Total body irradiation (TBI) has been utilized as a component of the treatment of blood cancers in form of conditioning regimen before bone marrow transplantation. The aim of conditioning is to suppress the immune system so that the patient will not reject the new bone marrow, make room in the bone marrow for the donor marrow stem cells to grow and to destroy any residual cancer cells.
TBI involves irradiation of the entire bone marrow of the body and it is a very complex process. The main challenges include since it involves a very big treatment area with changing depths and densities and field junctions.
Conditioning with chemotherapy alone has not given gratifying results because radiation had additional advantages. These advantages include no sparing of “sanctuary” sites (testes, brain), dose homogeneity regardless of blood supply and ability to tailor the dose distribution by shielding specific organs or by “boosting” sites
Sparing of the normal tissues while maintaining dose conformity to the bone marrow is the need of the time. The currently used linear accelerators (like Elekta Synergy at Dharamshila Hospital) have the potential for limiting these side effects especially by utilisation of techniques like Volumetric Arc Therapy (VMAT).
VMAT can deliver radiation in a single arc rotation of the linear accelerator around the patient. For some complex structures, more than one arc rotation might be necessary. VMAT has been reported to improve the dose conformity, reduce the dose to the organs at risk (OARs), and decrease the treatment time considerably.
VMAT is capable of delivering upto nine arc treatments within approximately 18 min. This can be compared with the 45–50-min treatment time reported for fixed gantry linear accelerator and tomotherapy IM-TMI. Hence VMAT can substantially decrease the treatment duration which is especially important in paediatric patients who are unable to lie still for long duration.
The planning for TBI is also very tedious .The entire body has to be immobilised in Vac cushion so as to reduplicate the position during treatment. Multiple CT scans are acquired as single CT scan cannot accommodate the full length of body. For the purpose of planning target volumes where radiation has to be given are defined. This consists of all the bone marrow in the body from the head to the toe. The individual bones are outlined and the organs to be spared like lungs, brain, eyes, oral cavity, heart, kidneys, liver, and bowels are also defined.
The delivery of TBI is also an intense procedure as it warrants treatment in the sterile environment. The treatment machine and the passages though which patient is mobilised are fumigated .The patient is wheeled in early morning to the radiation machine and treatment is delivered in infection free environment and then patient is wheeled back to BMT suite.
At DHRC, the Department of Haematology and Bone marrow transplant in conjunction with Radiation Oncology department have developed the reduced intensity conditioning protocol .This protocol comprises of low dose whole body radiation therapy along with low dose chemotherapy prior to the bone marrow transplant.
So far more than 20 patients have undergone TBI as a component of reduced intensity conditioning regiment with minimal toxicity and morbidity. The frequently used dose schedule has been single fraction of 2Gy to 4Gy.
Majority of patients have been treated with VMAT based TBI with good clinical outcome and tolerance. Hence, with the advancement in treatment technology, the indications of radiation have been redefined and the areas most feared are no more the forbidden zones.