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PTV Margin for Reduced Normal Tissue Dose VMAT Cranionspinal Irradiation

M Lin*, Z Xiong, E Chambers, N Desai, A Godley, The University of Texas Southwestern Medical Ctr, Dallas, TX


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: evaluate setup errors, assess adequate planning-target-volume (PTV) margins and reduce the treatment time for a kV-image-guided volumetric-modulated-arc-therapy (VMAT) craniospinal irradiation (CSI) technique.

Methods: patients treated with kV-image guided VMAT CSI were recruited. Patient setup began with kV images to straighten spine as needed. kV images were acquired at each isocenter, starting from the lower spine. In order to preserve the dose gradient at the matched region, no rotational adjustments were allowed. The daily kV images were analyzed to derive the inter-fractional shifts as systematic(S) and random(s) errors in the antero-posterior(AP), lateral(LR), and superior-inferior(SI) directions. PTV margins were calculated for a minimum CTV dose of 95% for 90% of patients. The treatment time was analyzed and compared with previous conventional 3D CSI treatment.

Results: average setup errors are within 1 mm in all isocenters and directions. The brain isocenter exhibited the smallest setup error in all directions due to the thermoplastic mask. The RL setup error is more frequently seen in the spine isocenters. The PTV margins are asymmetric and isocenter-specific: 1.7 mm(SI), 0.8 mm(RL), 1.7 mm(AP) for the brain isocenter, 2.2 mm(SI), 2.2 mm(RL), 1.7 mm(AP) for the upper isocenter, 1.5 mm(SI), 1.6 mm(RL), 0.7 mm(AP) for the lower spine isocenter. Comparing to the conventional 3D CSI treatment, the kV-image-guided VMAT CSI reduced the treatment time by 45% and 12% for the first and the subsequent fractions.

Conclusion: VMAT CSI was shown to be more efficient. kV-image acquisition and patient alignment starting from the lower spine isocenter is more efficient since this area is relatively more unstable than the brain isocenter. Potentially, only the lower spine needs to be imaged after initial setup, further reducing on table time. Exact PTV margins derived allow refining the optimization to spare more normal tissue.


Image Guidance, Setup Errors


TH- RT Interfraction Motion Management: General (most aspects)

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