First linear accelerator in the NKI

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In 1968, the first linear accelerator is installed in the Netherlands Cancer Institute on the Sarphatistraat. The machine is a Philips MEL SL75/super, with an 8 MV photon beam and a single 10 MeV electron beam.

Professor Klaas Breur is faced with the necessity of enlarging the treatment capacity and of improving the facilities for megavolt treatment. In The Netherlands, radiotherapy is developing fast. From 1964, the Rotterdam Radiotherapy Institute (RRTI) and later the University of Utrecht are provided with a 6 MV linear accelerator These machines offer improved treatment quality compared to the techniques possible with the older cobalt units. The advantages are in skin sparing, sharper beam definition and deeper penetration in tissue. Better treatment techniques become available for many tumours. The linear accelerator becomes the standard therapy machine in radiation oncology and remains so until today.

At the NKI the large image guided cobalt machine built by Henri Lokkerbol often breaks down. The technical maintenance becomes questionable as Philips, the new owner of Smit Röntgen, cannot maintain support of this machine. Since the installation the activity of the cobalt source has decayed from 2500 Ci to 1100 Ci. Fewer patients can be treated per day, even though patients are treated in the evenings. In 1968 the cobalt unit is replaced by the SL75 linear accelerator.

The NKI-AVL starts to catch up with modern developments. The X-ray units from 1953 are taken out of service. In 1967 the megavolt radiation capacity is expanded by replacing the outdated Siemens Convergence X-ray machine with a compact, powerful cobalt unit. With this new machine the treatment capacity can be expanded. In 1968 Lokkerbol's cobalt machine is replaced by the latest Philips type of linear accelerator, the SL75/super. Philips installs this linear accelerator with an 8 MV photon beam and a 10 MV (10 million volt) electron beam.  

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The compact cobalt unit of Philips-Smit. It is a product of the factory of Smit Röntgen, which has become a part of Philips Medical Systems.  


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The control room of the linear accelerator. 


For communication and a good view of the patient, the radiation therapist has a sound and video connection. For communication with colleagues in the department, a telephone and an intercom are available. The control panel for the radiation therapist is clear and compact. Technical systems and the technical control system of the accelerator are installed in the so-called modulator room, see the picture below. 

The linear accelerator is a further development of the accelerator type SL75, which is used in Rotterdam and Utrecht since 1964 and 1965. The producer is Mullard Equipment Ltd (MEL), located in Crawley, Sussex, England. This is a Philips company. In this SL75/super unit, the photon energy is increased to  8MV.Also, an electron beam of 10 MeV is available.  

The main part of the machine is an accelerator tube, in which electrons are accelerated over a distance of 2 meters by high frequency oscillating electro-magnetic fields. The electrons are accelerated to an energy of 8 MeV and are concentrated in a radiation beam of 3 mm in diameter that strikes a golden target plate. The absorption of the electrons in this target produces a photon beam with an energy spectrum that has a maximum value of 8MV and a mean energy of about 2.6 MV. This is an improvement in comparison with the cobalt source, which emits monochromatic photon radiation at energies of 1.17 and 1.33 MeV and has a source diameter of 20 mm. The linear accelerator can rotate 360 degrees around the patient. Because the machine is so compact, there is a lot of space to position the patient properly for treatment. 

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Installation in 1968.

The linear accelerator is transported through a breach in the thick concrete walls of the old cobalt treatment room.

Expansion of the physics and technical group. 

The linear accelerator is a machine that is more complex than what the hospital was used to. It is a technically complicated device of which the radiation source has many similarities with a powerful radar transmitter. The radiation is not continuous as from a Cobalt source, but is emitted in short pulses of 2 microseconds and a repetition frequency of typically 100 to 500 per second. The hospital appoints engineers and physicists with skills in the field of high frequency techniques and physics of pulsed radiation. In some respects, the device is still experimental and has teething problems. The electronic controls in the machine are analog and are based on simple semiconductor devices that are sometimes unstable.

In order to be able to use this machine safely, new methods are developed for technical maintenance, the radiation dosimetry and treatment planning.

A new generation of physicists and technicians are taken on, including Rob van der Laarse, the physicist who develops software for the radiation planning; Iain Bruinvis, physicist who makes the electron beams clinically useful; Theo Wilmering, the engineer who realizes many medical and technical devices and organises the work of the technical group; and Henk van der Gugten, engineer who specializes in the accelerator technology and designs improvements to the machine which makes the radiation treatment more stable and safe.  

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1970, Henk van der Gugten works on the adjustments of the electronic controls of the linear accelerator.