Understanding intensity modulated radiotherapy

One of the greatest challenges of radiotherapy is to minimise damage to normal cells, while delivering an accurate and adequate dose to destroy tumour cells. This is what Intensity Modulated Radiotherapy (IMRT) aims to do.

Traditionally linear accelerator (LINAC) machines use two collimators to produce a constant rectangular beam of radiotherapy, delivering the same dose across the beam. Rapid advancements in LINAC technology, dose mapping (dosimetry) and medical imaging have enabled the development of much more accurate radiotherapy delivery.

Intensity Modulated Radiotherapy uses a multi-leaf collimator (MLC) to vary, or modulate, the intensity of the beam so that patients receive different doses from different parts of the beam. Cutting edge computer technology is used to produce complex, sculpted dose distributions that increase the dose to the tumour and decrease the dose to surrounding healthy tissue and organs. A computer controlled MLC in the output head of the LINAC allows the shape of each beam to be tailored to deliver the optimal high dose volume.


The planning phase of Intensity Modulated Radiotherapy requires sophisticated software and skilled dosimetric and medical physics support. The clinical oncologist will define the target and prescribe a dose to be delivered to that target. They may also identify organs at risk to be avoided. The planner then designs a treatment plan with the clinical oncologist, using forward planning or inverse planning.

Forward planning - the planner creates a plan by inputting the variables into a radiotherapy treatment planning system (TPS) including the number of radiation beams and the angles they should be delivered from, if attenuating wedges should be used and the MLC configuration to use. The TPS then calculates the predicted dose to the patient. The planner can change the variables until the TPS predicts the result they want.

Inverse planning - the planner and clinical oncologist define a set of dose objectives including the dose to be delivered to the target volume, and the maximum dose to be received by organs at risk. The TPS then creates a plan to achieve the dose objectives.

Inverse planning is used for complex IMRT where the clinical oncologist can define and delineate a target volume and organs at risk in the TPS, as with the prostate. Forward planning is used where this is not necessary or possible, for example with breasts.

CT Planning, V-Sim and TPS are all used in the planning process to increase accuracy and calculate the optimum dose distribution.

Delivery of intensity modulated radiotherapy

IMRT can be delivered in a number of ways:

Step and shoot

Static fields with multiple sub-fields in each to modulate dose intensity. For example prostate treatment uses 7 fields and 70 sub-fields and breast treatment uses 2 fields and 6 to 8 sub fields.

Dynamic MLC

Static fields with MLC leaves moving during beam delivery to modulate dose intensity. 

Dynamic Arc

Gantry, collimators and MLC all move during beam delivery to modulate dose intensity. 

GenesisCare currently provides both step and shoot and dynamic arc to deliver Intensity Modulated Radiotherapy. Both can deliver equal complexity and effective treatment, while enabling us to be more confident that treatment is delivered precisely as planned.

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On 8th January 2016, we changed our name from Cancer Partners UK, to GenesisCare.