A new method for output factor determination in MLC shaped narrow beams

Commissioning of a PTW 34070 large‐area plane‐parallel ionization chamber for small field megavoltage photon dosimetry

“…Under knowledge of DAP w , it is then possible to derive the output factor for a small field from the relative dose distribution, as demonstrated by Sanchez‐Doblado et al8 A further use may be that of using the LAC as a transfer dosimeter to enable the calibration of small field detectors in a small reference field against a primary standard in primary standards laboratories, as outlined by Dufreneix et al15 …”

“…To improve the accuracy of the film dosimetry in the low‐dose region, we employed a method similar to that presented by Sanchez‐Doblado et al8 Two pieces of film were exposed at a high (4000 MU) and at a low (400 MU) setting, to ensure that the out‐of‐field and in‐field dose were within the dynamic range of EBT3 in one of the exposures. To combine the two exposures, the dose map obtained at 4000 MU was divided by 10, and both dose maps were aligned to each other using their respective beam centers.…”

“…Djouguela et al first described remarkable properties of DAP w measurements of small fields, such as the similarity of depth‐dose curves at constant source‐to‐detector distance ( DAP w ‐TPR) and constant source‐to‐surface distance ( DAP w ‐PDD) and the invariance of those curves with field size 7. Sanchez‐Doblado et al derived OR of small fields by combining measurements of DAP w and two‐dimensional dose distributions measured with radiochromic film, and found good agreement with diode measurements and Monte Carlo calculations 8. The Bragg Peak chamber has even found use as an upstream in‐line beam monitor by Lechner et al;9 and Heidorn et al have introduced the LAC as a quality assurance device in order to verify the constancy of the beam area for a Cyberknife Iris Variable Aperture Collimator 10…”

“…Several publications describe the use of a LAC, the PTW Bragg Peak chamber type 34070‐2.5 (PTW Freiburg, Germany), for measuring integral dose in small megavoltage photon fields 7, 8, 9, 10. Djouguela et al first described remarkable properties of DAP w measurements of small fields, such as the similarity of depth‐dose curves at constant source‐to‐detector distance ( DAP w ‐TPR) and constant source‐to‐surface distance ( DAP w ‐PDD) and the invariance of those curves with field size 7.…”

“…Since the PTW type 34070‐2.5 chamber has shown some potential useful applications for small field dosimetry measurements,7, 8 it was chosen as one of the detectors to measure DAP w at the Australian Radiation Protection and Nuclear Safety Agency's (ARPANSA) linear accelerator, which is the reference source for dosimetric calibration and audit services provided by ARPANSA.…”