Correction method for in-air output ratio for output variations occurring with changes in backscattered radiation

• Published in: Medical physics (Lancaster) Vol. 39; no. 2; pp. 671 - 675
• Main Authors: Tajiri, Minoru, Tokiya, Yuji, Watanabe, Kazuhiro
• Format: Journal Article
• Language: English
• Published: United States American Association of Physicists in Medicine 01.02.2012
• Subjects: Air; ASYMMETRY; backscatter; BACKSCATTERING; COLLIMATORS; Computer Simulation; Conformal radiation treatment; CORRECTIONS; equivalent square field; Field size; in-air output ratio; Ionization chambers; Lead; Linear accelerators; Models, Theoretical; monitor-backscatter effect; output variation; Photons; Polynomials; radiation therapy; RADIOLOGY AND NUCLEAR MEDICINE; Radiometry - methods; RADIOTHERAPY; Radiotherapy, Computer-Assisted - methods; Radiotherapy, Conformal - methods; rectangular field; Scattering, Radiation; SYMMETRY; Telescopes; X‐ray scattering; X‐rays; output variation; rectangular field; monitor-backscatter effect; equivalent square field; in-air output ratio
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.3675405

Purpose: The in-air output ratio (Sc) for a rectangular field is usually obtained using an equivalent square field formula. However, it is well-known that Sc obtained using an equivalent square field formula differs slightly from the measured Sc. Though several correction methods have been suggested for the monitor-backscatter effect, the authors propose a more simple correction method for a rectangular field. Methods: For rectangular fields and equivalent square fields, the authors assumed that the output variation was the product of six output variations for each backscattering area at the top of the collimator jaws, and the correction factor was the ratio of the output variation for a rectangular field to the output variation for an equivalent square field. The output variation was measured by using a telescope measurement. Results: The differences between the measured and corrected Sc ranged from −0.20% to 0.28% for symmetric rectangular fields by applying the correction factor to Sc obtained using an equivalent square field formula. This correction method is also available for asymmetric rectangular fields. Conclusions: The authors propose a method to correct Sc obtained using an equivalent square field formula, and a method to obtain the output variation for a field defined by collimator jaws.