THE RATIONALE AND BENEFITS OF IMAGE-GUIDED RADIOTHERAPY

Written by Melissa McClement

Application Specialist - Oncology, Tecmed Africa

April 2013

INTRODUCTION

The vital importance of imaging techniques in radiation oncology now extends beyond diagnostic evaluation and treatment planning.

IGRT is currently a solid tool to tackle the problem of radiotherapy accuracy (reduction in systematic errors).

Imaging is central to radiation oncology practice, with advances in radiation oncology occurring in parallel to advances in imaging. Targets to be irradiated and normal tissues to be spared are delineated on CT scans in the planning process. Recent technical advances have enabled the integration of various imaging modalities into the everyday practice of radiotherapy directly at the linear accelerator.

IGRT is used to delineate target volume and organs at risk. Identify and correct problems arising from inter- and intrafractional variation in patient setup, anatomy, target volume and organs at risk.

WHAT IS IGRT?

IGRT (Image-Guided Radiotherapy)is a way of using x-rays and scans before and during radiotherapy treatment.

The radiation treatment units are now recognized as state-of-the-art robotics capable of three-dimensional soft tissue imaging immediately before, during or after radiation delivery, improving the localization of the target at the time of radiation delivery, to ensure that radiation therapy is delivered as planned. Frequent imaging in the treatment room during a course of radiation therapy with decisions made on the basis of imaging, is referred to as IGRT.

IGRT can broaden the application of proven therapies, and also permit new therapies that are intolerant to geometric imprecision. It enables the application of special radiotherapeutic techniques with narrow safety margins in the vicinity of radiosensitive organs.

IGRT uses advanced imaging techniques to verify patient and tumour position. Knowing exactly where the tumour is, allows clinicians to reduce the volume of tissue irradiated, targeting only the tumour and sparing the surrounding normal tissue.

Anatomical changes that take place over the course of radiotherapy, such as weight loss and tumour shrinkage can be detected as they occur and can be accounted for in dosimetric calculations. Particularly during courses of treatment extending over a number of weeks, substantial changes can occur; with no modification of the original treatment plan, there may be pronounced deviations in dose distribution, tumour control, and the likelihood of adverse effects.

IGRT is very important in the following circumstances:

- Modern high-precision techniques with individual dose distribution

- Escalation of dose in the target volume

- Sparing of adjacent radiosensitive organs

In these situations small deviations in positioning can lead to large deviations in dose distribution.

ORGAN MOBILITY

The mobility of organs is a variable. Even with perfect positioning of skeletal structures, the position of the kidney, to name but one organ, can vary up to several centimeters.

In the case of ventral displacement, radiotherapy of the para-aortic lymph nodes can result in renal damage.

In the thorax, the oesophagus can show enormous variation in position. A highly conformal treatment plan that does not involve targeting of the whole mediastinum is sensitive to this lateral displacement of the oesophagus.

In radiotherapy of the prostate, variation in the filling status of the rectum can have a considerable effect on the prostate dose and on rectal exposure. In the case of high risk prostate cancers, the desired therapeutic dose is as high as 80Gy. To minimize acute and chronic gastrointestinal toxicity, the rectum is spared by means of techniques such as intensity- modulated radiotherapy or volumetric modulated arc therapy. An excessively full rectum is displaced forward into the high-dose zone, possibly resulting in much higher exposure of the anterior rectal wall. With image guidance this can be recognized in time and the treatment can be carried out later under better conditions.

ANATOMIC VARIATIONS DURING TREATMENT

As well as weight loss, changes in tumour volume can also have a significant impact. Lymphomas, for instance, may decrease rapidly in size after the commencement of radiotherapy. Should such a lymphoma lie directly adjacent to a radiosensitive structure, tumour shrinkage may reduce attenuation of the x-ray beam and conceivably lead to increased exposure of

the neighbouring entity. Growth in tumour size during radiotherapy also has consequences. An increase in volume owing to an inflammatory reaction or bleeding may lead to expansion of the tumour out of the target area, so that it does not receive the envisaged dose. If true tumour progression occurs during a course of irradiation, consideration should be given to discontinuing treatment. Without image guidance in such a situation, the treatment would be continued and the patient subjected to ineffective therapy with inappropriate exposure to radiation.

PATIENT BENEFITS

Through more precise targeting of the beam, dosage levels can be increased and target volumes can be reduced - so tumours get a higher dose of radiation and healthy surrounding tissues get very little. Irradiating less normal tissue reduces the toxicity of radiotherapy, improving the patient's quality of life. In some cases, improved targeting may make it possible to deliver higher radiation doses to the tumour and thereby increase the likelihood of local tumour control.

Increased precision and accuracy of radiotherapy are expected to augment tumour control, reduce incidence and severity of toxic effects after radiotherapy, and facilitate development of more efficient shorter schedules than currently available.

State-of-the-art motion management techniques allow patient to breathe naturally during treatment sessions, increasing treatment accuracy, reducing stress and increasing patient comfort.

Thus for the patient, IGRT increases both the quality and probability of successful treatment.

IGRT PROCESSES

IGRT reduces, but does not eliminate geometric uncertainties. Reduced geometric uncertainties may allow reduced PTV margins. But, that being said, there are parallel or related processes to this: IGRT has to be the treatment method and quality assurance or uncertainty management must be in place.

Generation 1 technology includes kV imaging (such as Brainlab's ExacTrac x-ray), and Portal Imaging Devices. Generation 2 technology includes kV imaging with On Board Imagers (OBI) and Cone - Beam CT's (CBCT).

OBI is fully automated real-time imaging system which enables clinicians to pinpoint tumour sites, adjust positioning, and complete a treatment all within the standard time slot. OBI operates along 3 axes of motion for optimum positioning. The OBI is capable of radiographic 2D imaging, fluoroscopy and 3D CBCT for fine tuning patient position.

After positioning the patient, the image is acquired. This image is aligned to the reference image used in planning. The error is estimated, and if larger than the tolerance, adjustments are made.

Positioning is a crucial factor. Accuracy and reproducibility of position are strongly dependent on the anatomic region involved and on the positioning aids used. Positioning methods depend on surface markings, but the actual position inside the body of, for instance, a lung tumour or an abdominal organ can change considerably.

Indexed immobilization reduces set-up times and ensures reproducibility.

Where the dose of IGRT is concerned, currently it is not possible to compare or combine effective doses from imaging and therapeutic procedures. The American Association of Physicists in Medicine states: "Because this comparison appears to be of great interest to the therapy community, we consider that theoretical and/or empirical estimates of effective dose from the therapy beam during treatment should be made".

INTEGRATION INTO CLINICAL RADIOTHERAPY

IGRT is a complex modality. It requires special technical facilities and represents a challenge in terms of financial and personnel resources. The current standard consists in reducing the variations that occur to a minimum and accounting for them when selecting safety margins.

According to many, IGRT does not to be done on every patient. In my opinion, it should. Yes, there are certain anatomical sites, mentioned earlier, which are very prone to movement, but each and every treatment site, each and every patient deserves to have the most precise and correct treatment possible. This entails that each and every patient should have IGRT if there is the capability. The additional dose enables narrower safety margins and guarantees precise administration of the therapeutic radiation at the correct site, the overall exposure will be lower and the risk to the patient smaller. A reduction of safety margins can reduce the exposure of adjacent structures and thus lower the likelihood of adverse effects.

CONCLUSION

The whole chain of interventions in the RT process should be prospectively assessed. This is particularly important because other steps in the RT process (eg contouring or valid measurements of toxicity) are at least as important as high geometric precision.

IGRT seeks to address geometric uncertainties in dose placement for target and normal tissues. It has become a routine part of current RT practice. Safe application of IGRT technology requires additional training and careful integration into the clinical process. IGRT reveals changes in anatomy during treatment which challenges conventional practices.

Developments in medical imaging are integral to radiation oncology, both for design of treatment plans and to localise the target for precise administration of radiation. At planning, definition of the tumour and healthy tissue is based on CT. At treatment, 3D soft tissue imaging can also be used to localise the target . These developments allow changes in tumour position, size and shape that take place during radiotherapy to be measured and accounted for to boost geometric accuracy and precision of radiation delivery.

IGRT facilitates the precise application of specialized irradiation techniques with narrow safety margins to radiosensitive organs.

REFERENCES

1) Image - Guided Radiotherapy: A New dimension in Radiation Oncology. Sterzing, F. Engenhart - Cabillic, R., Flentje, M. et al.

www.ncbi.nlm.nih.gov/pmc/articles/PMC3097488/

2)Image Guided Radiation Therapy: Benefits and Limitations. Khan, F. Professor University of Minnesota, Minneapolis, Minnesota.

3) Advances in Image-Guided Radiation Therapy. Dawson, L. and Jaffray, D.

jco.ascopubs.org/content/25/8/938.abstract

4) Image Guided Radiotherapy: Rationale, Benefits and Limitations. Dawson, L. and Sharpe, M. www.ncbi.nlm.nih.gov/pubmed/17012047

5) IGRT Treatment Process

www.varian.com/us/oncology/treatments/treatment_techniques/IGRT/benefits.html#

6) Image Guided Radiation Therapy: A refresher. Jaffray, D. Princess Margaret Hospital / Ontario Cancer Institute, University of Toronto.

7) What is Image Guided Radiotherapy? www.cancerresearchuk.org/cancer-help/about-cancer/cancer questions/what-is-image-guided-radiotherapy#benefits.