UA-35946176-3
2016
 

MR-Linac next step in Image Guided Radiotherapy

MR-Linac compleet toestel 

Manufacturer Elekta delivered one of their new MR-Linac "Unity" therapy units to the Netherlands Cancer Institute in April 2016. This image guided radiation treatment unit consists of a 7 MV linear accelerator with a multileaf collimator and a 1,5 Tesla diagnostic MRI scanner.    Elekta Unity provides the ability to reshape the dose based on daily changes in shape, size and position of the tumor and surrounding healthy anatomy and then enables accurate dose delivery with real-time visualization of the tumor.

From 1999 the concept of an MRI-Linac has been developed by professor Jan Lagendijk and his team in the radiotherapy division of the University Medical Centre in the town of Utrecht (NL). Based on his findings, the MR-Linac Unity has been designed by Elekta (linear accelerator and system integration) and PHILIPS (MRI). In 2009 a prototype unit in Utrecht demonstrated that MRI scans can be made while the accelerator beam is on. 1) 2)

MR-Linac constructie overzicht 

 

 fig.1 Assembly drawing of  the MR-Linac . copyright Elekta AB.

  • The linear accelerator is mounted in a ring structure that rotates around the MRI and the patient  during treatment.
  • The high fieldstrength of 1,5 Tesla makes clear MR images of soft tissue and organs.
  • The accelerator with 7 MV photon beam and multileaf collimator is capable of accurate conformal and IMRT treatment.

International cooperation.

In 2012 the Netherlands Cancer Institute (NKI) joined the international Elekta MR-Linac Consortium. In the consortium the manufactures Elekta and Philips join forces with seven leading cancer centres in the world to realise the clinical implementation of the Elekta MR-Linac. Clinical members are the Royal Marsden Hospital in London (UK), Christie hospital in Manchester ( VU), Froedtert/MCW in Wisconsin (USA),  M D Anderson Cancer Centre in  Houston (USA),  Sunnybrook  Odette Cancer Centre in Toronto (Can), and the NKI in Amsterdam (NL).

 

Quality of image guided radiotherapy improved by MRI imaging.

The quality of radiotherapy depends on the localisation of the target in the human body and the precise adjustment of the radiation treatment unit. The information on which the treatment has been planned is often compromised during the actual treatment, due to set up errors, patient movement, internal organ movement and changes in tumour shape. To make sure that all of the  target region is receiving the planned dose the radiation beams are made wider to compensate for  deviation. As a consequence more healthy tissue will be irradiated and sparing of critical organs is compromised.

Early in the history of radiotherapy it has already been attempted to improve accuracy by image guidance. From about 1953 some experimental therapy units were fitted with diagnostic quality x-ray tubes, to obtain images on film, under fluoroscopy, or with the first generation of X-Ray image intensifiers connected to a television monitor. In the 1970's portal film megavolt imaging became widely used, but fell short in poor image quality and time needed for analyses. In the 1990's the development of megavolt electronic portal image detectors (EPID)  and X-ray cone beam imaging for the first time introduced effective image guidance in Megavolt radiotherapy. Yet, this technology has limitations when the tumour is localised in soft tissue.

MR-Linac imaging the prostate

 

fig.2 Example of rectal imaging by the MR-Linac

At 1,5 Tesla the MRI makes clear and accurate images in soft tissue regions. For instance tumours in the abdomen and pelvic region can be localised better. In addition physical changes like the effects of breathing and bladder filling can be detected.  This makes it possible to  use smaller radiation beams, thereby reducing the dose to healthy tissue, resulting in a reduction of side effects.

The high quality of the MRI makes functional MRI (fMRI) possible. This will create a new method to determine the effect of radiation on the tumour. For instance by looking at the blood flow in tissue. fMRI will be a subject of study on the MR-Linac in the NKI .

More information:

1)  Lagendijk JJ, Raaymakers BW, Raaijmakers AJ et al. MRI/linac integration. Radiother Oncol 2008; 86: 5-9.

2)  Raaymakers BW, Lagendijk JJ, Overweg J et al. Integrating a 1.5 T MRI scanner with a 6 MV accelerator:      proof of concept. Phys Med Biol 2009; 54: N229-37.