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Evaluation of Off-Target Isocenter Placement On TArget Coverage and Critical Structure Dose

R Sadagopan

R Sadagopan*, P Balter, UT MD Anderson Cancer Center, Houston, TX

SU-E-T-715 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the effect of placing the isocenter away from the PTV (so that collision is avoided during VMAT treatment) on tumor coverage or critical structure doses in SBRT.
Method: CT images of lung SBRT patients were acquired for pre-treatment alignment using CT-on-rails and imported back in to the planning system. The daily images were registered with the original simulation CT using the GTV as the alignment structure. We re planned the patient (on original CT) using 2 additional scenarios 1) isocenter at the midline of the patient at the AP and Sup-Inf level of the GTV and 2) The isocenters at the center of the vertebral body at the Sup-Inf level of the GTV. In both cases, the plans were normalized so that PTV coverage is similar to the original clinical plan. The treatment plans were recomputed on the daily CTs using original isocenter and the two off target isocenters and doses to target and serial critical structure cord were tabulated. The MU units remained the same between the plans on original CT and daily CT.
Result: The target doses were observed to be not affected by the off target placement of isocenter. On average, the dose to 99% GTV and 95% PTV differed from planned value by 0.25 and 0.35 Gy respectively. The 0.1 and 1cc cord doses differed by .23 and .85 Gy respectively.
Conclusions: Off center placement of isocenter does not affect target coverage and serial critical structure dose such as cord. However, care should be exercised not to beam penumbra adjacent to critical structure.

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