The primary aim of radiotherapy in the treatment of cancer is to deliver sufficient radiation dose to the cancer-bearing region and avoiding treating the adjacent normal tissue.

However, because of anatomical constraints and conventional external beam radiotherapy, it is often not possible to achieve these aims with great precision, and thus the adjacent normal tissue tolerance to radiation damage becomes a major limiting factor.

In the treatment of gynaecologic cancers situated within the pelvis, the bladder and the rectum and small intestines within the pelvis are the main dose limiting organs i.e. they determine how much radiation can be delivered safely. This limitation will either result in compromising the dose to the target volume or result in long term and often-permanent morbidity if the doses to these critical organs are exceeded.

Intensity modulated radiotherapy (IMRT) is a novel way of delivering external beam radiation therapy allowing for adjustment (hence modulation) of the dose delivered within the region of the body receiving radiation so that a steep dose gradient can be achieved in specified regions within it.

Hence, critical organs lying adjacent to the target can be selectively under-treated whist maintaining a good dose delivery to the tumour-bearing region.

This has been made possible mainly through the use of:
1)	an automated and computerized dynamic motion of small radiation beam blocking devices (multi-leaf collimators).
2)	combining multiple fields (often between 4 to 8 treatment fields).
3)	revolutionary treatment planning system (inverse planning system).

The multiple field arrangements aid the creation of a “sculptured” treatment volume, conforming better to the often-irregular cancer-bearing region. Multiple small blocking metal devices will selectively and dynamically maneuver to either allow the radiation beam to pass or be blocked over a certain area with the radiation field.

The summation of these two processes gives rise to sharp dose delivery with the area receiving radiation. The inverse planning system allow radiation oncologist to specify or predetermine how much dose is allowed within a critical volume whilst maintaining a therapeutic dose level in the target, before the planning process starts. In essence, it allows the radiation dosimetrist to virtually “paint” the radiation dose pattern on the irradiated region with in the body.

IMRT therefore has improved tremendously the means of delivering radiation therapy, and in particular regions where critical structures exist e.g. head and neck, thorax and pelvis.

Cervical carcinoma would ideally be one cancer to consider using IMRT. Though radiotherapy is established as an effective modality in achieving cures even in advanced staged disease, the rectum and bladder bear the brunt of potential long-term radiation damage.

Hence, if increased relative sparing of their organs can be achieved, it could result in safer treatment and possibly allow dose escalation to further improve tumour control and cure rates. A number of reports have shown reduction in doses delivered to the bone marrow of the pelvis and also lesser acute and chronic side effects of the gastrointestinal system with IMRT when compared to conventional external beam therapy.

However, there are also limitations of IMRT that must not be disregarded. Immobilisation of the treated area is critical in such a complex and precision treatment system. While this is not a problem in treating head and neck tumours, pelvic immobilization is less often less rigid.

Secondly, there is the problem of organ movement and variation in size of the critical organs resulting from filling or emptying of the bowel and bladder.

Hence, creating precise “dose contouring” maps in the pelvis to treat a cervical cancer may only be accurate based on the initial planning CT volumes of target and critical organs, and does not necessarily mean it will remain accurate when the dose is delivered for the duration of 5 to 6 weeks daily treatment.

Contouring of the treatment volume is also in question as many areas treated in the conventional fields are assumed to have microscopic disease e.g. normal sized pelvic nodes and paracervical tissues and not detected on CT or MRI images. Exclusion or under dosing these areas may lead to increase treatment failure rates.

Currently, brachytherapy is used to deliver a very high dose of radiation to the cervix and is an integral part of the curative treatment with radiotherapy. Using the physics principle of "inverse square law"(where dose intensity falls inversely proportional to the distance away from the radiation source), allows a very intense delivery of radiation to the primary cervical tumour over a short period of time, while differentially sparing the adjacent bladder and rectum.

There is no data to suggest that IMRT can match what can be achieved with brachytherapy either in terms of efficacy or toxicity profile. There is also the concern that the use of multiple fields also results in more adjacent normal tissues receiving low radiation doses that can potentially lead to malignant change in years to come.

And lastly, is the complexity of designing and implementing an IMRT planned treatment for a patient and maintaining a high level of quality assurance, can impact heavily on the workload within the radiotherapy department, requiring hours of input from physicians, dosimetrists, oncologist and technical support staff.

Inevitably, it also results in a hefty financial price for both the institution aiming to provide this technology as well as for the patients. Whether IMRT can be justified in the routine use of cervical cancer treatment remains to be proven.

Khoo-Tan Hoon Seng
Senior Consultant
Therapeutic Radiology
National Cancer Centre, Singapore