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Entrance Dose Evaluation During KV-CBCT Scans for Different KVp and FOV Settings


B Sanchez- Nieto

B Sanchez- Nieto1*, P Caprile2, F Lagos3, F Lucic4, Z Gabriel5, (1) Pontificia Universidad Catolica de Chile, Santiago de Chile,Chile,(2) Pontificia Universidad Catolica de Chile, Santiago de Chile,Chile,(3) Pontificia Universidad Catolica de Chile, Santiago de Chile,Chile,(4) Clinica Alemana de Santiago, Santiago de Chile, Chile,(5) Clinica Alemana de Santiago, Santiago de Chile,Chile

SU-E-J-21 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
The aim of this study is to evaluate the entrance dose of an imaged cylindrical phantom with a KV-CBCT, using thermoluminescence dosimetry, for different settings.

Methods:
A KV-CBCT unit, integrated in the Elekta Synergy linear accelerator, was used to image the commercial phantom Catphan 504. Dose measurements were performed with TLD-100 chips distributed around the phantom surface for 100 and 120 KVp values, for different collimator cassettes and nominal values of mA (from 25 to 100) and ms (from 40 to 10), keeping the product constant.

Results:
The dose was not uniform over the surface for the same parameters, with differences up to 20%, being the largest dose at the posterior position. As expected, the dose increased with KVp, being the dose values for the 120 KVp around 60% larger than for the 100 KVp. Different superficial doses were obtained for collimators with the same projected field size at the isocenter. An average 25% increase in dose was observed for the S20 collimator cassette with respect to the M20, for the complete rotation of the unit. The largest relative dose was obtained for the posterior position, with 120KVp, using the S20 collimator. Eventhough mA*ms kept constant, for each KVp, entrance dose increased with mA.

Conclusion:
The variation over the surface could be attributed to the different relative proximity to surrounding scatter material (e.g., floor, walls, etc.). There is a clear impact of collimator cassette selection on the dose to the surface, being larger for smaller FOVs. This effect is minimized clinically; as when using smaller FOVs, a complete rotation is not required for image reconstruction (the S20 collimator is centered). Effectively, the collimator displacement to obtain larger FOVs, produces lesser dose. This result could be explained by the heel effect, as it generates a reduced intensity towards the anode.

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