Maximar-100

The Maximar-100 is a radiation therapy device that was made by General Electric to deliver superficial x-rays.

Maximar 100 X-Ray Unit Superficial Therapy

The Maximar-100's x-ray source is the GE SRT-1,[1] an x-ray tube specifically designed for use in the Maximar-100. This is a fixed-anode, reflection-type x-ray tube, whose name is composed of three parts: "SR" meaning self-rectified, "T" meaning therapeutic, and "1" indicating a 1-mm diameter nominal focal spot size. Its beryllium window produces a beam with very low inherent filtration but it is subject to beam non-uniformity due to heel effect.

The generator produces a voltage that is continuously adjustable between 30 and 100 kVp and a continuously adjustable current between 0 and 5 mA.[2] An analog indicator and rheostat is provided allowing for feedback, so adjustments can be made to compensate for drift during treatment.

Maximar 100 X-Ray Unit Superficial Therapy - Cones

The unit uses a set of interchangeable aluminum filters of 0.25, 0.5, 1, 2, and 3 mm thickness which filter the beam's spectrum, but also substantially effects its intensity.[3] Collimation is achieved using two sets of open ended circular applicators. The first are cylindrical and provide fields of 1, 1.5, 2, 3, 4, and 5 cm diameter at a 15 cm source-to-skin distance. The second are conical and provide fields of 5, 10, 15, and 20 cm diameter at a 30 cm source-to-skin distance.

The unit was first manufactured in 1953 and proved durable. It was rated for a 50% duty cycle,[4] i.e. 4 hours of operation at full power, 100 kVp and 5 mA, in any 8 hour period. As of 2022, at least one unit had been in clinical service continually since 1953.[5] Where this unit has been replaced by newer technology, it remains a clinical touchstone and basis for comparison.[6]

References
  1. Oak Ridge Associated Universities (2023, May 23). General Electric SRT-1 X-ray Tube. Museum of Radiation and Radioactivity. Retrieved July 7, 2023, from https://orau.org/health-physics-museum/collection/x-ray-coolidge/miscellaneous-x-ray-tubes/general-electric-srt-1-x-ray-tube.html
  2. General Electric (1953). MaximaR 100 X-Ray Unit—Superficial Therapy—Operation & Maintenance. Milwaukee, Wisconsin.
  3. Gupta, M. L. (1982). Relation between filter thickness and exposure rate for Maximar-100 X-ray therapy machine. Health Phys., 43(3), 420-422. https://doi.org/6171476
  4. Stovall, J. W. (1970). A Study of the Thermal Characteristics of X-rayTube Housings [Master's Thesis, Oregon State University]. https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/zw12z897q
  5. Strauss, M. (2022, July 25). Who Says Radiotherapy Equipment Can’t Last Forever? Articles and Insights. Retrieved July 7, 2023, from https://www.oncologysystems.com/blog/useful-life-of-radiotherapy-equipment
  6. Merz, B., Feldmeier, J., & Parsai, E. (2009). SU-FF-T-46: Dosimetric Comparison of Superficial X-Rays and a Custom HDR Surface Applicator for the Treatment of Superficial Cancers. Medical Physics, 36(6-9), 2529. https://doi.org/10.1118/1.3181518
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