Click here to


Are you sure ?

Yes, do it No, cancel

A New Formula for Calculating Standoff Factors for Orthovoltage X-Ray Therapy Unit

R ELGENDY1*, A Elawadi2 , M Alshanqity3 , S Almohsen4 , N ElQathamii5 , R Mohamed6 , (1) King Fahad Medical City Hospital, Riyadh, (2) Riyadh, (3) King Fahad Medical City Hospital, Riyadh, ,(4) Saihat Kingdom, (5) Riyadh, (6) Consultant radiation oncology, associate prof. NCI, Riyadh


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To introduce simple, accurate and applicable method to calculate standoff factor (SF) used for the patient’s dose calculations that treated with the orthovoltage machine.

Methods: Xstrahl 300 orthovoltage machine, produces 9 energies of x-ray beam from 60 to 300 kVp.A). Open Applicator a)SF Measurements: Small acrylic slab phantom and soft x-ray chamber 0.02cc connected to electrometer. Ruler fixed to Gantry to determine standoff distances (SD) up to 5cm with increment 0.5cm. Calculate the measured SF as:(reading at surface / reading at SD). Plot SF as a function of SD for all applicator and energies. Add trend line to get the general fitting equation.b)SF calculations using Inverse Square Law (ISL) at extended FSD (EFSD): given as: Reading at SD. (FSD / (FSD+SD))2.B)Closed Applicator a)SF Measurements: PTW water phantom, 0.6cc PTW farmer chamber fixed at 2 cm depth and connected to electrometer. Calculate SF corrected to the surface dose. Plot SF as in open get the general fitting equation . b)SF Calculations using ISL at extended FSD and effective FSD (Eff FSD).

Results: Open circular Applicator: The max absolute percentage %DD are 1.2%, 2.2% and 1.4% using general fitted equation comparing to 1.8%, 3.4%, and 2.7% using EFSD for 60, 80, and 100 kev respectively.Closed Applicator: The max absolute %DD are 2.5%, 1.7%, 1.5%, 1.7%, 1.3% and 1.4% for general fitted equation in comparing to 4.2%, 3.5%, 3.2%, 2.8%, 2.2% and 2.5% for EFSD and 4%, 2.8%, 2.5%, 2.5%, 1.7%, and 1.7 % for Eff FSD for 120 150, 180, 200, 250, and 300 kev respectively.

Conclusion: The general fitting equation gives very good agreement for SF calculations at standoff distances ≤ 5 cm for all energies and applicators for this type of orthovoltage machine. For other types of machines its recommend to verify it by some measurements.


Not Applicable / None Entered.


TH- External beam- photons: Kilovoltage therapy

Contact Email