Radiation oncology towards Intensity-Modulated Radiation Therapy (IMRT)

Radiotherapy aims to deliver a high radiation dose to a tumour, killing all tumour cells. However, from the physical and technical point of view, a difficult task because malignant tumours are often located close to radiosensitive organs such as the eyes, optic nerves and brainstem, spinal cord, bowels or lung tissues.

During radiotherapy, these so-called organs at risk (OAR) must not be damaged. The situation is even more complicated when the tumour itself is radio-resistant and very high is needed to reach a therapeutic effect.

This is where new technologies in radiation oncology, especially Three-Dimensional Conformal Radiotherapy (3DCRT), come into play. Any 3DCRT plan conforms the spatial distribution to the prescribed dose to the target, concomitantly excluding critical normal tissue from the volume receiving high radiation doses.

Reducing the dose to normal tissue permits tumour dose escalation compared to conventional methods. In addition, advances in computer technologies have significantly changed radiotherapy practice toward Intensity Modulated Radiation Therapy (IMRT). IMRT is considered an extension of an advanced form of 3DCRT. Instead of using uniform fields as in 3DCRT, IMRT uses intensity-modulated fields to generate dose distributions that are more conformal to the target. IMRT requires a higher level of precision compared to 3DCRT. This is because the generation of modulated fields by the inverse treatment planning algorithm is directly based on the Computerised Tomography (CT).

In radiotherapy practice, over the years of experience and research have confirmed that escalation of radiation dose to tumours results in better tumour control; with the routine radiotherapy technique, it would not be possible to increase the tumour dose due to the associated increase in dose to typical structures around the tumour.

On the other hand, higher doses to the normal structure have exhibited unacceptable complications. Thus, a balance was sought between tumour control (cure) and morbidity (difficulties); with improved imaging modalities, better delineation of the target volume is possible alone with critical structures around.

Development of computerised treatment planning systems and facility to transfer computerised tomography (CT) images to planning systems improved escalation of dose to various structures and the uniformity of dose to the entire target volume.

The development of various dose calculation algorithms could define dose precisely and point to point in the entire patient's body. This enables volumetric studies and critical dose analysis of various structures by Dose Volume Histograms (DVH) for target and other critical structures. All facilities are available in Bangladesh.

The writer is the Head of the Department of Oncology, Delta Medical College and Hospital.

E-mail: manzur2001bd@yahoo.com