ESTRO 2024 - Abstract Book

S3186

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

monitor units being lower than required. The graph demonstrates a change in gradient between the measured and effective SSD approaches. The MicroDiamond agreed well with the Semiflex; 180 kV to within 0.9% and 220 kV to within 0.5%. Table 1 shows how the effective SSD varies with the range of data used to calculate it, demonstrating that the data does not follow a perfect inverse square relationship, implying a linear or higher power component, and that data measured over increased stand off distances gives a longer effective SSD.

Figure 1. Stand off factors for 180 and 220 kV for a 10x10 cm applicator on an XStrahl 200 unit.

Table 1. Effective SSD variation with measurement range.

Effective SSD (cm)

Measurement range (cm)

180 kV

220 kV

0.15-2.15

31.2

34.9

0.15-3.15

33.2

37.0

0.15-4.15

35.0

38.6

0.15-5.15

36.9

39.7

Conclusion:

It has been shown that the use of effective SSD for stand off corrections for closed ended kilovoltage applications is imprecise with errors up to -1.9% against measurement, with potential for underdose. It has also been shown that effective SSD determination is very sensitive to the measurement range used to calculate it. Therefore, to mitigate this uncertainty, directly measured stand off factors are preferable for the derivation of clinical data rather than effective SSD derived.

Keywords: kilovoltage, dosimetry

References: