High dose rate brachytherapy for oral cancer

Brachytherapy results in better dose distribution compared with other treatments because of steep dose reduction in the surrounding normal tissues. Excellent local control rates and acceptable side effects have been demonstrated with brachytherapy as a sole treatment modality, a postoperative method, and a method of reirradiation. Low-dose-rate (LDR) brachytherapy has been employed worldwide for its superior outcome. With the advent of technology, high-dose-rate (HDR) brachytherapy has enabled health care providers to avoid radiation exposure. This therapy has been used for treating many types of cancer such as gynecological cancer, breast cancer, and prostate cancer. However, LDR and pulsed-dose-rate interstitial brachytherapies have been mainstays for head and neck cancer. HDR brachytherapy has not become widely used in the radiotherapy community for treating head and neck cancer because of lack of experience and biological concerns. On the other hand, because HDR brachytherapy is less time-consuming, treatment can occasionally be administered on an outpatient basis. For the convenience and safety of patients and medical staff, HDR brachytherapy should be explored. To enhance the role of this therapy in treatment of head and neck lesions, we have reviewed its outcomes with oral cancer, including Phase I/II to Phase III studies, evaluating this technique in terms of safety and efficacy. In particular, our studies have shown that superficial tumors can be treated using a non-invasive mold technique on an outpatient basis without adverse reactions. The next generation of image-guided brachytherapy using HDR has been discussed. In conclusion, although concrete evidence is yet to be produced with a sophisticated study in a reproducible manner, HDR brachytherapy remains an important option for treatment of oral cancer.     

A phantom for verification of dwell position and time of a high dose rate brachytherapy source

Accuracy of dwell position and reproducibility of dwell time are critical in high dose rate (HDR) brachytherapy. A phantom was designed to verify dwell position and dwell time reproducibility for an Ir-192 HDR stepping source using Computed Radiography (CR). The central part of the phantom, incorporating thin alternating strips of lead and acrylic, was used to measure dwell positions. The outer part of the phantom features recesses containing different absorber materials (lead, aluminium, acrylic and polystyrene foam), and was used for determining reproducibility of dwell times. Dwell position errors of <1?mm were easily detectable using the phantom. The effect of bending a transfer tube was studied with this phantom and no change of clinical significance was observed when varying the curvature of the transfer tube in typical clinical scenarios. Changes of dwell time as low as 0.1?s, the minimum dwell time of the treatment unit, could be detected by choosing dwell times over the four materials that produce identical exposure at the CR detector.     

Monte Carlo dosimetry of the IRAsource high dose rate <Superscript>192</Superscript>Ir brachytherapy source

High-dose-rate (HDR) brachytherapy is a common method for cancer treatment in clinical brachytherapy. Because of the different source designs, there is a need for specific dosimetry data set for each HDR model. The purpose of this study is to obtain detailed dose rate distributions in water phantom for a first prototype HDR ¹⁹²Ir brachytherapy source model, IRAsource, and compare with the other published works. In this study, Monte Carlo N-particle (MCNP version 4C) code was used to simulate the dose rate distributions around the HDR source. A full set of dosimetry parameters reported by the American Association of Physicists in Medicine Task Group No. 43U1 was evaluated. Also, the absorbed dose rate distributions in water, were obtained in an along-away look-up table. The dose rate constant, Λ, of the IRAsource was evaluated to be equal to 1.112 ± 0.005 cGy h^?1 U^?1. The results of dosimetry parameters are presented in tabulated and graphical formats and compared with those reported from other commercially available HDR ¹⁹²Ir sources, which are in good agreement. This justifies the use of specific data sets for this new source. The results obtained in this study can be used as input data in the conventional treatment planning systems.     

Dosimetric characterisation of the optically-stimulated luminescence dosimeter in cobalt-60 high dose rate brachytherapy system

This study investigates the characteristics and application of the optically-stimulated luminescence dosimeter (OSLD) in cobalt-60 high dose rate (HDR) brachytherapy, and compares the results with the dosage produced by the treatment planning system (TPS). The OSLD characteristics comprised linearity, reproducibility, angular dependence, depth dependence, signal depletion, bleaching rate and cumulative dose measurement. A phantom verification exercise was also conducted using the Farmer ionisation chamber and in vivo diodes. The OSLD signal indicated a supralinear response (R²?=?0.9998). It exhibited a depth-independent trend after a steep dose gradient region. The signal depletion per readout was negligible (0.02%), with expected deviation for angular dependence due to off-axis sensitive volume, ranging from 1 to 16%. The residual signal of the OSLDs after 1 day bleached was within 1.5%. The accumulated and bleached OSLD signals had a standard deviation of ±?0.78 and ±?0.18 Gy, respectively. The TPS was found to underestimate the measured doses with deviations of 5% in OSLD, 17% in the Farmer ionisation chamber, and 7 and 8% for bladder and rectal diode probes. Discrepancies can be due to the positional uncertainty in the high-dose gradient. This demonstrates a slight displacement of the organ at risk near the steep dose gradient region will result in a large dose uncertainty. This justifies the importance of in vivo measurements in cobalt-60 HDR brachytherapy.     

Ionizing radiation alters organoid forming potential and replenishment rate in a dose/dose-rate dependent manner

Intestinal organoids are an in vitro cultured tissue model generated from intestinal stem cells, and they contain a mixture of epithelial cell types. We previously established an efficient ‘one cell/well’ sorting method, and defined organoid-forming potential (OFP) as a useful index to evaluate the stemness of individual cells. In this study, we assessed the response to radiation dose and dose-rate by measuring both OFP and the percentage of stem cells in the crypts. After high-dose-rate (HDR, 0.5 Gy/min) irradiation in vivo, the percentage of stem cells in the harvested crypt cells decreased, and the replenishment of cycling stem cells originating from dormant cells was enhanced, but OFP increased in cells irradiated with a total dose of >1 Gy. In contrast, at a total dose of 0.1 Gy the percentage of stem cells reduced slightly, but neither replenishment rate nor OFP changed. Furthermore, the response to 1 Gy of low-dose-rate (LDR) irradiation was similar to the response to 0.1 Gy HDR irradiation. These results suggest that 0.1 Gy HDR irradiation or 1 Gy LDR irradiation does not alter stemness. Additionally, the OFP increase in the colon in response to irradiation was smaller than that in the duodenum, similar to the percentage of stem cells. Understanding the differences in the response of stem cells between the colon and the duodenum to radiation is important to clarify the mechanisms underlying the development of radiation-associated intestinal cancers.     

Long-term results of high-dose-rate brachytherapy and external-beam radiotherapy in the primary treatment of endometrial cancer

In this report we update our long-term follow-up results of the prospective study whose aim was to evaluate the efficacy of high-dose-rate (HDR) brachytherapy in combination with external-beam radiotherapy (EBRT) in the treatment of medically inoperable endometrial cancer. Between 1995 and 1998, 29 patients with stages I-III medically inoperable carcinoma of endometrium were treated with definitive irradiation. All patients underwent combined intracavitary HDR brachytherapy and EBRT. The EBRT dose was 50 Gy with midline shield from 16 Gy. The HDR brachytherapy dose was 50 Gy, delivered in 5 fractions in a weekly schedule. Overall survival (OS) at 5 and 10 years was 48.3% and 20.7%, respectively. Disease-specific survival (DSS) at 5 and 10 years was 73.5% and 67.9%, respectively. The 10-year DSS rate was 73.5% for all stages, 85.7% for Stage I disease, 71.4% for Stage II, and 16.7% for stage III disease. Late Grade 1-2 radiation complications were observed in 4 patients (13.8%). Our long-term follow-up confirms that HDR brachytherapy with EBRT appears to be an effective and safe treatment for stage I and II medically inoperable endometrial cancer.     

192Ir近距离腔内放疗联合体外逆向调强放射治疗宫颈癌160例疗效观察

目的 研究¹⁹²Ir高剂量率腔内放疗加体外逆向调强放疗（IMRT）治疗宫颈癌的疗效及放射反应。方法 160例宫颈癌患者采取¹⁹²Ir高剂量率腔内放疗加体外逆向调强放疗（IMRT）放射治疗，体外全盆腔照射IMRT，6MV X射线DT 45~50 Gy，5次/周，1.8~2 Gy/次。全盆腔放疗结束后行¹⁹²Ir腔内治疗1次/周，5~8 Gy/次，总剂量21~24 Gy。结果 全组1年、3年生存率分别为100%和78.75%，Ⅱ、Ⅲ期3年生存率分别为80%和63.89%，患者3年生存率I期与Ⅱ期、Ⅲ期与IV期有明显统计学差异（100%、80%，63.89%、16.67%，P<0.05），Ⅱ期、Ⅲ期无明显统计学差异（80%、63.89%，P>0.05）。病理I级与Ⅲ级的Ⅱ期患者生存率有明显差异（100%、53.33%，P<0.05）。病理I级与Ⅲ级的Ⅲ期患者生存率有明显差异（100%、33.33%，P<0.05）。IV期各病理分级的患者生存率均无明显差异（P>0.05）。Ⅱ、Ⅲ期患者中肿瘤直径<4 cm与肿瘤直径>4 cm的3年生存率有明显差异（P<0.05），而IV期患者肿瘤直径<4 cm与肿瘤直径>4 cm的3年生存率无明显差异（P>0.05）。早期放射性直肠炎反应发生率21.8%，膀胱反应发生率7.5%，晚期放射性直肠炎发生率7%，晚期膀胱炎反应发生率5%，直肠阴道瘘0.625%。结论 ¹⁹²Ir高剂量率腔内加体外适形调强放疗治疗宫颈癌疗效好，毒副反应较少。     

High-dose-rate brachytherapy and hypofractionated external beam radiotherapy combined with long-term hormonal therapy for high-risk and very high-risk prostate cancer: outcomes after 5-year follow-up

The purpose of this study was to report the outcomes of high-dose-rate (HDR) brachytherapy and hypofractionated external beam radiotherapy (EBRT) combined with long-term androgen deprivation therapy (ADT) for National Comprehensive Cancer Network (NCCN) criteria-defined high-risk (HR) and very high-risk (VHR) prostate cancer. Data from 178 HR (n = 96, 54%) and VHR (n = 82, 46%) prostate cancer patients who underwent ¹⁹²Ir-HDR brachytherapy and hypofractionated EBRT with long-term ADT between 2003 and 2008 were retrospectively analyzed. The mean dose to 90% of the planning target volume was 6.3 Gy/fraction of HDR brachytherapy. After five fractions of HDR treatment, EBRT with 10 fractions of 3 Gy was administered. All patients initially underwent ≥6 months of neoadjuvant ADT, and adjuvant ADT was continued for 36 months after EBRT. The median follow-up was 61 months (range, 25–94 months) from the start of radiotherapy. The 5-year biochemical non-evidence of disease, freedom from clinical failure and overall survival rates were 90.6% (HR, 97.8%; VHR, 81.9%), 95.2% (HR, 97.7%; VHR, 92.1%), and 96.9% (HR, 100%; VHR, 93.3%), respectively. The highest Radiation Therapy Oncology Group-defined late genitourinary toxicities were Grade 2 in 7.3% of patients and Grade 3 in 9.6%. The highest late gastrointestinal toxicities were Grade 2 in 2.8% of patients and Grade 3 in 0%. Although the 5-year outcome of this tri-modality approach seems favorable, further follow-up is necessary to validate clinical and survival advantages of this intensive approach compared with the standard EBRT approach.     

Thermoluminescence dosimetry for in-vivo verification of high dose rate brachytherapy for prostate cancer

It was the aim of the study to verify dose delivered in urethra and rectum during High Dose Rate brachytherapy boost (HDRBB) of prostate cancer patients. During the first fraction of HDRBB measurement catheters were placed in the urethra and rectum of prostate cancer patients. These contained LiF:Mg,Ti Thermoluminescence Dosimetry (TLD) rods of 1 mm diameter, with up to 11 detectors positioned every 16 mm separated by radio-opaque markers. A Lorentzian peak function was used to fit the data. Measurements from 50 patients were evaluated and measured doses were compared with predictions from the treatment planning system (Plato Vs 13.5 to 14.1). Prospective urinary and rectal toxicity scores were collected following treatment. In more than 90% of cases, the Lorentzian peak function provided a good fit to both experimental and planning urethral data (r2 > 0.9). In general there was good agreement between measured and predicted doses with the average difference between measured and planned maximum dose being 0.1 Gy. No significant association between dose and any clinical endpoints was observed in 43 patients available for clinical evaluation. An average inferior shift of 2 mm between the plan and the measurement performed approximately 1 hour after the planning CT scan was found for the dose distribution in the cohort of patients for the urethra measurements. Rectal measurements proved to be more difficult to interpret as there is more variability of TLD position between planning and treatment. TLD in-vivo measurements are easily performed in urethra and rectum during HDR brachytherapy of prostate patients. They verify the delivery and provide information about the dose delivered to critical structures. The latter may be of particular interest if higher doses are to be given per fraction such as in HDR monotherapy.     

Dose reduction trial from 60 Gy in 10 fractions to 54 Gy in 9 fractions schedule in high-dose-rate interstitial brachytherapy for early oral tongue cancer

To compare the effects of 60 Gy/10 fractions (twice a day) with those of 54 Gy/9 fractions in high-dose-rate interstitial brachytherapy (HDR-ISBT) for early tongue cancer, we performed a matched-pair analysis of patients with early tongue cancer (T1-2N0M0), who were treated with 60 or 54 Gy of radiation between 1996 and 2004. Seventeen patients treated with 54 Gy and 34 matched-pair patients treated with 60 Gy were extracted and analyzed. Local recurrence occurred in two patients in the 54-Gy arm and five patients in the 60-Gy arm. The 2-year local control rates were 88% for both the 54-Gy arm and 60-Gy arm (not significant). The 2-year overall survival rates were 88% in the 60-Gy arm and 82% in the 54-Gy arm. Two-year actuarial complication-free rates were 91% in the 60-Gy arm and 83% in the 54-Gy arm (not significant), respectively. There was no significant association between the total dose and local control rate and late complications. The outcome of 54 Gy/ 9 fractions was similar to that of 60 Gy/ 10 fractions in patients with early tongue cancer.     

The effects of two HDR brachytherapy schedules in locally advanced cervical cancer treated with concurrent chemoradiation: a study from Chiang Mai, Thailand

Efficacy of different schedules of HDR brachytherapy in concurrent chemoradiotherapy was evaluated. The study compared the effectiveness of the two HDR brachytherapy schedules which have the same Biological Effective Dose (BED) in locally advanced cervical carcinoma that was treated with concurrent chemoradiotherapy. Included in the study were 377 randomly selected patients with advanced carcinoma of the cervix uteri who were treated during the period 2004-2006. Patients were divided into Group I: 7.2 Gy × 3 fractions and Group II: 6 Gy × 4 fractions. With a median follow-up time of 35 months, local control, disease-free survival and overall survival rates were 80.8%, 63.4%, 98.8% in group I and 86.7%, 63.8%, 97.3% in group II, respectively. There was no statistical significance in terms of local control, disease-free survival, overall survival and complication rates between the two treatment schedules which could be observed. Seven patients in group I developed acute grade 2-4 GI toxicities and two patients in group II. In GU toxicities, there were three patients in group I and three patients in group II who developed grade 2-4 toxicities. In late toxicity, no patient developed grade 3-4 GU toxicities in group I while two patients developed grade 3-4 GU toxicities in group II. In GI toxicities, there were five and six patients in group I and group II, respectively, who developed grade 3-4 severity. Both HDR schedules seem to be safe and effective for the treatment of locally advanced cervical cancer.     

6MeV电子线和铱192治疗瘢痕疙瘩的剂量学比较

目的 比较6 MeV电子线和铱192(192Ir)高剂量率(HDR)三维后装近距离敷贴两种放射治疗技术在瘢痕疙瘩术后平整表面靶区、曲面靶区、超长靶区(长度>25 cm)模体中的剂量学差异.方法 在胸部仿真模体上,采用激光灯标记拟照射瘢痕疙瘩术后位置(包括平整表面和曲面),固体水模体模拟超长靶区,分别获取电子线放射治疗C...     

Lack of backscatter factor measurements in HDR applications with MOSkins

Measurements of backscatter correction factors for intra operative (IOBT) HDR brachytherapy applicators were made using Centre for Medical Radiation Physics (CMRP), MOSFET devices. In clinical use there is an absence of backscatter material above the IOBT applicator, leading to a lower dose than predicted by conventional TG-43 dose calculations. To estimate the uncertainty in the MOSFET measurements, the dosimetric characteristics, including reproducibility, stability, linearity, and angular and energy response were measured using a HDR Ir-192 source, kilovoltage treatment unit and a high energy linac. Measurements were compared with previously published Monte Carlo data. Variability of the response of the MOSFETs due to angular variation contributed the largest uncertainty in dose measurements. Using the IOBT applicator without adequate scatter material resulted in a reduction of delivered dose of on average 10%, but was dependent on the location on the applicator and the treatment field size. Theoretical calculations based on previously published study indicated an expected reduced dose of on average 4%. MOSFET devices provide an ideal measurement tool in the presence of high dose gradients, however, the dosimetric characteristics of the detector must be accounted for when estimating the uncertainty.     

Analysis of prognostic factors in localized high-risk prostate cancer patients treated with HDR brachytherapy,hypofractionated 3D-CRT and neoadjuvant/adjuvant androgen deprivation therapy (trimodality therapy)

Trimodality therapy consisting of high dose rate (HDR) brachytherapy combined with external beam radiation therapy (EBRT), neoadjuvant hormonal therapy (NHT) and adjuvant hormonal therapy (AHT) has been used to treat localized high-risk prostate cancer. In this study, an analysis of patients receiving the trimodality therapy was performed to identify prognostic factors of biochemical relapse-free survival (bRFS). Between May 2005 and November 2008, 123 high-risk prostate cancer patients (D''Amico classification) were treated with NHT prior to HDR brachytherapy combined with hypofractionated EBRT. Among these patients, 121 had completed AHT. The patients were assigned by time to be treated with a low-dose or high-dose arm of HDR brachytherapy with subsequent hypofractionated 3D conformal radiation therapy (3D-CRT). Multivariate analysis was used to determine prognostic factors for bRFS. With a median follow-up of 60 months, the 5-year bRFS for all patients was 84.3% (high-dose arm, 92.9%; low-dose arm, 72.4%, P = 0.047). bRFS in the pre-HDR PSA ≤ 0.1 ng/ml subgroup was significantly improved compared with that in the pre-HDR PSA > 0.1 ng/ml subgroup (88.3% vs 68.2%, P = 0.034). On multivariate analysis, dose of HDR (P = 0.045, HR = 0.25, 95% CI = 0.038–0.97) and pre-HDR PSA level (P = 0.02 HR = 3.2, 95% CI = 1.18–10.16) were significant prognostic factors predicting bRFS. In high-risk prostate cancer patients treated with the trimodality therapy, the dose of HDR and pre-HDR PSA were significant prognostic factors. The pre-HDR PSA ≤ 0.1 subgroup had significantly improved bRFS. Further studies are needed to confirm the relevance of pre-HDR PSA in trimodality therapy.     

High-dose-rate interstitial brachytherapy with computed tomography-based treatment planning for patients with locally advanced uterine cervical carcinoma

The aims of this study were to carry out a dose volume analysis of high-dose-rate interstitial brachytherapy with computed tomography-based treatment planning and to investigate the treatment outcome of patients with locally advanced bulky and/or irregularly shaped uterine cervical carcinoma. Between July 2003 and December 2007, 15 patients were treated with external beam radiation therapy and high-dose-rate interstitial brachytherapy with or without intracavitary brachytherapy. Seven patients were treated with interstitial brachytherapy alone, and 8 were treated with combined use of intracavitary and interstitial brachytherapy. A comparison of the volume and dose parameters with intracavitary and interstitial brachytherapy in patients who received both treatments showed that the median D90 of the high-risk clinical target volume per fraction was 4.4 Gy with intracavitary brachytherapy and 5.6 Gy with interstitial brachytherapy, and the median V100 was 66% with intracavitary brachytherapy and 85% with interstitial brachytherapy. The median D2cc of the bladder with intracavitary and interstitial brachytherapy per fraction was 5.5 Gy and 4.7 Gy, respectively, and the median D2cc of the rectum with intracavitary and interstitial brachytherapy was 5.9 Gy and 4.1 Gy, respectively. The median follow-up time was 37 months, and the overall and progression-free survival rates for all patients at 3 years were 78% and 51%, respectively. The actuarial 2-year and 3-year locoregional control rates were 80% and 71%, respectively. Dose distribution was improved with image-based interstitial brachytherapy, and satisfactory local control was achieved for patients with locally advanced uterine cervical carcinoma in which intracavitary brachytherapy may result in a suboptimal dose distribution.     

Preliminary results of conformal computed tomography (CT)-based intracavitary brachytherapy (ICBT) for locally advanced cervical cancer: a single institution's experience

Intracavitary brachytherapy using tandem and ovoids is an important component of definitive treatment for cervical cancer. In the present study, we analyzed the dose-volume histograms (DVHs) of the tumor volume and organs at risk including the sigmoid colon by CT-based treatment planning for high dose rate (HDR) intracavitary brachytherapy (ICBT) in cervical cancer. Seventeen patients with carcinoma of the cervix uteri were treated with external beam radiotherapy plus concurrent chemotherapy. For brachytherapy, the planning procedure started by performing a conventional plan which prescribed a dose of 6.5-7 Gy per fraction to point A, then optimized the dose based on CT imaging. Volumes and DVHs were calculated for the HR-CTV, bladder, rectum and sigmoid colon. The mean BED(2Gy) total doses of post-optimized plans of HR-CTV, bladder, rectum and sigmoid colon were: 89.6, 94.1, 74.0 and 69.8 Gy, respectively. For conventional plans, the calculated mean BED(2Gy) total doses of HR-CTV, bladder, rectum and sigmoid colon were 92.2, 120.1, 75.7 and 78.3 Gy, respectively. This study showed statistical significant higher BED(2Gy) total doses for bladder and sigmoid colon (p < 0.001) using conventional plans versus post-optimized, CT-based plans, while no difference between HR-CTV and rectum BED(2Gy) total doses could be detected. After a median follow-up of nineteen months, all seventeen patients had a clinical complete response. Two patients developed distant metastasis. Compared with conventional treatment, CT based brachytherapy planning was very effective in reducing doses to OARs, especially bladder and sigmoid colon whilst maintaining a high therapeutic dose for tumor target volumes in the treatment of cervical carcinoma.     

The emerging role of high-dose-rate (HDR) brachytherapy as monotherapy for prostate cancer

High-dose-rate (HDR) brachytherapy as monotherapy is a comparatively new brachytherapy procedure for prostate cancer. In addition to the intrinsic advantages of brachytherapy, including radiation dose concentration to the tumor and rapid dose fall-off at the surrounding normal tissue, HDR brachytherapy can yield a more homogeneous and conformal dose distribution through image-based decisions for source dwell positions and by optimization of individual source dwell times. Indication can be extended even to T3a/b or a part of T4 tumors because the applicators can be positioned at the extracapsular lesion, into the seminal vesicles, and/or into the bladder, without any risk of source migration or dropping out. Unlike external beam radiotherapy, with HDR brachytherapy inter-/intra-fraction organ motion is not problematic. However, HDR monotherapy requires patients to stay in bed for 1–4 days during hospitalization, even though the actual overall treatment time is short. Recent findings that the α/β value for prostate cancer is less than that for the surrounding late-responding normal tissue has made hypofractionation attractive, and HDR monotherapy can maximize this advantage of hypofractionation. Research on HDR monotherapy is accelerating, with a growing number of publications reporting excellent preliminary clinical results due to the high ‘biologically effective dose (BED)’ of >200 Gy. Moreover, the findings obtained for HDR monotherapy as an early model of extreme hypofractionation tend to be applied to other radiotherapy techniques such as stereotactic radiotherapy. All these developments point to the emerging role of HDR brachytherapy as monotherapy for prostate cancer.     

A semi-analytic approach to determine dose rate constant of brachytherapy sources in compliance with AAPM TG 60 formalism

Values of dose rate constant (DRC) in compliance with AAPM TG 60 formalism recommended for intravascular brachytherapy (IVBT) were calculated for different point isotropic mono-energetic photon sources in the energy range E = 20-1000 keV using a semi-analytic model. Based on these DRC values, DRC of some existing models of 192Ir and 125I brachytherapy sources were then calculated using (1) bare energy spectra and (2) a single energy parameter which represents mean energy (photon number weighted or air-kerma weighted) for bare and actual sources or the most probable energy of the spectra (energy line with the highest probability of emission) of the investigated sources (192Ir and 125I). Applicability of the semi-analytic approach was examined by also computing the values of DRC of the investigated sources using MCNP Monte Carlo simulation code (Version 3.1) that involved modeling of the sources accurately. A comparison of values of DRC resulting from MCNP calculations with those resulting from the semi-analytic approach showed that for 192Ir sources the agreement was within 0.40% and for 125I sources it was within 2.3%.