Third-party brachytherapy source calibrations and physicist responsibilities: report of the AAPM Low Energy Brachytherapy Source Calibration Working Group.

The AAPM Low Energy Brachytherapy Source Calibration Working Group was formed to investigate and recommend quality control and quality assurance procedures for brachytherapy sources prior to clinical use. Compiling and clarifying recommendations established by previous AAPM Task Groups 40, 56, and 64 were among the working group's charges, which also included the role of third-party handlers to perform loading and assay of sources. This document presents the findings of the working group on the responsibilities of the institutional medical physicist and a clarification of the existing AAPM recommendations in the assay of brachytherapy sources. Responsibility for the performance and attestation of source assays rests with the institutional medical physicist, who must use calibration equipment appropriate for each source type used at the institution. Such equipment and calibration procedures shall ensure secondary traceability to a national standard. For each multi-source implant, 10% of the sources or ten sources, whichever is greater, are to be assayed. Procedures for presterilized source packaging are outlined. The mean source strength of the assayed sources must agree with the manufacturer's stated strength to within 3%, or action must be taken to resolve the difference. Third party assays do not absolve the institutional physicist from the responsibility to perform the institutional measurement and attest to the strength of the implanted sources. The AAPM leaves it to the discretion of the institutional medical physicist whether the manufacturer's or institutional physicist's measured value should be used in performing dosimetry calculations.

American College of Radiology white paper on radiation dose in medicine.

The benefits of diagnostic imaging are immense and have revolutionized the practice of medicine. The increased sophistication and clinical efficacy of imaging have resulted in its dramatic growth over the past quarter century. Although data derived from the atomic bomb survivors in Japan and other events suggest that the expanding use of imaging modalities using ionizing radiation may eventually result in an increased incidence of cancer in the exposed population, this problem can likely be minimized by preventing the inappropriate use of such imaging and by optimizing studies that are performed to obtain the best image quality with the lowest radiation dose. The ACR, which has been an advocate for radiation safety since its inception in 1924, convened the ACR Blue Ribbon Panel on Radiation Dose in Medicine to address these issues. This white paper details a proposed action plan for the college derived from the deliberations of that panel.

Theoretical study of the influence of a heterogeneous activity distribution on intratumoral absorbed dose distribution.

Radioimmunotherapy of hematopoeitic cancers and micrometastases has been shown to have significant therapeutic benefit. The treatment of solid tumors with radionuclide therapy has been less successful. Previous investigations of intratumoral activity distribution and studies on intratumoral drug delivery suggest that a probable reason for the disappointing results in solid tumor treatment is nonuniform intratumoral distribution coupled with restricted intratumoral drug penetrance, thus inhibiting antineoplastic agents from reaching the tumor's center. This paper describes a nonuniform intratumoral activity distribution identified by limited radiolabeled tracer diffusion from tumor surface to tumor center. This activity was simulated using techniques that allowed the absorbed dose distributions to be estimated using different intratumoral diffusion capabilities and calculated for tumors of varying diameters. The influences of these absorbed dose distributions on solid tumor radionuclide therapy are also discussed. The absorbed dose distribution was calculated using the dose point kernel method that provided for the application of a three-dimensional (3D) convolution between a dose rate kernel function and an activity distribution function. These functions were incorporated into 3D matrices with voxels measuring 0.10 x 0.10 x 0.10 mm3. At this point fast Fourier transform (FFT) and multiplication in frequency domain followed by inverse FFT (iFFT) were used to effect this phase of the dose calculation process. The absorbed dose distribution for tumors of 1, 3, 5, 10, and 15 mm in diameter were studied. Using the therapeutic radionuclides of 131I, 186Re, 188Re, and 90Y, the total average dose, center dose, and surface dose for each of the different tumor diameters were reported. The absorbed dose in the nearby normal tissue was also evaluated. When the tumor diameters exceed 15 mm, a much lower tumor center dose is delivered compared with tumors between 3 and 5 mm in diameter. Based on these findings, the use of higher beta-energy radionuclides, such as 188Re and 90Y is more effective in delivering a higher absorbed dose to the tumor center at tumor diameters around 10 mm.

Daily ultrasound-based image-guided targeting for radiotherapy of upper abdominal malignancies.

PURPOSE: Development and implementation of a strategy to use a stereotactic ultrasound (US)-based image-guided targeting device (BAT) to align intensity-modulated radiotherapy (IMRT) target volumes accurately in the upper abdomen. Because the outlines of such targets may be poorly visualized by US, we present a method that uses adjacent vascular guidance structures as surrogates for the target position. We assessed the potential for improvement of daily repositioning and the feasibility of daily application. METHODS AND MATERIALS: A total of 62 patients were treated by sequential tomotherapeutic IMRT between October 2000 and June 2003 for cholangiocarcinoma and gallbladder carcinoma (n = 10), hepatocellular carcinoma (n = 10), liver metastases (n = 11), pancreatic carcinoma (n = 20), neuroblastoma (n = 3), and other abdominal and retroperitoneal tumors (n = 8). The target volumes (TVs) and organs at risk were delineated in contrast-enhanced CT data sets. Additionally, vascular guidance structures in close anatomic relation to the TV, or within the TV, were delineated. Throughout the course of IMRT, US BAT images were acquired during daily treatment positioning. In addition to the anatomic structures typically used for US targeting (e.g., the TV and dose-limiting organs at risk), CT contours of guidance structures were superimposed onto the real-time acquired axial and sagittal US images, and target position adjustments, as indicated by the system, were performed accordingly. We report the BAT-derived distribution of shifts in the three principal room axes compared with a skin-mark-based setup, as well as the time required to perform BAT alignment. The capability of the presented method to improve target alignment was assessed in 15 patients by comparing the organ and fiducial position between the respective treatment simulation CT with a control CT study after US targeting in the CT suite. RESULTS: A total of 1,337 BAT alignments were attempted. US images were not useful in 56 setups (4.2%), mainly because of limited visibility due to daily variations in colonic and gastric air. US imaging was facilitated in intrahepatic tumors and asthenic patients. The mean +/- SD shift from the skin mark position was 4.9 +/- 4.35, 6.0 +/- 5.31, and 6.0 +/- 6.7 mm in the x, y, and z direction, respectively. The mean magnitude vector of three-dimensional alignment correction was 11.4 +/- 7.6 mm. The proportion of daily alignments corrected by a magnitude of >10, >15, and >20 mm was 48.9%, 25.1%, and 12.7%, respectively. The magnitude of shifts in the principal directions, as well as the three-dimensional vector of displacement, was statistically significant (test against the zero hypothesis) at p <0.0001. The guidance structures that were the most valuable for identification of the TV position were the branches of the portal vein, hepatic artery, and dilated bile ducts in intrahepatic lesions and the aorta, celiac trunk, superior mesenteric artery, and extrahepatic aspects of the portal vein system in retroperitoneal and extrahepatic lesions. The mean total setup time was 4.6 min. The correlation of BAT targeting with target setup error assessment in the control CT scans in 15 patients revealed setup error reduction in 14 of 15 alignments. The average setup error reduction, assessed as a reduction in the length of setup error three-dimensional magnitude vector, was 54.4% +/- 26.9%, with an observed mean magnitude of residual setup error of 4.6 +/- 3.4 mm. The sole worsening of an initial setup was by a magnitude of <2 mm. US targeting resulted in statistically significant improvements in patient setup (p = 0.03). CONCLUSION: Daily US-guided BAT targeting for patients with upper abdominal tumors was feasible in the vast majority of attempted setups. This method of US-based image-guided tumor targeting has been successfully implemented in clinical routine. The observed improved daily repositioning accuracy might allow for individualized reduction of safety margins and optional dose escalation. Compared with the established application of the BAT device for prostate radiotherapy, in which the target can be directly visualized, the TV in the present study was predominantly positioned relative to guidance vascular structures in close anatomic relation. We perceived an enormous potential in improved and individualized patient positioning for fractionated radiotherapy and also for stereotactic extracranial radiotherapy and radiosurgery, especially for tumors of the liver and pancreas.

Repositioning accuracy of a commercially available thermoplastic mask system.

BACKGROUND AND PURPOSE: To evaluate the repositioning accuracy of a commercially available thermoplastic mask system for single dose radiosurgery treatments and fractionated treatment courses. PATIENTS AND METHODS: The repositioning accuracy of the Raycast-HP mask system (Orfit Industries, Wijnegem, Belgium) was analyzed. Twenty-two patients that were treated by intensity-modulated radiation therapy (IMRT) or intensity modulated radiosurgery (IMRS) for 43 intracranial lesions, underwent repeated CT imaging during their course of treatment, or as a positional control immediately before radiosurgery. We evaluated multiple anatomical landmark coordinates and their respective shifts in consecutive repeated CT-controls. An iterative optimization algorithm allowed for the calculation of the x, y and z-components of translation of the target isocenter(s) for each repeated CT, as well as rotation in the respective CT data sets. In addition to absolute target isocenter translation, the total magnitude vector (i.e. sum-vector) of isocenter motion was calculated along with patient rotations about the three principle axes. RESULTS: Fifty-five control CT datasets were analyzed for the target isocenter's respective position relative to the original treatment planning CT simulation. Mean target isocenter translation was 0.74+/-0.53, 0.75+/-0.60 and 0.93+/-0.78 mm in x, y and z-directions, respectively. Mean rotation about the x, y and z-axes was 0.67+/-0.66, 0.61+/-0.63 and 0.67+/-0.61 degrees, respectively. The respective median and mean magnitude vectors of isocenter translation were 1.28 and 1.59+/-0.84 mm. Analysis of the accuracy of the first setup control, representative of setup accuracy for radiosurgery treatments, compared with setup accuracy throughout a fractionated radiation treatment course were statistically equivalent (P= 0.15) thus indicating no measurable deterioration of setup accuracy throughout the treatment course. CONCLUSIONS: The analyzed Orfit thermoplastic mask system performed favorably compared with other mask immobilization systems for which peer-reviewed repositioning data exist. While the performance of the system for fractionated treatment courses was considered to be excellent, use of this mask system for radiosurgery immobilization in our clinic is subject to additional quality assurance measures to prohibit the delivery of treatments with target dislocations larger than 2 mm. The measured data in the present study should enable the users of this system to assign appropriate margins for the generation of planning target volumes.