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

Abstract

Background: The 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 methods: TBI 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.

Results: For 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%.

Conclusion: TBI 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.

Figure 1

Twelve beams with equally spaced angles were used for the head-first-plan. The beams were expanded (yellow part of the beam) laterally on both edges by a maximum of 5 leaves each (3,125 cm at isocenter). This expansion is limited if leaves on the end of the MLC are already used because of the patient size.

Figure 2

Dose volume histograms of total body (head-first plan), lung and inner lung as a function of modulation factor (MF), pitch and field width of Y-jaws (FW). Values, if not varied are: MF = 1.5, pitch = 0.25, FW = 2.5 cm.

Figure 3

Results of a dose measurement of a TBI plan with the dose verification system Delta4® in the thorax area. The target was truncated below the stomach for this measurement to allow the use of this system without undesired irradiation of the radiosensitive electronic parts of the Delta4®.

Figure 4

Dose distribution (in Gy) of a TBI head-first treatment plan. Isodoses: 14 Gy (red), 13 Gy (orange), 12 Gy (green), 11 Gy (green), 10 Gy (light blue), 9 – 8 – 6 – 4 – 2 Gy blue (stepwise).

Figure 5

Comparison of dose-volume-histograms for the translational method with lung blocks and TomoDirect ^TM . The DVH of the treatment plan of the translational technique was calculated with Oncentra, the comparison is done with Oncentra as well.