Open AccessAssessing the deviation from the inverse square law for orthovoltage beams with closed‐ended applicators
Author(s) -
Gräfe James,
Poirier Yannick,
Jacso Ferenc,
Khan Rao,
Liu HongWei,
Eduardo VillarrealBarajas J.
Publication year - 2014
Publication title -
journal of applied clinical medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.83
H-Index - 48
ISSN - 1526-9914
DOI - 10.1120/jacmp.v15i4.4893
Subject(s) - inverse square law , divergence (linguistics) , computation , square (algebra) , inverse , beam divergence , standard deviation , beam (structure) , field size , optics , scattering , physics , field (mathematics) , mathematics , geometry , statistics , beam diameter , algorithm , classical mechanics , laser beams , linguistics , philosophy , gravitation , laser , pure mathematics
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 cm 2 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 cm 2 and 10 × 10 cm 2 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. PACS numbers: 87.55.‐x, 87.55.kh