New standoff-factor formula for orthovoltage radiotherapy treatments

Orthovoltage x-rays are useful for the treatment of some superficial cancers and benign conditions. An orthovoltage machine has numerous different applicators (open and closed ended) and energies that require measurements for all different applicator-energy combinations in addition to patient-specific Standoff Factor (SF) measurements, which is arduous and time-consuming. This study aimed to introduce a simple, accurate, and practical method to calculate SF. This factor is usually calculated based on the inverse square law (ISL), which is not an accurate approximation for closed-ended applicators. In this work, we introduced a simple, accurate, and practical method to calculate SF that is valid for both open-ended and closed-ended applicators. Xstrahl 300 therapy unit was used with two sets of Open-ended and Closed-ended applicators with energies up to 300 kVp. The proposed SF empirical formula and ISL were evaluated against the measurements. For open-ended applicators, the maximum Percentage Differences (PD) in calculated SF using the suggested formula and ISL were 0.84% and 1.97% relative to the measurement, respectively. For closed-ended applicators, the maximum PD was 2.53% and -8.12% using the suggested formula and ISL relative to the measurement, respectively. The results demonstrated satisfactory accuracy compared to the measured standoff factor values and superior accuracy when compared to the commonly used ISL method, particularly for closed-ended applicators. The study concluded that SF calculated using the proposed formula was in agreement with measured SF at clinically relevant standoff distances for all energies and applicators combinations. Thus, we recommend using this proposed formula for SF calculations.