Intensity-modulated Total Body Irradiation (TBI) with TomoDirect™

Abstract

BackgroundThe new TomoDirect™ modality offers a non-rotational option with discrete beam angles. We have investigated this mode for TBI with the intention to test the feasibility and to establish it as a clinical routine method. Special foci were directed onto treatment planning, dosimetric accuracy and practical aspects.Patients and methodsTBI plans were calculated with TomoDirect™ for a Rando™ phantom and all patients with an intended fractionated total body irradiation between November 2013 and May 2014 (n = 8). Finally, four of these patients were irradiated with TomoDirect™. Additionally we studied variations in the modulation factor, pitch, field width of Y-jaws and dose grid during optimization. Dose measurements were performed using thermoluminescent rods in the Rando™ phantom, with the Delta4® and with ionization chambers in a solid water phantom.ResultsFor all eight calculated plans with a prescribed dose of 12 Gy Dmean was 12.09-12.33 Gy (12,25 ± 0.08 Gy), D98 11.2-11.6 Gy (11.45 ± 0.12 Gy) and D2 12.6-13.1 Gy (12.94 ± 0.13 Gy). Dmean of inner lungs was 8.73 ± 0.22 Gy on the left side and 8.69 ± 0.27 Gy on the right side.When single planning parameters are varied with otherwise constant parameters, the modulation factor showed the greatest impact on dose homogeneity and treatment time. The impact of the pitch was marginally, and almost equal homogeneity can be obtained with field width of Y-jaws 5 cm and 2.5 cm.Measurements with thermoluminescent rods (n = 25) in the Rando™ phantom showed a mean dose deviation between measured and calculated dose of 0.66 ± 2.26%. 18 of 25 TLDs had a deviation below 3%, seven of 25 TLDs between 3% and 5%.ConclusionTBI with TomoDirect™ allows a superior homogeneity compared to conventional methods, where lung blocks are widely accepted. The treatment is performed only in supine position and is robust and comfortable for the patient.TomoDirect™ allows the implementation of organ-specific dose prescriptions. So the discussion about the balance between the need for aggressive treatment and limited toxicity can be renewed with the new potentials of TomoDirect™ - for children as well as for adults – and possibly yield a better clinical outcome in the future.

Dose evaluation indices for total body irradiation using TomoDirect with different numbers of ports: A comparison with the TomoHelical method

“…For the TomoDirect plans, we generated plans with 2–12 ports with approximately equally spaced angles; in all, 11 TomoDirect plans were created for each patient. The detailed beam angles of each TomoDirect plan are shown in Table .…”

“…Total body irradiation (TBI) is widely used in conjunction with chemotherapy as a part of the conditioning regimen of hematopoietic stem cell transplantation (HSCT) for hematologic malignancies . The most commonly applied total dose and fractionation schedule of myeloablative TBI is 12 Gy, twice daily, over 3 days .…”

“…The former delivers treatment with 360° gantry rotation while the couch is translating through the gantry. In contrast, the latter delivers treatment with 2–12 predetermined, discrete angles using a fixed gantry as the couch passes through …”

“…Some groups have reported the roles and feasibility of TBI using the TomoHelical mode; these studies also showed that TomoHelical offers advantages compared with conventional linear accelerator‐based approaches in terms of dose homogeneity at the target lesion and organ at risk (OAR) sparing . There are few reports of TBI using TomoDirect; however, Salz et al . investigated TBI using TomoDirect with 12 static ports with equally spaced angles.…”

“…Therefore, even if a set‐up error up to 2 cm of the surface occurs, sufficient dose distribution is ensured. (c) The dose heterogeneity in the circulating blood is improved …”