IGRT (Image-Guided Radiotherapy) uses x-rays and scans before and during radiation therapy to more precisely target tumors and reduce radiation exposure to healthy tissues. IGRT allows doctors to detect and correct errors in patient positioning and account for changes in tumor size or position during treatment. This improves accuracy and allows higher radiation doses to tumors or reduced margins around tumors, lowering toxicity risks and improving patient outcomes and quality of life. While requiring additional resources, IGRT has become a standard part of radiation therapy by improving precision and reducing uncertainties.

1. THE RATIONALE AND BENEFITS OF IMAGE-GUIDED
RADIOTHERAPY
Written by Melissa McClement
Application Specialist - Oncology, Tecmed Africa
April 2013

2. INTRODUCTION
The vital importance of imaging techniques in radiation oncology now extends
beyond diagnostic evaluation and treatment planning.
IGRTis currently a solid tool to tackle the problemof 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 normaltissues to be spared are delineated on CT scans in the planning
process. Recent technical advances haveenabled the integration of various
imaging modalities into the everyday practiceof radiotherapy directly at the
linear accelerator.
IGRTis used to delineate target volume and organs at risk. Identify and correct
problems arising frominter- and intrafractional variation in patient setup,
anatomy, target volume and organs at risk.

3. WHATIS 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-artrobotics
capable of three-dimensional softtissueimaging immediately before, during or
after radiation delivery, improving the localization of the target at the time of
radiation delivery, to ensurethat radiation therapy is delivered as planned.
Frequent imaging in the treatment room during a courseof radiation therapy
with decisions made on the basis of imaging, is referred to as IGRT.
IGRTcan broaden the application of proven therapies, and also permit new
therapies that are intolerant to geometric imprecision. Itenables the
application of special radiotherapeutic techniques with narrow safety margins
in the vicinity of radiosensitiveorgans.
IGRTuses advanced imaging techniques to verify patient and tumour position.
Knowing exactly where the tumour is, allows clinicians to reduce the volume of
tissueirradiated, targeting only the tumour and sparing the surrounding
normal tissue.
Anatomical changes that take place over the courseof radiotherapy, such as
weight loss and tumour shrinkagecan be detected as they occur and can be
accounted for in dosimetric calculations. Particularly during courses of
treatment extending over a number of weeks, substantialchanges can occur;
with no modification of the original treatment plan, there may be pronounced
deviations in dosedistribution, tumour control, and the likelihood of adverse
effects.
IGRTis very important in the following circumstances:
- Modern high-precision techniques with individual dose distribution
- Escalation of dosein the target volume
- Sparing of adjacentradiosensitiveorgans

4. In these situations small deviations in positioning can lead to large deviations
in dose distribution.
ORGANMOBILITY
The mobility of organs is a variable. Even with perfect positioning of skeletal
structures, theposition of the kidney, to name but one organ, can vary up to
severalcentimeters.
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 conformaltreatment plan that does not involvetargeting of the whole
mediastinum is sensitiveto this lateral displacement of the oesophagus.
In radiotherapy of the prostate, variation in the filling status of the rectum can
have a considerableeffect on the prostatedose and on rectal exposure. In the
case of high risk prostate cancers, the desired therapeutic doseis 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-dosezone, possibly resulting in much higher exposureof
the anterior rectal wall. With image guidance this can be recognized in time
and the treatment can be carried out later under better conditions.
ANATOMICVARIATIONS DURING TREATMENT
As well as weight loss, changes in tumour volumecan also have a significant
impact. Lymphomas, for instance, may decrease rapidly in sizeafter the
commencement of radiotherapy. Should such a lymphoma lie directly
adjacent to a radiosensitivestructure, tumour shrinkagemay reduce
attenuation of the x-ray beam and conceivably lead to increased exposure of

5. the neighbouring entity. Growth in tumour sizeduring 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 courseof 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
inappropriateexposure to radiation.
PATIENTBENEFITS
Through more precisetargeting of the beam, dosagelevels can be increased
and target volumes can be reduced - so tumours get a higher dose of radiation
and healthy surrounding tissues getvery 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 possibleto 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 moreefficient shorter schedules
than currently available.
State-of-the-artmotion management techniques allow patient to breathe
naturally during treatment sessions, increasing treatmentaccuracy, reducing
stress and increasing patient comfort.
Thus for the patient, IGRTincreases both the quality and probability of
successfultreatment.

6. IGRTPROCESSES
IGRTreduces, 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: IGRThas to be the treatment
method and quality assuranceor 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 systemwhich enables clinicians to
pinpoint tumour sites, adjustpositioning, 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 referenceimage used in planning. The error is estimated, and if larger
than the tolerance, adjustments aremade.
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 surfacemarkings, butthe 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 IGRTis concerned, currently it is not possibleto compareor
combine effective doses fromimaging and therapeutic procedures. The
American Association of Physicists in Medicine states: "Becausethis
comparison appears to be of great interest to the therapy community, we
consider that theoretical and/or empirical estimates of effective dose fromthe
therapy beam during treatment should be made".

7. INTEGRATIONINTO CLINICAL RADIOTHERAPY
IGRTis a complex modality. Itrequires special technical facilities and
represents a challenge in terms of financial and personnelresources. The
currentstandard consists in reducing the variations that occur to a minimum
and accounting for them when selecting safety margins.
According to many, IGRTdoes not to be done on every patient. In my opinion,
it should. Yes, there are certain anatomical sites, mentioned earlier, which are
very proneto movement, but each and every treatment site, each and every
patient deserves to havethe most preciseand correcttreatment possible. This
entails that each and every patient should have IGRTif there is the capability.
The additional doseenables narrower safety margins and guarantees precise
administration of the therapeutic radiation at the correct site, the overall
exposurewill be lower and the risk to the patient smaller. A reduction of
safety margins can reduce the exposureof adjacent structures and thus lower
the likelihood of adverseeffects.

8. CONCLUSION
The whole chain of interventions in the RT process should be prospectively
assessed. This is particularly important becauseother steps in the RT process
(eg contouring or valid measurements of toxicity) are at least as important as
high geometric precision.
IGRTseeks to address geometric uncertainties in doseplacement for target
and normaltissues. Ithas become a routine part of currentRT practice. Safe
application of IGRTtechnology requires additional training and careful
integration into the clinical process. IGRTreveals 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 tissueis based
on CT. At treatment, 3D softtissue imaging can also be used to localise the
target . These developments allow changes in tumour position, sizeand shape
that take place during radiotherapy to be measured and accounted for to
boostgeometric accuracy and precision of radiation delivery.
IGRTfacilitates the precise application of specialized irradiation techniques
with narrow safety margins to radiosensitiveorgans.

9. 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)ImageGuided 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) ImageGuided Radiotherapy: Rationale, Benefits and Limitations. Dawson,
L. and Sharpe, M. www.ncbi.nlm.nih.gov/pubmed/17012047
5) IGRTTreatment Process
www.varian.com/us/oncology/treatments/treatment_techniques/IGRT/
benefits.html#
6) ImageGuided Radiation Therapy: A refresher. Jaffray, D. Princess
MargaretHospital / Ontario Cancer Institute, University of Toronto.
7) Whatis ImageGuided Radiotherapy? www.cancerresearchuk.org/cancer-
help/about-cancer/cancer questions/what-is-image-guided-
radiotherapy#benefits.