ESTRO 2024 - Abstract Book

S3185

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Keywords: gamma index analysis, Octavius4D, Verisoft

229

Digital Poster

Deviations from effective SSD for kilovoltage radiotherapy stand off factors

David Nash, John R Kearton, Antony L Palmer

Portsmouth Hospitals University NHS Trust, Medical Physics, Portsmouth, United Kingdom

Purpose/Objective:

It is known that the dose per MU deviates from an inverse square law (ISL) relationship with stand-off distance from closed-ended applicators for kilovoltage radiotherapy. AAPM guidance [1] recommends the use of the effective SSD when correcting for stand off. In this work deviations in actual stand off factors from those derived from the effective SSD will be described for an XStrahl (Walsall, UK) 200 kilovoltage x-ray unit at 180 and 220 kV (HVL of 0.55 and 2.13 mmCu respectively), with an alternative method proposed.

Material/Methods:

A PTW (Freiberg, Germany) 3D 31021 Semiflex chamber was positioned vertically in an empty IBA (Schwarzenbruck, Germany) BluePhantom 2 watertank under a 50 cm SSD 10x10 cm applicator, initially in contact with the end of the applicator (0.15 cm to effective point of measurement (EPOM)). The charge was measured at a range of stands offs up to 5 cm (5.15 cm from front face to EPOM). Stand off factors were calculated from this data via two methods: (1) calculating the effective SSD by determining the intercept from a graph of stand off versus the square root of the charge and (2) calculating the quadratic fit to the readings and determining the reading at the surface of the applicator. The stand off factor could then be calculated according to charge at applicator surface/charge at stand off. The theoretical stand off according to the ISL was also calculated for reference. The stand off factors were plotted for both energies and both calculation methods. The stand off factors were also measured with a PTW MicroDiamond for confirmation via the quadratic fit method. To demonstrate the challenges with measurement, the effective SSD was calculated using different ranges of distance from the end of the applicator (e.g. 0.15 cm to 2.15cm or 0.15 cm to 5.15 cm).

Results:

Figure 1 is a plot of the measured and derived stand off factors. For 180 kV the stand off factors calculated via the effective SSD were up to -1.9% different to measured and -6.4% different to the ISL, and for 220 kV the stand off factors were up to -1.6% different to measured and up to -4.6% different to ISL. This would result in the calculated