Dose falls-off faster than the inverse square law (ISL) for orthovoltage beams with closed-ended applicators. This work investigates the discrepancy for 30 cm FSD applicators. When using the ISL alone, the maximum dosimetric error would be 3% and 5% at 10 mm and 20 mm from the applicator, respectively, and increases with larger distances. The effective source position was found to be 22.5 cm and reduces the dosimetric error to less than 1.6% for distances less than 20 mm.