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Comparison of PTV Margins Determined by In-Room Stereoscopic Image Guidance and by On-Board Cone Beam Computed Tomography Technique for Brain Radiotherapy Patients

T Ganesh

T Ganesh*, s Paul , A Munshi , B Sarkar , S Krishnankutty , J Sathya , S George , K Jassal , S Roy , B Mohanti , Fortis Memorial Research Institute, Gurgaon, Other (Non U.S.)


SU-E-J-39 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Stereoscopic in room kV image guidance is a faster tool in daily monitoring of patient positioning. Our centre, for the first time in the world, has integrated such a solution from BrainLAB (ExacTrac) with Elekta's volumetric cone beam computed tomography (XVI). Using van Herk's formula, we compared the planning target volume (PTV) margins calculated by both these systems for patients treated with brain radiotherapy.

For a total of 24 patients who received partial or whole brain radiotherapy, verification images were acquired for 524 treatment sessions by XVI and for 334 sessions by ExacTrac out of the total 547 sessions. Systematic and random errors were calculated in cranio-caudal, lateral and antero-posterior directions for both techniques. PTV margins were then determined using van Herk formula.

In the cranio-caudal direction, systematic error, random error and the calculated PTV margin were found to be 0.13 cm, 0.12 cm and 0.41 cm with XVI and 0.14 cm, 0.13 cm and 0.44 cm with ExacTrac. The corresponding values in lateral direction were 0.13 cm 0.1 cm and 0.4 cm with XVI and 0.13 cm, 0.12 cm and 0.42 cm with ExacTrac imaging. The same parameters for antero-posterior were for 0.1 cm, 0.11 cm and 0.34 cm with XVI and 0.13 cm, 0.16 cm and 0.43 cm with ExacTrac imaging. The margins estimated with the two imaging modalities were comparable within ± 1 mm limit.

Verification of setup errors in the major axes by two independent imaging systems showed the results are comparable and within ± 1 mm. This implies that planar imaging based ExacTrac can yield equal accuracy in setup error determination as the time consuming volumetric imaging which is considered as the gold standard. Accordingly PTV margins estimated by this faster imaging technique can be confidently used in clinical setup.

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