Single Institute Experience – Linear Accelerator Based Total Body Irradiation Using Volumetric Modulated Arc Therapy Planning in Standard Size Linear Accelerator Room

Abstract

<h3>Purpose/Objective(s)</h3> To share experience of linear accelerator based total body irradiation (TBI) with volumetric modulated arc therapy (VMAT) in a standard size linear accelerator room <h3>Materials/Methods</h3> 30 patients aged 2-18 years (between 2018-2021) were treated with TBI as conditioning for bone marrow transplant (BMT)using VMAT based plan on our linear accelerator in a standard sized linear accelerator room. Indication of BMT was – acute lymphoblastic leukemia, acute myelogenous leukemia, chronic myeloid leukemia, myelodysplasia, aplastic anemia etc. Patient selection involved assessment regarding need of anesthesia, details of previous radiation, details of conditioning regimen for scheduling, organs dysfunction, adequate counselling and written consent from parents. With or without anesthesia, patients were immobilized using full body vacloc, head rest, brain orfit cast and knee rest. Position was supine, hands by side. Both head first (HF) and feet first (FF) scans were acquired if body length was more than 120 cm (for 120 cm, HF scan alone was acquired). A wire was placed on thigh to aid fusion of HF and FF scans for plan evaluation. Fiducials at three levels and optically stimulated luminescent dosimeter for dose monitoring were placed. Dose was 2-4 Gy single fraction and 12-13.2Gy/6 fractions (twice daily over 3 days, with 6 hours gap) based on chemotherapy conditioning regimen- delivered to 14 and 16 patients respectively. The planning objective was 90-100% coverage and dose max less than 120%. For multifraction plans, target was mean dose to lungs and kidney less than 10 Gy and 8Gy respectively. VMAT plans were generated with 3-5 iso-centers with fixed couch vertical and lateral values and changing longitudinal value, ensuring only longitudinal couch movement. Plan was made using 6MV energy and 5-9 arcs and 3 cm overlap between fields (avoid over joints to prevent effusion and over lungs to reduce lung dose) <h3>Results</h3> Mean PTV V95 – 93% (92-96%), Dmax – 115% mean (109%-120%). Mean lung dose (for 12-13.2Gy plans) was 9.4Gy/ (9-10 Gy), kidney mean dose was 7.8Gy (7.5-8 Gy). Mean monitor units in the plans were 1120 for complete arc and 690 for partial arcs. Treatment was delivered in strict aseptic precautions, under cover of steroids and antiemetics - in presence of radiation oncologist and physicist. Cone beam CT scan was used for position verification before and after treatment. The mean duration of treatment was 75 mins (range 60-90 minutes). 18 patients had grade 2 nausea and vomiting, one had knee joint effusion. No other side effects were reported. All transplants were successful except one reported graft versus host disease. <h3>Conclusion</h3> The procedure needs careful planning and delivery. It is feasible and possible to deliver TBI using VMAT in a standard sized linear accelerator room and should be offered to patients requiring the same.