1. MAGNETIC RESONANCE IMAGING IN RADIATION TREATMENT
Abbas ADESINA ABDUS-salam

2. Radiation therapy is a localized treatment like surgery
It involves the use of ionizing radiation to eliminate tumour cells “in situ” without apparent removal of tissue
Results of radiation treatment in terms of disease control is sometimes broadly similar to surgical intervention
Surgery has the advantage of complete control over the excision margin
Radiotherapy has the advantage of reaching places and levels where surgical knives can not reach or not desirable
The aim of modern radiation treatment practice is to combine the precise control of the surgical knives and the reach of radiation

3. Achievement of this lofty objective is built on 2 important fulcrums
Precision in tumour localization
Precision in dose delivery
These were important barriers to effective radiation treatment in the past
Imprecise tumour localization meant more guesswork with resultant bigger field sizes and more side effects

4. Radiodiagnosis has repeatedly come to the aid of its sister in resolving these problem and helping to strengthen these 2 pivots
Plain X-ray helped introduced what is later to be know as 2D treatment planning
With 2D planning, bony landmarks became important guides in tumour localization and treatment volume delineation
But bony landmarks are at better approximation of the disease volume
Using bony landmarks in Cervical Cancer for example will mean that the entire pelvic brim should be involved in external beam treatment in an effort to include all the pelvic nodes even when there is no evidence of nodal involvements
The limitation of plain x-ray for tumour localization and treatment quickly became obvious

5. Ultrasound scan improved detection of extent of disease, presence or absence and extent of metastases, most especially the lymph nodes
Combined with sentinel node identification, USS can greatly improve the determination of disease extent
It however found no use in radiation treatment planning
Improvement in this area had to await the introduction of the CT Scan
CT scan gave birth to the era of 3D conformal treatment planning
With CT, radiation oncologist could start dreaming of the precision of the surgical knives
Our radiation fields do not have to be square, circle, oval, rectangle or triangle anymore
It can follow the precise shapes of the tumours, follow them round tight curves in the human anatomy and spare unaffected organs or in fact parts of organs

6. CT-aided radiotherapy is the current gold standard
Most radiation treatment centres across the world have dedicated CT for treatement planning
Planning CT are basically like a regular CT

7. The use of MRI is beginning to attract attention and commentaries from practitioners
MRI has better image resolution compared to CT
This better is imaging resolution is most pronounced for soft tissue
Less radio-diagnosis competence is required

10. Additional benefits of MRI over CT planning
MRI does not use ionizing radiation, making its repeated use possible both in children and adults where there is a need to keep a firm grip on the total radiation dose
MRI images do not have to be acquired with trans- axial slices. The slices can be sagittal, coronal or using any plane desired for the relevant anatomy or proposed treatment fields
MRI is also better suited to assess intra-treatment variations in tumour/treatment volume positions

11. Modification to MR techniques can also improve the quality of output
Such include the use of contrasts
MR Angiography
Dynamic Contrast-Enhanced MR
Blood Oxygen Level Dependent MR
Diffusion Imaging
MR Spectroscopy

12. Radiation treatment planning system is designed to use CT images
So, why has MRI not replaced CT as the imaging of choice in radiation treatment planning?
Years of research and technological development have gone into CT- treatment planning
Radiation treatment planning system is designed to use CT images
Electron density, which many modern treatment planning depends on is not easily or accurately measured using MRI
MRI geometric image distortion is also a serious problem that will need to be significantly resolved
System, Patient and presence of metal artifacts
Problem with compatibility between radiotherapy treatment positions and MRI acquisition positions
Integration of MRI and RT equipment is also creating technical challenges that are currently being worked on

13. In concluding
MRI is poised to play increasingly important role in radiation treatment planning,
It’s superior image clarity in tumour localization and treatment delivery is enticing to the radiation oncologists
However, the current technical challenges has limited the use of MRI in radiation oncology to mainly “offline” diagnosis. Integration with live treatment system is still experimental
It is safe to predict however that once some of these technological hurdles are crossed, MRI will take its rightful as a superior technology in radiation treatment planning and dose delivery

14. Important Resources
Maria A Schmidt and Geoffrey S Payne; Radiotherapy planning using MRI, IOP Science 2015
Yue Cao and Lili Chen; Mri in radiation treatment planningand assessment,
Mary Koshy, Michael Pentaleri, Shyam Paryani; The Role of MRI in Radiation Therapy Planning; hwem_ssxa_websites-context- ownload/mdaw/mtc1/~edisp/the_role_of_mri_in_radiation_therapy_ planning pdf