Electron applicator connected to the old SL75/super accelerator.
The new SL75/20 linear electron accelerator has an adjustable energy control, able to set the energy of the accelerator between 4 and 20 MeV. In this range photon beams of 8 and 16 MV and electron beams of 4, 6, 8, 10, 12, 14, 17, and 20 MeV are available.
Radiation oncologists are keen to apply the new electron beams clinically, in particular for the treatment of tumours situated close to the skin. Electrons have a limited range in tissue; the electron beam is fully absorbed after approximately 0.5 cm per MeV. A 4 MeV beam reaches up to 2 cm in tissue while a 20 MeV beam reaches 10 cm. Deeper lying tissue receives only a small amount of radiation. This is different compared to X-rays, which penetrate much deeper. The electron beams of the SL75/20 accelerator do not, however, fulfill the state-of-the-art requirements for clinical use. The applicators are still based on the design of the old SL75 type accelerator.
Clinical physicist Herbert Marcuse, chief physicist in the department of radiotherapy, is well known for his colourful choice of words. He describes electron beams as a "pot of fleas", indicating that they are easily scattered and jump in all directions. It is therefore important to apply a properly designed electron beam collimating system. At that time such a system would consist of a simple tube type of applicator positioned on the skin of the patient. However, these tubes generate a lot of scattered electrons that affect the beam characteristics unfavourably. At higher energies these applicators produce too much "bremsstrahlung" (X-rays), which results in unwanted dose deep in the patient.
In 1973, physics student Iaïn Bruinvis is appointed in the physics group of the NKI. First as MSc student and later as doctoral student he investigates the possibilities to "tame the fleas", and develops ionization chambers and phantoms to perform specific measurements in electron beams. Treatment planning software is developed in cooperation with physicist Rob van der Laarse to calculate the dose distribution of electron beams in patients.
An applicator construction kit is manufactured in the workshop of the Radiotherapy Department in the Wilhelmina Gasthuis (teaching hospital of the University of Amsterdam) for the study of scattered electrons in an applicator. Parts of the applicator can be added or removed to study the effect on the dose distribution.
The ionization chamber and water tank used for these measurements have been made in the workshop of the NKI. Water is used to imitate soft tissue in a patient with respect to radiation interaction.
The ionization chamber is waterproof and is made from polystyrene. It is a parallel plate chamber of 3 mm x 8 mm, with 2 mm distance between the electrodes. The chamber volume is 0.05 cc.
An important result of the study of Bruinvis is an improved design of the electron flattening filters and of the construction of the applicators, combined with adaptations in the control of the (photon) collimator blocks of the accelerator. Around 1976 Philips installs the improved electron collimating system on both accelerators in the NKI-AVL.
Chief radiation therapist Riet van der Heide - Schoon shows one of the new applicators made by Philips according to the design of physicist Iaïn Bruinvis.
Iaïn Bruinvis obtains his doctorate in 1987 with his work on the application of electron beams in radiotherapy practice. His thesis is dedicated to the study of beam characteristics, irregular treatment fields, and the development of measuring equipment and treatment planning software for electron beams .
Dr. Iaïn Bruinvis in 1989, standing next to the newly installed SL25 accelerator of Philips. It shows a further development of electron applicators by Philips.
By using various frames that function as apertures, the wall of the applicator can be omitted, resulting in a considerable improvement in beam characteristics. The size and shape of the treatment field is determined with a custom frame that is inserted at the end of the applicator. In most cases these frames are cast in the mouldroom for each individual patient. For rectangular fields a range of standard frames is available.