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Purpose Ionization chambers are widely used for dosimetry with megavoltage photon beams. Several properties of ionization chambers, including the cable effect, polarity effect, and ion recombination loss, are described in standard dosimetry protocols. The cable effect is categorized as the leakage current and Compton current, and careful consideration of these factors has been described not only in reference dosimetry but also in large fields. However, the mechanism of Compton current in the cable has not been investigated thoroughly. The cable effect of ionization chambers in 6 MV X-ray beam was evaluated by measurement, and the mechanism of Compton current was investigated by Monte Carlo simulation. Materials and methods Four PTW ionization chambers (TM30013, TM31010, TM31014, and TM31016) with the same type of mounted cable, but different ionization volumes, were used to measure output factor (OPF) and cable effect measurement. The OPF was measured to observe any variation resulting from the cable effect. The cable effect was evaluated separately for the leakage current and Compton current, and its charge per absorbed dose to water per cable length was estimated by a newly proposed method. The behavior of electrons and positrons in the core wire was analyzed and the Compton current for the photon beam was estimated by Monte Carlo simulation. Results In OPF measurement, the difference in the electrometer readings by polarity became obvious for the mini- or microchamber and its difference tended to be larger for a chamber with a smaller ionization volume. For the cable effect measurement, it was determined that the contribution of the leakage current to the cable effect was ignorable, while the Compton current was dominant. The charge due to the Compton current per absorbed dose to water per cable length was estimated to be 0.36 ± 0.03 pC Gy-1 cm-1 for PTW ionization chambers. As a result, the contribution of the Compton current to the electrometer readings was estimated to be 0.002% cm-1 for the Farmer-type, 0.011% cm-1 for the scanning, and 0.088% cm-1 for microchambers, respectively. By the simulation, it was determined that the Compton current for MV x-ray could be explained by not only recoil electrons due to Compton scattering but also positron due to pair production. The Compton current estimated by the difference in outflowing and inflowing charge was 0.45 pC Gy-1 cm-1 and was comparable with the measured value. Conclusion The cable effect, which includes the leakage current and Compton current, was quantitatively estimated for several chambers from measurements, and the mechanism of Compton current was investigated by Monte Carlo simulation. It was determined that the Compton current is a dominant component of the cable effect and its charge is consistently positive and nearly the same, irrespective of the ionization chamber volume. The contribution of Compton current to the electrometer readings was estimated for chambers. The mechanism of Compton current was analyzed and it was confirmed that the Compton current can be estimated from the difference in outflowing and inflowing charge to and from the core wire.

The dual-layer multi-leaf collimator (MLC) in Halcyon involves further complexities in the dose calculation process, because the leaf-tip transmission varies according to the leaf trailing pattern. For the volumetric modulated arc therapy (VMAT) treatment, the prescribed dose for the target volume can be sensitive to the leaf-tip transmission change. This report evaluates the dosimetric consequence due to the uncertainty of the dual-layer MLC model in Eclipse through the dose verifications for clinical VMAT. Additionally, the Halcyon leaf-tip model is empirically adjusted for the VMAT dose calculation with the Acuros XB.For this evaluation, an in-house program that analyzes the leaf position in each layer was developed. Thirty-two clinical VMAT plans were edited into three leaf sequences: dual layer (original), proximal single layer, or distal single layer. All leaf sequences were verified using Delta4 according to the dose difference (DD) and the global gamma index (GI). To improve the VMAT dose calculation accuracy, the dosimetric leaf gap (DLG) was adjusted to minimize the DD in single-layer leaf sequences.The mean of DD were -1.35%, -1.20%, and -1.34% in the dual-layer, proximal single-layer, and distal single-layer leaf sequences, respectively. The changes in the mean of DD between leaf sequences were within 0.2%. However, the calculated doses differed from the measured doses by approximately 1% in all leaf sequences. The tuned DLG was increased by 0.8 mm from the original DLG in Eclipse. When the tuned DLG was used in the dose calculation, the mean of DD neared 0% and GI with a criterion of 2%/2 mm yielded a pass rate of more than 98%.No significant change was confirmed in the dose calculation accuracy between the leaf sequences. Therefore, it is suggested that the dosimetric consequence due to the leaf trailing was negligibly small in clinical VMAT plans. The DLG tuning for Halcyon can be useful for reducing the dose calculation uncertainties in Eclipse VMAT and required in the commissioning for Acuros XB.

PURPOSE The ICRU has published new recommendations for ionizing radiation dosimetry. In this work, the effect of recommendations on the water-to-air and graphite-to-air restricted mass electronic stopping power ratios (sw, air and sg, air ) and the individual perturbation correction factors Pi was calculated. The effect on the beam quality conversion factors kQ for reference dosimetry of high-energy photon beams was estimated for all ionization chambers listed in the Addendum to AAPM's TG-51 protocol. METHODS The sw, air , sg, air , individual Pi, and kQ were calculated using EGSnrc Monte Carlo code system and key data of both ICRU report 37 and ICRU report 90. First, the Pi and kQ were calculated using precise models of eight ionization chambers: NE2571 (Nuclear Enterprise), 30013, 31010, 31021 (PTW), Exradin A12, A12S, A1SL (Standard imaging), and FC-65P (IBA). In this simulation, the radiation sources were one 60 Co beam and ten photon beams with nominal energy between 4 MV and 25 MV. Then, the change in kQ for ionization chambers listed in the Addendum to AAPM's TG-51 protocol was calculated by changing the specification of the simple-model of ionization chamber. The simple-models were made with only cylindrical component modules. In this simulation, the radiation sources of 60 Co beam and 24 MV photon beam were used. RESULTS The significant changes (p

Respiratory‐gated volumetric modulated arc therapy (gated VMAT) involves further complexities to the dose delivery process because the gantry rotation must repeatedly stop and restart according to the gating signals. In previous studies, the gantry rotation performances were evaluated by the difference between the plan and the machine log. However, several reports pointed out that log analysis does not sufficiently replicate the machine performance. In this report, a measurement‐based quality assurance of the relation between the gantry angle and gate‐on or gate‐off using triggered kilovoltage imaging and a cylinder phantom with 16 ball bearings is proposed. For the analysis, an in‐house program that estimates and corrects the phantom offset was developed. The gantry angle in static and gated arc delivery was compared between the machine log and the proposed method. The gantry was set every 5 deg through its full motion range in static delivery, and rotated at three speeds (2, 4 and 6 deg s‐1) with different gating intervals (1.5 or 3.0 s) in gated arc delivery. The mean and standard deviation of the angular differences between the log and the proposed method was −0.05 deg ± 0.12 deg in static delivery. The mean of the angular difference was within ±0.10 deg and the largest difference was 0.41 deg in gated arc delivery. The log records the output of the encoder so that miscalibration and mechanical sagging will be disregarded. However, the proposed method will help the users to detect the mechanical issues due to the repeated gantry stops and restarts in gated VMAT.

The ICRU has published new recommendations for ionizing radiation dosimetry. In this work, the effect of recommendations on the water-to-air and graphite-to-air restricted mass electronic stopping power ratios (sThe sThe significant changes (p 0.05) were observed for sThe effects of ICRU-90 recommendations on k

In Japan and North America, different dosimetry protocols have been implemented to determine the absorbed dose to water: JSMP Standard Dosimetry 12 and AAPM TG-51 addendum. In this study, Japanese and Canadian reference dosimetries for high energy photon beams were compared theoretically, and then they were verified experimentally. We estimated the theoretical differences of the ion recombination correction factors, the leakage correction factors, the radial dose distribution correction factors, the calibration factors, the beam quality correction factors and the absorbed dose to water. When an influence of the radial dose distribution is negligible, the ratios of Canadian to Japanese absorbed dose in reference dosimetries ranged from 0.995 to 1.007 for all the reference-class-Farmer-type ionization chambers. This discrepancy was mainly caused by the wall correction factor included in the beam quality correction factor. Subsequently, to verify the theoretical approaches, we calibrated the same ionization chamber in

Flattening filter-free (FFF) beams generated by linear accelerators have been widely adopted in many hospitals recently for radiation therapy. FFF technology can provide higher dose rates so that shortening of the treatment time and less intra-fraction motion error are expected.In Japan, the current way of determining absorbed dose to water for FFF beams is to follow the Standard Dosimetry 12 protocol which was developed for flattened beams. Since it has been reported that the flattened beams and FFF beams have different beam properties, it is necessary to evaluate the usefulness of Standard Dosimetry 12 protocol for FFF beam dosimetry.This report reviews physical and dosimetric properties of FFF beams especially in terms of the effect on absorbed dose to water dosimetry using an ionization chamber. From the review, it became evident that the absorbed dose to water is underestimated by volume averaging effect of the ionization chamber. On the other hand, the absorbed dose to water is overestimated by using the beam-quality specifier TPR

When a brass compensator is set in a treatment beam, beam hardening may take place. This variation of the energy spectrum may affect the accuracy of dose calculation by a treatment planning system and the results of dose measurement of brass compensator intensity modulated radiation therapy (IMRT). In addition, when X-rays pass the compensator, scattered photons are generated within the compensator. Scattered photons may affect the monitor unit (MU) calculation. In this study, to evaluate the variation of dose distribution by the compensator, dose distribution was measured and energy spectrum was simulated using the Monte Carlo method. To investigate the influence of beam hardening for dose measurement using an ionization chamber, the beam quality correction factor was determined. Moreover, to clarify the effect of scattered photons generated within the compensator for the MU calculation, the head scatter factor was measured and energy spectrum analyses were performed. As a result, when X-rays passed the brass compensator, beam hardening occurred and dose distribution was varied. The variation of dose distribution and energy spectrum was larger with decreasing field size. This means that energy spectrum should be reproduced correctly to obtain high accuracy of dose calculation for the compensator IMRT. On the other hand, the influence of beam hardening on k(Q) was insignificant. Furthermore, scattered photons were generated within the compensator, and scattered photons affect the head scatter factor. These results show that scattered photons must be taken into account for MU calculation for brass compensator IMRT.

This article is intended to improve the certainty of the absorbed dose determination for reference dosimetry in CyberKnife beams. The CyberKnife beams do not satisfy some conditions of the standard reference dosimetry protocols because of its unique treatment head structure and beam collimating system. Under the present state of affairs, the reference dosimetry has not been performed under uniform conditions and the beam quality correction factor kQ for an ordinary 6 MV linear accelerator has been temporally substituted for the kQ of the CyberKnife in many sites. Therefore, the reference conditions and kQ as a function of the beam quality index in a new way are required. The dose flatness and the error of dosimeter reading caused by radiation fields and detector size were analyzed to determine the reference conditions. Owing to the absence of beam flattening filter, the dose flatness of the CyberKnife beam was inferior to that of an ordinary 6 MV linear accelerator. And if the absorbed dose is measured with an ionization chamber which has cavity length of 2.4, 1.0 and 0.7 cm in reference dosimetry, the dose at the beam axis for a field of 6.0 cm collimator was underestimated 1.5%, 0.4%, and 0.2% on a calculation. Therefore, the maximum field shaped with a 6.0 cm collimator and ionization chamber which has a cavity length of 1.0 cm or shorter were recommended as the conditions of reference dosimetry. Furthermore, to determine the kQ for the CyberKnife, the realistic energy spectrum of photons and electrons in water was simulated with the BEAMnrc. The absence of beam flattening filter also caused softer photon energy spectrum than that of an ordinary 6 MV linear accelerator. Consequently, the kQ for ionization chambers of a suitable size were determined and tabulated as a function of measurable beam quality indexes in the CyberKnife beam.

This report describes the update of the beam quality conversion factor k(Q,Q0) of the standard dosimetry protocol in Japan. The k(Q,Q0) corrects for the difference between the response of an ionization chamber in the reference beam quality Q0 used for calibrating the chamber and in the actual user beam quality Q. All changes of k(Q,Q0) were caused by the perturbation correction factors which were recalculated by Monte Carlo simulation. With a calculation process, unsolved problems in this update are also discussed here.

河川線形のフラクタル次元を求める際, 地図から求めることが多いが, 本研究では, 衛星リモートセンシングデータを用い, ボックスカウンティング法で河川線形のフラクタル次元を求めた. 衛星数値データは, LANDSAT/MSS, LANDSAT/TM, SPOT/HRVの3種類を使用した. 九州北部の河川を対象として, そのフラクタル次元を各衛星数値データから求めたものと2種類および3種類の衛星数値データを併用して求めたものとで比較した. また, 中国松花江で起きた洪水時の河川の変化をフラクタル次元を用いて比較した. 得られた結論は次のとおりである. (1) 対象とした河川のフラクタル次元は1.0に近い値であり, 河川線形はそれほど複雑でなかった. (2) それぞれの衛星数値データから求めたものと併用したものとで, フラクタル次元にほとんど差はなかった. (3) 衛星数値データは河川線形のフラクタル次元の測定に有用である. (4) 洪水氾濫による河川流域の拡大は, 衛星数値データを用いたフラクタル次元によって, これまでよりも適切に表現できる.

This article is intended to improve the certainty of the absorbed dose determination for reference dosimetry in CyberKnife beams. The CyberKnife beams do not satisfy some conditions of the standard reference dosimetry protocols because of its unique treatment head structure and beam collimating system. Under the present state of affairs, the reference dosimetry has not been performed under uniform conditions and the beam quality correction factor kQ for an ordinary 6 MV linear accelerator has been temporally substituted for the kQ of the CyberKnife in many sites. Therefore, the reference conditions and kQ as a function of the beam quality index in a new way are required. The dose flatness and the error of dosimeter reading caused by radiation fields and detector size were analyzed to determine the reference conditions. Owing to the absence of beam flattening filter, the dose flatness of the CyberKnife beam was inferior to that of an ordinary 6 MV linear accelerator. And if the absorbed dose is measured with an ionization chamber which has cavity length of 2.4, 1.0 and 0.7 cm in reference dosimetry, the dose at the beam axis for a field of 6.0 cm collimator was underestimated 1.5%, 0.4%, and 0.2% on a calculation. Therefore, the maximum field shaped with a 6.0 cm collimator and ionization chamber which has a cavity length of 1.0 cm or shorter were recommended as the conditions of reference dosimetry. Furthermore, to determine the kQ for the CyberKnife, the realistic energy spectrum of photons and electrons in water was simulated with the BEAMnrc. The absence of beam flattening filter also caused softer photon energy spectrum than that of an ordinary 6 MV linear accelerator. Consequently, the kQ for ionization chambers of a suitable size were determined and tabulated as a function of measurable beam quality indexes in the CyberKnife beam.

Purpose: The purpose of this study is to investigate the tolerance limits of IMRT dose verification for establishing the Japanese IMRT QA guideline. For this purpose, we analyzed the results of IMRT dose verification of multicenter in Japan by statistical means. Methods: In this study, we studied head and neck field by field plan, head and neck hybrid plan, prostate field by field plan and prostate hybrid plan, separately. For 6 institutes in Japan, we analyzed the absorbed dose differences of IMRT dose verification plans between measurement and calculation which is expressed as percentage of calculation. In first, we evaluated the statistical nature of these dose differences. Then, to estimate the tolerance limit for IMRT absorbed dose verification, we calculated the confidence limit of the above dose differences, which was introduced by Palta. Results: For all of treatment sites and dose verification plan types, the frequency distributions of the dose differences showed the normal distribution. While the mean values of the dose differences were about equal in every case, the standard deviations for head and heck cases, in either plan type, were larger than prostate cases. The confidence limits of each case, head and neck field by field plan, head and neck hybrid plan, prostate field by field plan and prostate hybrid plan, were 4.5%, 4.0%, 3.0% and 2.4%, respectively. In head and neck cases, field by field plan, hybrid plan, the rate of the dose verification point passing criterion that 3.0% were 85.2% and 86.1%, respectively. Conclusions: The results of our study showed that 3% is an appropriate tolerance limit of IMRT absorbed dose verification for Japanese IMRT QA guideline. However, this tolerance limit may be slightly strict for head and neck cases. In the future, it may need to set the tolerance limit for every site. This research was supported by grant of Japanese Society of Medical Physics.

Purpose: The purpose of this study is to establish on‐site IMRT audit in Japan. Method and materials: We developed IMRT audit system using specialized water phantom. The phantom can be used to calibrate dose of reference condition and to validate monitor unit of conventional and IMRT field. In the specialized water phantom, detector can be set up at various positions, and various detectors can be used by replacing sleeves. The protocol of IMRT plan referred to shape and dose constraints of NCI “IMRT benchmark”. The planned IMRT dose was verified according to own QA protocol at each site. Then, audit items were evaluated by visit. The audit items were TPR20,10 for kQ determination, dose at reference condition (10 cm depth, 10×10 cm2 field), output factor (5×5, 10×10, 20×20 cm2 field), dose linearity (0.5, 1.0, 5.0 Gy) to evaluate basic commissioning status of RTP. Furthermore IMRT point dose at isocenter and geometrical arrangement of axial plane were evaluated with an ionization chamber and EDR2 verification films. The on‐site IMRT audit was done for 9 beams at 8 sites in 2009. Results: For reference condition, dose monitor unit (cGy/MU) was well adjusted within 1% at all sites. For IMRT point dose, difference between self and audit evaluation agreed within 1% at all sites. However dose difference between calculated and measured exceeded 3%, lack of commissioning was consequently pointed out at 2 sites. The reproducibility of geometrical arrangement was less than 3 mm at all sites. Conclusion: Feasibility study of on‐site IMRT audit was performed using specialized water phantom to reduce measurement uncertainty. It is confirmed that the on‐site IMRT audit combined reference and IMRT condition is reliable. To establish IMRT on‐site audit in Japan, visit site will be increased in 2010.

Purpose: The purpose of this study is to develop IMRT postal audit phantom using radiophotoluminescence glass dosimeter, and to report initial experiences of the phantom. Method and Materials:We developed the IMRT postal audit phantom. The phantom can be inserted with both RPL glass dosimeter, GD‐302M (Asahi Techno Glass) for point dose verification and Radiochromic film, EBT2 (ISP) for relative planar dose verification. In the dedicated phantom, RPL glass dosimeter can be placed in 4 positions corresponding to target and 1 position to OAR. The EBT2 film can be inserted on the central transverse plane in the phantom. The phantom was made of water substitute material, Tough Water WE‐211 (Kyoto kagaku). for the initial experiences, 7 fields segmental MLCIMRT was planned with XiO (Elekta) using 6 & 10 MV from Clinac iX‐S. In addition, to evaluate the measurement accuracy of RPL glass dosimeter, point doses with ionization chamber (PTW 30013 Farmer type and PTW 31016 Pinpoint 3D) were measured under the same conditions. Results: Differences between the point dose with RPL glass dosimeter and that with Pinpoint 3D chamber agreed within 2% at all positions (high and low dose regions). Point dose differences between with Farmer chamber and with Pinpoint 3D chamber agreed within 1% at target (high dose region). At OAR (low dose and steep dose gradient region), however, the dose difference was exceeded by 5%. This is due to volume averaging effect of Farmer chamber.Conclusion: Feasibility study of IMRT postal audit phantom was performed. The measurement dose with RPL glass dosimeter was good agreement with Pinpoint 3D ionization chamber. Therefore, the dedicated phantom using RPL glass dosimeter shows considerable potential for the IMRT postal audit. To establish IMRT postal audit in Japan, we are starting dummy run now.

Purpose: The radiophotoluminescence (RPL) glass dosimeter is suitable detector for postal dose audit program because it doesn't have fading effect and luminescence can be read repeatedly. However energy dependence cause of high‐Z is significant. Burlin cavity theory can be applied to dosimetry with RPL glass (r=2.4 g cm‐3) because it extended the Bragg‐Gray and Spencer‐Attix cavity theories to cavities of intermediate dimensions. Therefore, by using Burlin cavity theory, the methods to correct beam quality dependence is proposed and it was evaluated for several beam quality and field size in this report. Materials and methods: The energy spectrum of photons and electrons in water were simulated for several photon energy, depth and field size using the EGSnrc MonteCarlo code. The (μen/r)w,glass and (L/r)w,glass were determined with the spectrum. The physical quantity d is a weighting factor which gives the contribution to the total dose from electrons generated by photonin ter a (1 d) is the contribution to the total dose from electrons generated by photon interaction in the RPL glass. The d was determined using original user code of the EGSnrc. In addition, measurement using the RPL glass was performed in the same condition as simulation. Results: Beam quality correction factor by Burlin's, when RPL glass dosimeter was calibrated in 6 MV X‐ray, 10 cm depth in water and 10 cm by 10 cm field, was 0.994 for 4 MV and 1.010 for 10 MV. This correction factors were in good agreement with that of measurement within 0.5%. Conclusion: In this study, the correction methods of beam quality dependence of RPL glass dosimeter were investigated. As a result, good agreement between Burlin's and measurement was obtained, and it was shown that beam quality dependence of RPL glass dosimeter can be corrected by suggested method.

Purpose: The purpose of this investigation is to compare quality of plan, treatment time, monitor units (MUs) and dose delivery capability between dynamic and segmental IMRT for combination of Varian 21iX‐S and CMS XiO. Method and Materials: Head and neck (HN) and prostate IMRT were planned in both dynamic and segmental fashion using same dose objectives in the optimization process. Quality of plans was evaluated with isodose curves and dose‐volume‐histograms. Treatment time defined as time from the first beam‐on to the last beam‐off and MUs were compared. Dose rate of segmental and dynamic were 500 and 300 MU/min, respectively. Dosimetric verification was performed with 0.6 cc ionization chamber for point dose measurement and radiographic film for dose distribution. Results: In HN cases, dose conformity for PTV were slightly superior, however doses to parallel organs (e.g. parotid gland) were higher in dynamic delivery. MUs with dynamic delivery were lager by 20% for HN and prostate cases. Therefore, larger MUs lead more transmitted dose from MLC to parallel organs. Some reports mentioned that with dynamic delivery using other than XiO, treatment time could be shortened by half. But our results showed only 20% reduction for HN and comparable for prostate. The MLC sequence file of dynamic delivery was fairly large (320 control points/port), on the other hand, that of segmental was 1/6 of dynamic. Therefore, beam was kept off while treatment console read MLC sequence file (approximately 20 s/port for dynamic and 5 s/port for segmental, respectively). For HN cases, dose delivery capability was better in both point dose and dose distribution for segmental delivery. Due to higher MUs, dynamic delivery needs precise modeling especially for MLC transmission is required. Conclusion: Delivery performances are superior in segmental delivery, except for treatment time. To conclude, segmental delivery could be suitable for IMRT.