Precision conformation therapy with the multi-leaf collimator

Conformatie therapie.JPG 

Conformation therapy (Figure courtesy of Harm Meertens)

In conformation therapy, the shape of irradiation fields is matched as closely as possible  to the shape of the  localized tumour tissue (target volume).With the 3-D treatment planning system, a treatment plan is then designed with the dose distribution conforming as well as possible to the target volume, while the dose to healthy tissue is minimized. The aim is to destroy the tumour with a high dose without unacceptable side effects. When treating from multiple directions, it is often necessary to adjust the shape of the field. In modern radiotherapy machines, this is a continuous process whereby, depending on the beam direction (gantry angle), the collimator jaws, the multi-leaf collimator blades and the collimator rotation are automatically adjusted.

Starting in the late 1980s, in the research of the radiotherapy department at the Netherlands Cancer Institute (NKI) the concept of conformation therapy takes shape. These treatments are also described as a High Dose - High Precision (HDHP) approach to irradiation. In the 1990s, it gains momentum when new techniques and developments reinforce each other:

  • 3D imaging with CT and MRI scanners provides insight into the anatomy and organ motion.
  • 3D treatment planning with advanced software introduces complex geometry irradiations.
  • Chemotherapy renders ​​tumours more sensitive to radiation.
  • Computer controlled linear accelerators can be adjusted automatically and continuously.
  • The multi-leaf collimator produces automatically adjustable conformal irradiation fields.
  • The electronic portal imaging device (EPID) for real-time quality control becomes available.
  • Accurate methods of in-vivo dosimetry with diodes.

With these intensive forms of conformation therapy the target volume is delineated very accurately, based on CT and MRI scanner images.  The treatment planning system also performs accurate 3D dose calculations. The accuracy of the treatment is guaranteed by EPID verification of treatment field positioning and by in-vivo verification of dose delivery. Due to this exact determination of the target volume, dose distribution and control of dose delivery (=> high precision), the surrounding tissues and organs receive no or less radiation dose during the treatment, so that complications can be prevented or reduced. Patients may therefore undergo a more intensive irradiation (=> high dose), which has a positive effect on the chance of cure. 

As of 1993, new techniques, developed in research programmes, are gradually introduced in clinical patient care at the NKI. A specialized research group led by radiation oncologists Joos Lebesque and Ronald keus and medical fysicist Iain Bruinvis, the Top Clinical Care project, is responsible for these integrated, advanced treatments. In 1995, these are applied in some 100 prostate and parotid gland patients [70]. In 1996, already about 30% (900) of all patients were treated in an intensive or special way and 200 patients received an HDHP conformation therapy treatment. In addition to prostate cancer, an increasing number of cases of lung, bladder, salivary gland, and braintumours are treated with HDHP techniques[71].

Introduction of the Multi-Leaf Collimator (MLC)

1992 Philips SLi met portal armklem in B9 web.jpg 

SLi20 20 MeV linear accelerator (Philips) in Bunker 9 (current name A1).

The accelerator is installed by Philips in 1988. In 1995 this machine is upgraded and fitted with  one of the first 80-leaves, 40 cm fieldsize multi-leaf collimators (MLC) available from Philips. 

The MLC is a radiation diaphragm consisting of a large number of narrow blades or leaves.  It is used to shape the edges of the radiation beam to accurately match the shape of the clinical target area. The resulting radiation field is called conformal, ie it conforms to the irregular shape of the target area. With the first MLC on accelerator 9, the radiation field is shaped in steps of 1 cm.    

1995 MLC T9.jpg 

Multi-leaf collimator on the Philips SLi20 linear accelerator

With this collimator, a maximum field size of 40 cm x 40 cm at one meter distance can be set. The radiation field is shaped by setting the 2 groups of 40 "leaves" (tungsten leaves of 9 cm height) which lie opposite each other.  Each of the 80 leaves is motor driven and can be independently and accurately adjusted. The width of each leaf is equal to a step in the field border of 1 cm at one meter distance from the radiation source.

Bronnen & Publicaties

  • [70] Annual Report, NKI 1995 ,
  • [71] Annual Report, NKI 1996 ,
  • Lebesque portret 

    Dr. Joos Lebesque


    Joos Lebesque obtains his doctorate in physics and mathematics in 1979, receives his medical diploma in 1981 and starts his training in radiotherapy in 1981 under Prof. Breur. In 1985 he is appointed radiation oncologist in the NKI. Because of his unique combination of knowledge in physics and medicine he contributes in a special way to the department and acts as a bridge between physics and the clinic. He introduces e.g. an exact technique of field matching for breast irradiation, a simultaneus boost technique for the prostate and biological parameters in treatment planning. From 1990 onwards he is project leader of many research projects in Conformal, Intensity Modulated and Image-Guided Radiotherapy. Subjects are for example lung damage and treatment, dose verification and geometrical margins, effects of organ movement and dose escalation for prostate cancer. From 1990 to 1998 he is head of the research division of radiotherapy.