The New Way of Total Body Irradiation “Total Marrow and Lymphoid Irradiation (TMLI)”
Total Body Irradiation (TBI)
has been used for the treatment of hematologic malignancies (blood cancers) and as a conditioning regimen before bone marrow transplantation. The dose of radiation required to eradicate cancer cells in TBI can results in unavoidable toxicities to the surrounding normal organ/tissues of the host, leading to adverse consequences and side effects. This is especially true for the older conventional TBI methods
. Therefore, techniques that could provide safer dose escalation for these patients have been highly sought after.
Known Side Effects of TBI
The Acute Toxic Effects
last for about 4 -6 weeks and they include nausea & vomiting, dryness of mouth and skin, mouth ulcers, mild redness of skin fatigue and fall in blood counts.
The Chronic Toxic Effects
have included cataracts, learning difficulties and Growth Retardation in Children, and, most importantly lung complications in form of-Interstitial Pneumonitis. The risk of Secondary Malignancy later on in life is potentially a serious problem.
In order to reduce these side effects and at the same time delivering a good radiation dose to the desired area (target) newer techniques have been evolved.
The New Way
With the technological advancements in radiation therapy machines and treatment delivery methods (like intensity modulation) it is now possible to deliver radiation to a desired target which is the bone marrow and the lymphoid organs (Total Marrow Irradiation-TMI, Total Marrow Lymphoid Irradiation-TMLI)
and protect surrounding normal structures and reduce long term as well as acute side effects
Volumetric Arc Therapy (VMAT)
is one such technique which achieves this goal.
VMAT makes use of customized algorithms to deliver IMRT in an arc rotation of the linear accelerator around the patient.
How Do We Actually Achieve Our Target
The areas which are to treated (Bone marrow and Lymphoid tissue) are defined as the Clinical Target Volume (CTV)
. An additional margin is then added to the CTV to obtain the Planning Target Volume (PTV)
The organs to be spared
included the lungs, brain, eyes, oral cavity, heart, kidneys, liver, and bowel. They are contoured by a radiation oncologist. The total time required for contouring is around 4 hours
for one patient.
The time for delivering the radiation therapy has also been considerably reduced by VMAT. If we compared VMAT with other technologies (tomotherapy IM-TMI), the treatment time with VMAT has been reduced from 40 – 50 min. to 18 min. only.