title: Assessing the deviation from the inverse square law for orthovoltage beams with closed-ended applicators.
abstract: In this report, we quantify the divergence from the inverse square law (ISL) of the beam output as a function of distance (standoff) from closed-ended applicators for a modern clinical orthovoltage unit. The divergence is clinically significant exceeding 3% at a 1.2 cm distance for 4 × 4 and 10 × 10 cm2 closed-ended applicators. For all investigated cases, the measured dose falloff is more rapid than that predicted by the ISL and, therefore, causes a systematic underdose when using the ISL for dose calculations at extended SSD. The observed divergence from the ISL in closed-ended applicators can be explained by the end-plate scattering contribution not accounted for in the ISL calculation. The standoff measurements were also compared to the predictions from a home-built kV dose computation algorithm, kVDoseCalc. The kVDoseCalc computation predicted a more rapid falloff with distance than observed experimentally. The computation and measurements agree to within 1.1% for standoff distances of 3 cm or less for 4 × 4 cm2 and 10 × 10 cm2 field sizes. The overall agreement is within 2.3% for all field sizes and standoff distances measured. No significant deviation from the ISL was observed for open-ended applicators for standoff distances up to 10 cm.
authors.affiliation: Tom Baker Cancer Centre University of Calgary. james.grafe@gmail.com.
authors.forename: James
authors.initials: J
authors.lastname: Gräfe
authors.forename: Yannick
authors.initials: Y
authors.lastname: Poirier
authors.forename: Ferenc
authors.initials: F
authors.lastname: Jacso
authors.forename: Rao
authors.initials: R
authors.lastname: Khan
authors.forename: Hong-Wei
authors.initials: HW
authors.lastname: Liu
authors.forename: J Eduardo
authors.initials: JE
authors.lastname: Villarreal-Barajas
mesh_terms: D000465:Algorithms; D004583:Electrons; D006801:Humans; D010315:Particle Accelerators; D011878:Radiotherapy / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted