In advanced stage tumour, radiation treatment
is complementary to surgery in obtaining maximal local regional
control. The aim of radiotherapy is to maximise the dose
of radiation to the tumour, and minimise dose to the surrounding
normal structures. In the past, this was achieved with heavy
blocks and shields.
With the advent of modern radiation equipment
and computer planning software technology, 3-dimensional
conformal radiotherapy (3DCRT) has become a reality. This
allows easy conformation of the target volume using CT computer
planning and electronically controlled mechanical blocks
called multileaf-collimators (MLCs).
Intensity Modulated Radiotherapy (IMRT)
takes 3DCRT a step further. 3DCRT conforms the radiation
field to the target volume and hence spares the surrounding
normal organs. However, 3DCRT is unable to achieve a good
conformity in targets of irregular shapes especially if
the tumours wrap around important normal structures.
IMRT overcomes this limitation. It is able
to deliver almost perfect conformity to tumour targets of
almost any shape with maximal sparing of the surrounding
structures. Moreover, IMRT has the capability of ‘dose-painting’,
a technique where different doses can be delivered to different
parts of the treated volume. This is achieved by the modulation
of the intensity of the radiation fields. Each field may
be considered to be made up of many infinite numbers of
pencil beams. These beams can be precisely controlled to
deliver desired dose to any target of any shape while keeping
the dose to the normal structures to a minimum. This is
achievable through a process called inverse-planning.
IMRT is especially useful in treating head
and neck cancers because of the multitude of vital organs
in this region that could be easily damaged by radiation.
The tumour often curves around the brainstem or spinal cord
and other important structures which are almost impossible
to avoid without compromising on the radiation dose and
hence on disease control and survival.
Recent trials have confirmed that IMRT improves
disease control and survival with decline in complications
by allowing dose escalation to the tumour and yet sparing
the normal tissue of excessive radiation. This is especially
useful in cases needing re-irradiation. In NCC, we have
started treating patients with nasopharyngeal cancers using
IMRT since 2002. The early results are encouraging with
good disease control and fewer side effects.
No doubt IMRT is superior to conventional
3DCRT but it is a very labour-intensive technique that relies
heavily on modern expensive equipment. Selection of patients
for this new promising treatment is important to best utilise
our resources.
Dr John Low Seng Hooi
Associate Consultant
Therapeutic Radiology
