Experimental study of humidity effect on charge measurement of reference ionization chambers in clinical high-energy photon beams.

PURPOSE: In the practice code of dosimetry, humidity effect is assumed to be constant as far as the measurements are performed in the relative humidity (RH) range of (20-80)%; thus, the humidity effect can be ignored with a dose uncertainty of 0.15%. This assumption is based on the previous experimental results by Niatel and Guiho. Rogers and Ross calculated the stopping power ratio of humid air and dry air for high-energy electron beams by using a Monte Carlo code. They demonstrate that the W value, the mean energy required to create an ion pair in air, is independent of the beam quality when the air is dry, and that the traditional humidity correction can be used also for high-energy photon and electron beams; however, this was only a computational study. In the present study, we measured the humidity correction of Farmer-type ionization chambers in high-energy photon beams and determined the W values of humid air using the calculated energy deposition of humid air with a Monte Carlo code. Furthermore, we proposed an analytical expression to determine a practical humidity correction for an ionization chamber as a function of absolute humidity. METHOD: Experiments were carried out using a clinical linear accelerator (linac, Elekta Precise) at the National Metrology Institute of Japan (NMIJ). A shield box was constructed downstream of the linac and connected to an air processor, which maintained the temperature around 22°C and controlled the humidity in the range of (10-70)% inside the box. We prepared two Farmer-type ionization chambers: PTW 30013 and Exradin A19. Each ionization chamber was placed inside the box and irradiated with 6-, 10-, and 15-MV high-energy photon beams from the clinical linac. The energy deposition to the humid air inside the ionization chamber was calculated using the Electron Gamma Shower Version 5 (EGS5) code system. RESULTS: Stabilization for the humidity of the ionization chamber was completed within 3 h. The polarity and ion recombination corrections did not show any change in the humidity range studied. The measured humidity correction and the evaluated W values of humid air in high-energy photon beams were in good agreement with those by Rogers in TG-21 and by Niatel in the range of RH (10-70)%. CONCLUSION: Humidity correction of ionization chambers in high-energy photon beams from the clinical linac was determined experimentally. Using the analytical expression for the energy depositions by EGS5, the analytical expression for the W values was also derived.

Experimental evaluation of conversion factors, N(D,w)/N(X), for Roos-type and Advanced-Markus-type plane-parallel ionization chambers.

A Japanese code of practice for clinical dosimetry, titled "Standard Dosimetry of Absorbed Dose in External Beam Radiotherapy" was published by the Japan Society of Medical Physics (JSMP) in 2002. It mostly followed IAEA Technical Reports Series No. 398, which was based on N(D,w), i.e., the calibration factor in terms of absorbed dose to water for a dosimeter. The Japanese primary standard dosimetry laboratory, however, has not supplied N(D,w) but N(X), as the calibration factor in terms of exposure. The unique feature of the Japanese code was provision of a data table of calculated conversion factors, N(D,w) / N(X) values, for many types of ionization chambers, excluding new plane-parallel ionization chambers. This paper describes the experimental evaluation of the conversion factors for the new plane-parallel ionization chambers, such as the Roos-type and Advanced Markus chambers. The obtained N(D,w) / N(X) values for PTW 34001, Wellhöfer PPC 40 and PTW 34045 were 37.96 +/- 0.19, 37.85 +/- 0.36 and 37.90 +/- 0.26 (Gy/C kg(-1)), respectively. They agreed with estimations based on Monte Carlo calculations.

[New method of estimating effective energy for X-ray CT scanners].

Because the exposure dose in X-ray computed tomography examinations is sometimes difficult to determine, it is important to be able to estimate the dose for these examinations. The effective energy of the X-ray CT scanner is required to estimate exposure dose. Although the half-value-layer (HVL) method has been used to calculate effective energy, it is not an easy method. This paper proposes a technique by which effective energy can be easily calculated. Certain details were found to cause change in effective energy, and the ratio (inner-metal center-air ratio: IMCAR) between air dose and dose in fixing the metallic pipe in the isocenter of an X-ray CT scanner was necessary. The IMCAR from a different X-ray CT scanner was required, and, when effective energy was calculated, it showed an error of less than 0.7% for the half-value-layer method. The effect of this error on dose estimation was slight (0.4%). This technique is useful, because effective energy can easily be calculated with a high degree of accuracy.

Determination of the depth of 50% of maximum ionization, I50, for electron beams by the divided difference method.

A new characterization of depth-ionization parameters for electron beams is empirically deduced from our data analysis based on the divided difference method (the DD method), which employs the numerical differential of an ionization curve. The important feature of the present method is that it does not necessarily require normalized percent depth-ionization (NPDI) data. The depth of 50% of maximum ionization, I50, which is an important parameter for electron beam dosimetry, can be deduced from the analysis of an unnormalized (or partial) depth-ionization (UDI) curve obtained over a short interval of depth. The values of I50 determined by the DD method are in agreement to within 0.1 mm for energies of 4, 6, and 9 MeV, compared with the ones determined by the TG-51 protocol method (or the conventional method), and the difference was 0.9 mm for 12 and 15 MeV. The dose at the reference depth, dref, calculated from I50 by the DD method, is found to be in agreement with TG-51 to within 0.1%. The field size dependence of the DD method using UDI data was studied for three field sizes: 6 x 6, 10 x 10, and 20 x 20 cm2. For all energies, the discrepancies of I50 as determined by both methods were 0.9 mm on average for the 6 x 6 cm2 fields and 0.6 mm for the other two field sizes. This dependence was remarkable for 6 x 6 cm2 fields for 12 and 15 MeV, and the discrepancies shown by the DD method were 1.2 mm for 12 MeV and 1.8 mm for 15 MeV, respectively. Since the reference field size in clinical dosimetry is usually 10 x 10 cm2, this dependence will not affect clinical dosimetry. The DD method could be an alternative option for checking beam quality in dose calibration.

[Comparative evaluation of absorbed dose and image quality in X-ray head CT scanning for hospital facilities in Gunma.].

Recently, the number of scans for X-ray computed tomography (CT) examinations has been rising due to the wide-spread use of multi-slice CT (MSCT) scanners. There is a concern that the total medical exposures will be increased by these examinations. In order to lower exposures, routine parameters for head CT examinations done at several hospitals in Gunma were investigated. In this study, the computed tomography dose index (CTDI(100, C)), noise, and low contrast resolution were measured. The CTDI(100, C) for all the hospitals exceeded the guideline (40mGy) suggested by the Japan Association of Radiological Technologists (JART). Low contrast resolution showed the coefficient of variation of +/-5% between hospitals. In conclusion, it was proposed that the technologists should reconsider the parameters of the head X-ray CTs, in cases where their output dose far exceeds the standard.

Maintained Salivary Function after Brachytherapy in Patients with Head and Neck Carcinomas - Evaluation Using Quantitative Salivary Gland Scintigraphy.

The purpose of this study was to determine whether or not salivary gland dysfunction occurs within the first three months after brachytherapy in patients with head and neck carcinoma. Of the 20 patients with head and neck squamous cell carcinoma included in this study, 11 were treated with brachytherapy and the remaining 9 patients received external irradiation. All the patients underwent a salivary gland scintigraphy before and after radiotherapy. The scintigraphic parameters of each major salivary gland were then compared before and after the radiotherapy. In the brachytherapy group, none of the scintigraphic functional parameters showed a significant change before and after the radiotherapy. In contrast, all of the parameters with the exception of the uptake ratio (UR) of the submandibular glands significantly decreased after external irradiation. This observation was to be expected owing to the different irradiation doses administered by the two techniques. The scintigraphic technique used to evaluate salivary gland function should be used in future intensity-modulated radiation therapy salivary-gland-sparing studies in order to evaluate both the acute and chronic effects of irradiation in head and neck cancer patients.

Maintained salivary function after brachytherapy in patients with head and neck carcinomas--evaluation using quantitative salivary gland scintigraphy.

The purpose of this study was to determine whether or not salivary gland dysfunction occurs within the first three months after brachytherapy in patients with head and neck carcinoma. Of the 20 patients with head and neck squamous cell carcinoma included in this study, 11 were treated with brachytherapy and the remaining 9 patients received external irradiation. All the patients underwent a salivary gland scintigraphy before and after radiotherapy. The scintigraphic parameters of each major salivary gland were then compared before and after the radiotherapy. In the brachytherapy group, none of the scintigraphic functional parameters showed a significant change before and after the radiotherapy. In contrast, all of the parameters with the exception of the uptake ratio (UR) of the submandibular glands significantly decreased after external irradiation. This observation was to be expected owing to the different irradiation doses administered by the two techniques. The scintigraphic technique used to evaluate salivary gland function should be used in future intensity-modulated radiation therapy salivary-gland-sparing studies in order to evaluate both the acute and chronic effects of irradiation in head and neck cancer patients.

[A monitor unit verification calculation independent on RTP systems and output factor]

In order to confirm quality assurance and quality control in radiation therapy, it is common to perform a monitor unit verification calculation independent on RTP system. Generally, this verification calculation is often to perform at manual calculation level separately from RTP system. Characteristics of output factors included in this calculation were explained on the basis of the situation in the world. Khan et al. had proposed the concept of collimator scatter factor and phantom scatter factor. The validity of these factors was addressed in of role played in dose monitor unit calculation. If we will utilize SMR, which was derived from zero-area phantom scatter factor, and zero-area TMR in dosimetry, a simple and accurate Clarkson integration is possible. These scatter factors and Clarkson integration is able to use not only for monitor unit calculation in a simple treatment technique, but also in an excellent technique as IMRT. These factors had been also used in the convolution method to improve an accuracy of dose calculation. These factors are not family in Japan, but it is expected to be got an agreement of the usage in future.