At the UMC Utrecht, The Netherlands, we have constructed a prototype MRI accelerator. The prototype is a modified 6 MV Elekta (Crawley, UK) accelerator next to a modified 1.5 T Philips Achieva (Best, The Netherlands) MRI system. From the initial design onwards, modifications to both systems were aimed to yield simultaneous and unhampered operation of the MRI and the accelerator. Indeed, the simultaneous operation is shown by performing diagnostic quality 1.5 T MRI with the radiation beam on. No degradation of the performance of either system was found. The integrated 1.5 T MRI system and radiotherapy accelerator allow simultaneous irradiation and MR imaging. The full diagnostic imaging capacities of the MRI can be used; dedicated sequences for MRI-guided radiotherapy treatments will be developed. This proof of concept opens the door towards a clinical prototype to start testing MRI-guided radiation therapy (MRIgRT) in the clinic.

Two p-type diode detectors, a parallel-plate ion chamber, and radiographic film were used to measure total scatter factors and tissue maximum ratios (TMRs) for a stereotactic radiosurgery system with circular fields ranging from 5 to 50 mm in diameter. One diode has a square detection diagonal of 2.3 mm and the other diode has a circular detection diameter of 1 mm. It is found that the two diodes measured essentially the same total scatter factors for all field sizes. Total scatter factors measured by film are within 3% of diode values. Our results also suggest that the parallel-plate ion chamber could underestimate total scatter factors for fields as large as 15 mm in diameter, although it is recommended for field diameters > or = 12.5 mm. The total scatter factors used in our clinic are combined from data measured with the ion chamber and the 2-mm-diam diode. The combined total scatter factors generally agree with published data. While film overestimates TMRs for the smallest fields at large depths because of energy dependence of the film, the measurements with the 1-mm-diam diode agree with published data measured with thermoluminescent dosimeters. It is demonstrated that the accurate measurements of total scatter factors and TMRs for small fields can be obtained by combining results of the commercially available detectors used in this study.

We report investigations into the feasibility of generating radioactive oxygen (15O, a positron emitter, with half-life 2.05 min) using a tuned Elekta SL25 accelerator, for the end purpose of imaging tumor perfusion. 15O is produced by the "gamma, neutron," (gamma,n) reaction between high-energy photons and normal oxygen (16O) in the body. As most in vivo 16O is bound in water molecules the 15O radio-marker is produced in proportion to water content in tissue. Imaging the washout of the 15O distribution using sensitive positron-emission-tomography (PET) technology can yield spatial information about blood perfusion in the tissue. The aim of this article was to determine the amount of 15O activity that could be produced by the tuned medical accelerator. A further aim was to model the activation process using Monte Carlo and to investigate ways to optimize the amount of 15O that could be generated. Increased activation was achieved by (i) tuning the beam to give higher-energy electrons incident on the target of the accelerator, (ii) increasing dose rate by removing the conventional filtration in the beam and reducing the source to object distance, and (iii) reducing low-energy photons by means of a carbon block absorber. The activity per-unit-dose produced by the tuned beam was measured by irradiating spheres of water to known doses and placing the spheres in a calibrated coincidence-counting apparatus. Peak energy of the tuned bremsstrahlung beam was estimated at 29 MeV, and generated activity up to 0.24/microCi/cc/3Gy in water. The measured amount of 15O agreed to within 10% of the prediction from the Monte-Carlo-computed spectrum, indicating reasonable ability to model the activation process. The optimal thickness of the carbon absorber was found to be about 25 cm. The insertion of a carbon absorber improved spectral quality for activation purposes but at the cost of reduced dose rate. In conclusion, the viability of generating 15O with an Elekta SL25 has been demonstrated. In conjunction with recent advances in high-sensitivity portable PET imaging devices, real potential exists for imaging in situ activated 15O washout as a surrogate measurement of macroscopic tumor perfusion.

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A RAD-8 Treatment Planning System located at University Hospitals in Madison, Wisconsin, is currently being used to calculate radiotherapy isodose distributions for seven hospitals in the Wisconsin region. This system was purchased by Project 6 of the Wisconsin Regional Medical Program, known as Wisconsin Radiological Physics Laboratory (WRPL). Interaction with the system is accomplished by using Xerox telecopiers. A software beam generation system has been developed which enhances the ability of this system to respond effectively to use of this type. Treatment beams may be defined from data collected by a water phantom beam scanning device which outputs data onto paper tape, from beam information already contained on the RAD-8 system tape and from 60Co treatment machine parameters. From a set of basic beam definitions, additional treatment beams may be generated as needed in a matter of a few minutes. At present between two and three treatment plans are calculated per day with response times varying between a few hours and one day. The overall utilization of this system was presented as well as the accuracy which can be expected for teletherapy and interstitial or intracavitary calculations performed.