Essential Radiotherapy Techniques – Intensity Modulated Radiation Therapy (IMRT)

What is Intensity Modulated Radiation Therapy (IMRT)

Intensity modulated radiation therapy is a radiotherapy technique that allows radiation to be more closely shaped to fit the tumour and spare nearby critical normal tissue.

Use of IMRT

The decision to use IMRT would depend on the clinical circumstances and the intent of the treatment. Not all patients will require IMRT; however there are circumstances where IMRT is increasingly the standard of care. When the radiation doses required to control the cancer are close to normal tissue radiation tolerance levels IMRT is indicated.

Consideration should be given to the impact on the quality of life, technical implementation and anatomical complexity.

The sparing of normal tissue achieved by IMRT results in fewer treatment-related toxicities and side effects. In addition, comparable or higher doses to the tumour with IMRT would result in equivalent or better tumour control and disease free intervals.

IMRT is also indicated where previous radiotherapy has been given to nearby tissues and conventional techniques of radiotherapy would result in unacceptable toxicities.

IMRT-capable equipment distribution across Australia¹⁴

C-arm linac: IMRT

IMRT is traditionally delivered by a C-arm Linac, with a number of static modulated beam positions around the tumour volume. According to the Faculty of Radiation Oncology facilities census, 85% of all linear accelerators in Australia are IMRT-capable and 97% of Australian radiotherapy centres have at least one IMRT –capable linac.

C-arm linac: Volumetric Modulated Arc Therapy (VMAT)

VMAT is a newer type of IMRT technique that uses the same hardware but delivers the radiotherapy using a rotational or arc geometry rather than static beams. Of the current linear accelerator pool 25% of machines are VMAT-capable. Arc techniques enable an improvement in the beam delivery time and may result in overall reduction in the treatment time.

Helical IMRT

Helical IMRT combines a ‘CT-like’ physical configuration with a radiotherapy delivery system (linac). One Helical IMRT linac is currently operating in Australia.

IMRT services across Australia in 2010¹⁴

Although IMRT-capable equipment is available in 49 centres (97%) nationally, in many centres the IMRT service is not offered. In 2010 Australian Capital Territory and Northern Territory did not offer any IMRT services. They both have since introduced the service, but the data on IMRT utilisation is not available. In South Australia IMRT is only available through a private service provider.

Although the majority of Australian centres have IMRT capability, 14 centres (29%) of those with IMRT capability do not deliver any IMRT treatments.

Of the 35 centres (71%) that deliver IMRT treatments in 2010:

• 12% treated 10 of fewer patients with IMRT

• 20% treated between 11 and 50 patients with IMRT

• 25% treated between 51 and 150 patients with IMRT

• 14% treated more than 151 patients with IMRT

Overall, out of the total new radiotherapy treatments delivered nationally, IMRT treatment courses comprised only 6.5%.

Total National IMRT courses delivered (2010)

Trends and issues arising

IMRT should be available in all centres that offer radiation therapy including rural, metropolitan, and in both public and private facilities. All patients who have radiation therapy should have access to IMRT where clinically appropriate.

Given that the equipment base to deliver IMRT in Australia already exists and that the sector is becoming more experienced in the use of this technique, it is estimated that between 30% and 50% of all radiation therapy patients will be treated with IMRT going forward. The fact that the IMRT potential of existing technology is not being used to benefit patients should be a significant concern to patients and service providers.

There are a number of barriers to IMRT uptake at present, these include:

• Professional – lack of capacity to undertake the training and learning required as most radiotherapy teams are devoted to meeting the existing patient load;

• Professional – lack of capacity to undertake the necessary Quality Assurance which is essential for this technique;

• Resourcing – lack of an appropriate Medicare rebate which would resource and encourage timely implementation.

IMRT treatment planning and delivery requires significantly longer preparation time and physics QA and therefore is more resource-intensive. As such, the cost of delivering IMRT treatment is higher than 3D conformal therapy. In the absence of appropriate public funding, patient access to IMRT is limited by the capacity of the radiotherapy departments to absorb the financial cost.

Delivering IMRT requires precise imaging to guide clinical decision-making. Image Guided Radiation Therapy (IGRT) is an essential component of delivering IMRT. The rapid evolution of IGRT technologies offers a high level of reassurance that IMRT cases can be done with high quality ¹⁵.