[Absorbed dose measurement of photon beam with Farmer-type ionization chambers in Japanese dosimetry protocols].

The Japan Society of Medical Physics (JSMP) has published a new dosimetry protocol "JSMP-01" (standard dosimetry of absorbed dose in external beam radiotherapy) which conforms to the recommendations of the International Atomic Energy Agency (IAEA TRS-398) and the American Association of Physicists in Medicine (AAPM TG-51) protocols for the calibration of radiotherapy beams. Since the new protocol offers the physical data for the Famer-type ionization chambers of the various wall materials, the user can measure the absorbed dose at reference point (D(r)) using most of the commercially available Famer-type ionization chambers. In this paper, the six Famer-type ionization chambers of the various wall materials are examined for photon beam by two ways. To verify the JSMP-01 protocol as the first way, D(r) was cross-measured based on the JSMP-01 protocol using a Farmer-type ionization chamber of the acrylic wall material which is called "JARP-chamber" and the Farmer-type ionization chambers of the various wall materials, and compared. To compare the basic data in previous and new protocols as the second way, D(r) was measured based on the previous protocol (JSMP-86) and the JSMP-01 protocol using the Farmer-type ionization chambers of the various wall materials. Dose calculation was made using common exposure calibration factor for (60)Co gamma-rays (Nc) for each of the Farmer-type ionization chambers. Measurement was made with each ionization chamber for 6 and 10 MV photon beams in two facilities. D(r) were found to agree to that of JARP-chamber within about +/- 1% despite significant differences of ratio of calibration factor (k(D,X)) and beam quality conversion factor (k(Q)) for photon beams. The ratios JSMP-01/JSMP-86 of the reference dose were found to lie on between 0.999 and 1.004 for 6 MV and on between 0.999 and 1.005 for 10 MV depending upon the Farmer-type ionization chambers used. The largest discrepancies between the previous and new protocols arise from the use of different data of k(D,x) x k(Q) and C(lambda) for the absorbed dose conversion factors of each ionization chamber.

Contribution of Cerenkov radiation in high-energy x-ray and electron beam film dosimetry using water-substitute phantoms.

The contribution of Cerenkov radiation in high-energy film dosimetry was investigated using commercially available water-substitute phantoms. Doses were evaluated using six phantoms: RMI-451, Mix-DP, WE-211, WE-Black, PMMA and PMMA-Black. The contribution of Cerenkov radiation was determined from the shielded and unshielded evaluation doses when a bare film was inserted into the phantom in a dark room and irradiated. For both x-ray and electron beams, Cerenkov radiation produced a phantom-dependent increase in the unshielded dose when compared with the shielded dose. We also found that the darker the phantom, the smaller the contribution of Cerenkov radiation. These results suggest that for film dosimetry using bare film, the accuracy of dose evaluation may be improved by using phantoms with high opacity.

[Trial production of water substitute phantom considering the effect of light].

We have produced a novel water substitute phantom suitable for film dosimetry, while retaining the radiological property of a conventional water substitute phantom. The novel phantom excludes the effect of light, which is known to affect the accuracy of results on conventional phantoms. The effect of light was eliminated by appropriately adjusting the quantum of the carbon black to that of a conventional phantom material. Through comparison of the novel phantom with a conventional phantom it was shown that the absorbed dose determined by conventional phantom was 15% higher for 10 MV X-rays and 18% higher for an 18 MeV electron beam, attributable to the contamination of Cerenkov light. Although the net optical density of the film increased with time owing to the optical permeability of the phantom, that of the novel phantom did not vary with time. The novel phantom was therefore shown to be unaffected by such local light and by the optical transmission of the phantom.