same source to aperture distance as the mMLC. The percentage depth dose values of square fields were found to be consistent with the fields collimated with the upper and lower jaws. Output factors for the very small fields were found to vary rapidly. Circular fields could be produced with depth dose characteristics similar to those produced using conical tertiary collimators, commonly used for radiosurgery, but with a broader penumbra.

Quality assurance for dynamic multileaf collimator modulated fields using a fast beam imaging system

A quality assurance procedure was developed for x-ray beam intensity modulated conformal radiotherapy (IMCRT) using dynamic multileaf collimators (MLC). The procedure verifies a prescribed intensity modulated x-ray beam pattern in the beam eyes view (BEV) before the treatment procedure is applied to a patient. It verifies that (a) the leaf sequencing computer files were transferred correctly to the linac control computer; (b) the treatment can be correctly executed without machine faults. A fast beam imaging system (BIS) consisting of a Gd2O2S scintillation screen, a charge-coupled device (CCD) camera, and a portable personal computer (Wellhöfer Dosimetrie, Schwarzenbruck, Germany) was commissioned for this purpose. Measurements for the BIS performance are presented in this work. Reference images were derived from MLC leaf sequencing files that were used to drive a dynamic MLC system (Varian Oncology Systems, Palo Alto, CA). A correlation method was developed to compare the BIS measurements with the calculated reference images. A correlation coefficient calculated using 26 correct intensity modulated fields was shown to be a reliable threshold to identify inaccurate treatment delivery files. The study has demonstrated the feasibility of using the BIS and the correlation method to carry out on-line quality assurance tasks for IMCRT treatment fields in the BEV.

Use of a multileaf collimator as a dynamic missing-tissue compensator

A dynamic multileaf collimator (D-MLC) was used to investigate the feasibility of producing missing-tissue compensators. The modulation of the x-ray field in two dimensions produced by conventional physical compensators was mimicked by delivering a sequence of D-MLC-shaped subfields. A method is introduced to calculate monitor units (MU) for dynamically compensated fields that is analogous to and expands upon methods used for conventional compensating filter MU calculations. In this investigation, the tissue deficit at the surface of an anatomical phantom was measured using a Moiré camera. The tissue deficit data were used to generate a series of D-MLC subfields that, delivered in sequence, provided the compensated treatment. Film was used to integrate the dose delivered to a specified depth of compensation. Isodose distributions were measured for uncompensated fields, fields compensated with a conventional lead or plastic filter, and fields compensated with the D-MLC. A comparison of the dose distributions shows the compensation achieved with the dynamic compensating filter is comparable to that achieved using conventional physical compensating filters.

Application of a video-optical beam imaging system for quality assurance of medical accelerators

Method validation techniques were developed and experiments were carried out using a beam imaging system (BIS, Wellhöfer Dosimetrie, Schwarzenbruck, Germany) for routine quality assurance of medical accelerators. The routine quality assurance tasks include x-ray beam flatness and symmetry check, light/radiation field congruence test, beam energy constancy for electrons and mechanical checks for couch and collimator rotations. Comparisons were made between the BIS application and conventional quality assurance methods that use radiographic films or detector arrays. In this work, we have demonstrated efficiency and accuracy of the BIS to perform some of the routine quality assurance tasks for medical linear accelerators.