ESTRO 2024 - Abstract Book

S3305

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

cm x 0.6 cm, 0.8 cm x 0.8 cm and 1 x 1 cm – 5 x 5 cm. The DLG was also measured in water using a IBA Farmer chamber and the standard sweeping gap test (5). All measurements were performed one week prior- and after a 6 MV target replacement of a Varian TrueBeam linac.

Results:

Lateral dose profiles of 0.6 cm x 0.6 cm field size before- and after x-ray target replacement, are shown in Figure 1. It is observed that, in the flattened region (above 80% of normalized dose in profile center), the profile in the X-direction is higher after the x-ray target replacement than before. This can be attributed to the new x-ray target providing smaller spot size and decreasing the occlusion effect (6). However, this trend is not obvious in the Y-direction. The penumbra for larger fields reflects the spot size (7), which is why penumbra widths from profiles of 5 cm x 5 cm field size, were investigated. Figure 2 shows the penumbra region in the X- and Y-directions of 5 cm x 5 cm field size. Also, Calculated penumbra widths are given in Table 1. It is seen that the penumbra width decreases in the X-direction after x-ray target replacement, while it increases in the Y-direction. This indicates that the spot size is smaller in the X-direction and larger in the Y-direction after the x-ray target replacement. This explains the results of measured OF before and after the x-ray target replacement, which shows no significant difference (within 0.4%). It can be argued that the effect of the increase/decrease of the spot size in both directions cancel each other out in terms of OFs. The DLG increased by 3.6% after the x-ray target replacement. It should be noted that the MLCs are aligned in the X direction. The increment of DLG can be attributed to the earlier finding of the spot size being smaller in the X-direction after the x-ray target replacement. Smaller spot size mainly affects the narrowest slits of the DLG test, reducing the occlusion effect and increasing the detector reading. This in turn results in higher DLG value. Consequently, calibration of the DLG was needed.