Room: AAPM ePoster Library
Purpose: The objective of this study is to investigate and compare dose distributions in 3D–Lattice radiotherapy (LRT) versus 2D–GRID-“mini”-beam in a treatment planning system.
Methods: LRT and GRID planning were employed for a deeply seated tumor on a patient CT using AAA-algorithm in Eclipse-v15.6. 18 Gy in a single-fraction was prescribed to the GTV in all plans. The GRID-plan was generated using a GRID-block and a single anterior-posterior (AP) 18-MV beam to maximize dose penetration. The LRT-plans were created with RapidArc using 6-arcs with both 6 MV-FFF and 10 MV-FFF beams. The 3D-Lattice structure consists of small 1cm-diameter spheres evenly spaced inside the GTV, with 3 cm center-to-center separations. Virtual structures based on the Lattice or GRID pattern was created and registered to the patient CT.
Results: For a pelvis-mass GTV of 13x9x13 cm³, the GRID-plan with SSD-setup and 9 cm prescription depth generated strong dose gradient along the mini beam axis at dmax, with hot-spots around 131.5%. The left-to-right lateral valley-to peak ratio was 32.1% at dmax.
In contrast, 3D-LRT plans created high dose vertices within the tumor target regardless of target size and location. The hot-spots were uniformly distributed within the designed spheres in LRT. The valley-to-peak ratios were obtained from the dose profiles of the plan. For LRT-plan with 6 MV-FFF, anisotropic valley-to-peak ratios centered in GTV were observed at 55.2% laterally, 13.6% longitudinally, and 68.3% along the AP direction, respectively. A low dose bath was exhibited to normal tissues. The 10MV-FFF LRT plan achieved slightly larger numbers.
Conclusion: Compared to the traditional GRID plan where the maximum dose resided superficially, LRT plans generated more conformal dose coverage to a deep target. LRT plans also achieved comparable, yet directional dependent valley-to-peak ratios. A low-dose-bath is observed in normal tissue in LRT.