Planning evaluation of step-and-shoot IMRT, RapidArcTM, and helical tomotherapy for hippocampal-avoidance whole brain radiotherapy (HA-WBRT) RTOG protocol 0933 provides guidance for planning hippocampal-avoidance whole-brain RT (HA-WBRT) using linac-based static field IMRT and helical tomotherapy1. Intensity modulated arc delivery has been shown feasible for HA-WBRT with an integrated boost2, however all 3 modalities have not yet been compared on a common set of patient data. This study compares HA-WBRT treatment plans calculated using static gantry angle non-coplanar IMRT, intensity modulated arc therapy, and helical tomotherapy on a common set of 8 patients that include the RTOG credentialing datasets as well as patients successfully accrued to the protocol. Two patient datasets are shown here as an example. The PTV consists of the whole brain minus the hippocampus volume expanded 5mm in 3-dimensions. Three treatment plans were generated for each patient: Pinnacle non-coplanar IMRT using 9 beams and optimization criteria recommended in the 0933 protocol’s Appendix VIII, Varian RapidArcTM using 2 coplanar arcs, and Accuray® TomoTherapy® using a 1cm jaw width. Three planners independently planned each modality without knowledge of the patient’s other plans. Plans were optimized to deliver 30 Gy to the PTV in 10 fractions according to 0933 requirements: target coverage V30Gy≥90%, D2%≤37.5Gy, D98%≥25Gy), hippocampus dose (min ≤ 9Gy & max ≤ 16Gy), and optic nerves and chiasm max dose < 37.5 Gy. Variation acceptable dose limits are reported in the table below. A target homogeneity index was also calculated as % % %⁄ for each plan.

Patient #1 Following planning, all dose maps were imported to Varian’s EclipseTM software for comparison. Axial images with an overlaid dose wash and DVH’s for the PTV and hippocampus are displayed for two patients here and the table below reports the dosimetric criteria and the

homogeneity index. While HA-WBRT has shown to be technically feasible using static field IMRT, concern has been raised regarding the volume of +15% (34.5 Gy) hotspots within the brain PTV3. TomoTherapy® provides the most homogeneous treatment plans. However using the 1cm jaw leads to longer treatment times. For the two RapidArcTM plans here, patient #1 exhibits superior homogeneity and patient #2 exhibits similar homogeneity compared to 9-field step & shoot IMRT. In addition, the coplanar arc technique will be more time efficient to deliver as they do not require 7 couch kicks like the 9-beam IMRT arrangement recommended by the 0933 protocol.

Patient #2

0933 Dosimetric Criteria (Variation acceptable limits)

Patient #1 Patient #2

9 Field RapidArc Tomothe

rapy 9 Field RapidArc

Tomotherapy

PTV V30Gy > 90% 94.7% 92.1% 94.3% 94.8% 93.6% 95.0% D2% ≤ 37.5 Gy 35.7 Gy 32.9 Gy 31.6 Gy 36.0 Gy 35.7 Gy 33.4 Gy D98% ≥ 25 Gy 27.9 Gy 26.9 Gy 28.0 Gy 28.2 Gy 27.4 Gy 27.7 Gy

Hippocampus D100% ≤ 10 Gy 8.7 Gy 8.0 Gy 8.1 Gy 8.4 Gy 8.7 Gy 6.8 Gy Max ≤ 17 Gy 15.8 Gy 15.5 Gy 14.1 Gy 16.4 Gy 14.6 Gy 16.2 Gy

Optic Nerves & Chiasm

Max ≤ 37.5 Gy 36.3 Gy 32.7 Gy 35.5 Gy 35.6 Gy 38.4 Gy 35.6 Gy

Homogeneity Index (HI) 0.24 0.19 0.12 0.24 0.25 0.19 1V. Gondi, R. Tolakanahalli, M. P. Mehta et al., Int J Radiat Oncol Biol Phys 78 (4), 1244-52. 2F. Hsu, H. Carolan, A. Nichol et al., Int J Radiat Oncol Biol Phys 76 (5), 1480-5. 3J. P. Knisely and J. B. Yu, Int J Radiat Oncol Biol Phys 79 (3), 957-8; author reply 958.