3652 - Single Institution Experience of Volumetric Modulated Arc Based Total Body Irradiation (VMAT-TBI)

Purpose/Objective(s):

Total body irradiation (TBI) involves delivery of a relatively homogenous dose of radiation to the entire body. It is given as a part of conditioning regimen prior to allogenic and autologous hematopoietic stem cell transplantation (HSCT). Here we share our experience of VMAT-TBI in a standard sized Linac room.

Materials/Methods: 101 patients aged 2-53 years were treated with TBI as a part of conditioning regimen for HSCT on Linac between January 2018 to January 2025. Indications for TBI included ALL, AML, CML, myelodysplasia and aplastic anaemia. Pre-RT assessment included detailed history and examination, previous radiation if any, conditioning regimen and need of anaesthesia. Adequate counselling of the patients and parents (in case of patients <18 years) was done and written consent was taken. 58 patients were prescribed 2Gy in single fraction and 43 were prescribed 12-13.2Gy in 6 fractions (delivered twice daily over 3 days). 14 patients (14%) received cranial boost prior to TBI (dose of 6Gy in 3 fractions) in view of CNS positive disease. Patients were simulated in supine position with hands by side and immobilized using a vacloc and head rest. Both head first and feet first scans of 2.5mm slice thickness were acquired for patients whose length exceeded 120cm. We followed ILROG TBI guidelines. The planning objectives included 90-100% coverage with a Dmax of less than 120%. Organs at risk (OARs) contoured included both lens, eyes, kidneys and lungs. Dose constraints for critical OARs were given for multifractionated plans which included Lungs Dmean <10Gy and Kidneys Dmean < 8Gy. Junctions over joints and lungs were avoided. VMAT plans were generated using 6MV beams with 3-5 isocentres. Treatment was given with fixed couch vertical/lateral values allowing only longitudinal translation. Surface dose monitoring was done by optically stimulated luminescence dosimeters (OSLDs).

Results:

Mean PTV V95 was 94% (92-96%) with Dmax of 115% (109-120%). For multi-fractionation plans mean dose for lungs was 9.4Gy (9-10Gy) and kidney was 7.8Gy (7.6-8Gy). Mean monitor units were 1120 for a complete arc and 690 for partial arc. Treatment was delivered with strict aseptic precautions with radiation oncologist and medical physicist present. Mean treatment time was 75 minutes (60-90 minutes). Two patients reported graft versus host disease. One of them underwent re-irradiation. 63 patients had grade I-II nausea and vomiting and one patient had knee joint effusion. There was no symptomatic radiation pneumonitis, cataract or late kidney injury till last follow-up.

Conclusion:

VMAT-TBI is safe, feasible, effective as well as practically deliverable in a standard Linac room with careful planning and diligent implementation. However, larger prospective study documenting late toxicity would be beneficial. This single institution data reiterates TBI as an integral part for HSCT program.