The value of the PinPoint ion chamber for characterization of small field segments used in intensity-modulated radiotherapy.

Volume averaging and lack of electronic equilibrium complicate accurate dosimetry of small photon fields. In this paper the performance of the PinPoint ion chamber for characterizing small fields used in intensity-modulated radiotherapy (IMRT) was investigated and the results were compared with those obtained using the Markus ion chamber and a diamond detector. Sharp beam penumbras were measured for a 5 x 5 cm field defined using a cerrobend block mounted on the accelerator head. In addition, output factors were measured for a 6 MV photon beam and a variety of small rectangular fields collimated widthwise using the multileaf collimator (MLC) in combination with the back-up jaws. From this study, a reference field of 5 x 5 cm and a measuring depth of 5 cm are recommended. This is related to the over-response of the PinPoint chamber to low-energy Compton scattered photons, an effect that was investigated rigorously and turned out to limit the scope of this ionization chamber. However, taking into account some limitations, the PinPoint chamber is an excellent detector for output measurements in small fields down to 2 cm. In profile measurements the chamber causes a broadening of the measured penumbras but its spatial resolution is superior to that of the Markus chamber.

[1]  B. Bjärngard,et al.  Doses on the central axes of narrow 6-MV x-ray beams. , 1990, Medical physics.

[2]  G. Hartmann,et al.  Tissue maximum ratios (and other parameters) of small circular 4, 6, 10, 15 and 24 MV x-ray beams for radiosurgery. , 1992, Physics in medicine and biology.

[3]  A R Smith,et al.  Evaluation of multileaf collimator design for a photon beam. , 1992, International journal of radiation oncology, biology, physics.

[4]  W A Beckham,et al.  Dose rate dependence of a PTW diamond detector in the dosimetry of a 6 MV photon beam. , 1994, Physics in medicine and biology.

[5]  Film dosimetry in water in a 23 MV therapeutic photon beam. , 1995 .

[6]  S. Rustgi.  Evaluation of the dosimetric characteristics of a diamond detector for photon beam measurements. , 1995, Medical physics.

[7]  P W Hoban,et al.  A comparison of dosimetry techniques in stereotactic radiosurgery. , 1996, Physics in medicine and biology.

[8]  Energy and dose rate dependence of a diamond detector in the dosimetry of 4-25 MV photon beams. , 1997, Medical physics.

[9]  S. Burch,et al.  A new approach to film dosimetry for high energy photon beams: lateral scatter filtering. , 1997, Medical physics.

[10]  S Cora,et al.  Use of a new type of radiochromic film, a new parallel-plate micro-chamber, MOSFETs, and TLD 800 microcubes in the dosimetry of small beams. , 1998, Medical physics.

[11]  D. Thwaites,et al.  Assessment of new small-field detectors against standard-field detectors for practical stereotactic beam data acquisition. , 1999, Physics in medicine and biology.

[12]  J M Slater,et al.  Dosimetry techniques for narrow proton beam radiosurgery. , 1999, Physics in medicine and biology.

[13]  H. Thierens,et al.  Dose measurements compared with Monte Carlo simulations of narrow 6 MV multileaf collimator shaped photon beams. , 1999, Medical physics.

[14]  How much does film sensitivity increase at depth for larger field sizes? , 1999, Medical physics.