SU-E-T-479: Skin Dose from Flattening Filter Free Beams: A Monte Carlo Investigation.

PURPOSE: Flattening filter free (FFF) beams in radiotherapy have advantages such as shorter treatment delivery time and lower out-of-field dose compared with conventional flattened beams. This study investigates in detail the skin dose induced by FFF beams from a TrueBeam accelerator (Varian Medical Systems, Palo Alto, CA) using Monte Carlo method. METHODS: Phase space files generated using real geometry of a TrueBeam accelerator above the jaws, were used as the input radiation source files in beam simulation for various field sizes using BEAMnrc. Phase space files for various field sizes were generated at the phantom surface. DOSXYZnrc was used for dose calculations in phantom and in patient using the generated phase space files as source input files. RESULTS: The calculated percentage depth dose curves and profiles in water agreed with measurements within ± 2% for the high dose region and ±2 mm in the penumbra. The peak fluence of a 6 MV FFF beam with the same electron beam incident on the target is about 3 times that of a flattened beam . The mean energy of a 6 MV FFF beam is 0.92-0.95 MeV while it is 1.18-1.30 MeV for the flattened beam. Due to the mean energy difference, the dose in a 6 MV FFF beam is about 6% (of the maximum dose, or 12% of local dose) higher at depth of 1 mm compared with a flattened beam. CONCLUSIONS: Due to the lower mean photon energy, in an FFF beam the surface (skin) dose is slightly higher compared to the conventional flattened beam of the same field size.

Needle imaging during ultrasound-guided permanent prostate implants.

As demonstrated by water phantom experiments and clinical observations, the reverberation artifact associated with the ultrasound needle image during permanent prostate implants is extremely useful in determining precise radioactive seed positioning. It also serves as an independent quality assurance check of the number of seeds in the strand.

Centralized multiinstitutional postimplant analysis for interstitial prostate brachytherapy.

PURPOSE: To investigate the feasibility and utility of performing centralized postimplant analysis for transperineal interstitial permanent prostate brachytherapy (TIPPB) by conducting a pilot study that compares the results obtained from 125I implants conducted at five different institutions. METHODS AND MATERIALS: Dose-volume histogram (DVH) analysis was performed on 10 postimplant CT scans from each of five institutions. This analysis included the total implanted activity of 125I, ultrasound, and CT volumes of the prostate, target-volume ratios, dose homogeneity quantifiers, prostate dose coverage indices, and rectal doses. As a result of the uncertainty associated with the delineation of the prostatic boundaries on a CT scan, the contours were redrawn by a single, study center physician, and a repeat DVH analysis was performed. This provided the basis for comparison between institutions in terms of implant technique and quality. RESULTS: By comparing total activity to preimplant ultrasound volume we clearly demonstrated that differences exist in implant technique among these five institutions. The difficulty associated with determining glandular boundaries on CT scans was apparent, based upon the variability in prostate volumes drawn by the various investigators compared to those drawn by the study center physician. This made no difference, of course, in the TVR or homogeneity quantifiers that are independent of target location. Furthermore, this variability made surprisingly little difference in terms of dose coverage of the prostate gland. Rectal doses varied between institutions according to the various implant techniques. CONCLUSIONS: Centralized, outcome-based evaluation of transperineal interstitial permanent prostate brachytherapy is viable and appropriate. Such an approach could be reasonably used in the conduct of multiinstitutional trials used to study the efficacy of the procedure.

Problems with rigid seed strand lodging during prostate implantation: a proposed mechanism and solution.

Since the introduction of rigid strands of radioactive seeds embedded in absorbable suture material, many brachytherapists have experienced problems with the strands lodging inside the implant needle during the deposition process. By using a scanning electron microscope, we examined some potential factors which could lead to this problem. It seems plausible that the lodging is due to two factors: prostate tissue hindering the motion of the strand initially, and friction between the strand braids and the inner surface of the needle. Both result in an "accordion effect" as the stylet applies pressure on the strand. Based on this assumption, a solution was found. A combination of using needles with a polished inner surface, and repeated clockwise and counterclockwise 360 degrees twisting of the needle about the stylet during the deposition process allows for smooth deposition of the strand at the intended location. By using this technique, one is able to exploit the potential dosimetric advantages of rigid seed strand implants without additional problems.

Improvement in dosimetry of ultrasound-guided prostate implants with the use of multiple stabilization needles.

The use of three stabilization needles for ultrasound-guided permanent prostate implants results in better dosimetric coverage of the prostate, all other variables being equal. This technique prevents rocking motion of the prostate when anterior needles are inserted and thus improves accuracy of seed placement and shortens the length of the procedure. On post-implant CT-based dosimetric evaluation, the three stabilization needles technique provides better coverage, with the prescription isodose line encompassing 95% of the target volume compared to 90% for the implant utilizing two stabilization needles. Further improvements in the technique are currently under investigation, with the goal of consistently achieving truly "conformal anatomical prostate brachyradiotherapy."

A simple method for verifying activity of iodine-125 seeds in rigid absorbable suture.

A simple system which facilitates the verification of the calibration of iodine-125 sources in rigid absorbable suture, on the remote traceability basis, was developed. It consists of a plastic jig accommodating a sterile closed-end 16 gauge plastic catheter. The iodine-125 source in rigid absorbable suture is placed into the sterile closed-end 16 gauge plastic catheter. The jig fits in a standard dose calibrator. The sterility of the strand is maintained while a reasonable number of seeds used for an actual implant can be easily measured. This is an improvement over the current recommended practice of assaying just one separate seed of the same strength designation. This system brings the calibration procedure for the rigid sterile seed strands in line with the AAPM TG-40 recommendation for the rest of radioactive seed products.

A spreadsheet technique for dosimetry of transperineal prostate implants.

Although conceptually straightforward, dosimetry of permanent 125I seed prostate implants is not necessarily easy to implement in clinical practice, especially for institutions that are unwilling or unable to modify their commercial RTP systems. Spreadsheet techniques that aid in both preimplant treatment planning and post-implant dosimetric evaluation have been developed. The first spreadsheet converts the seed distribution expressed in terms of template grid coordinates to the format suitable for input into the RTP system, and determines the positions and loading patterns of individual needles. The second spreadsheet macroprogram is designed, as a modification of the Roy et al. [Int. J. Radiat. Oncol. Biol. Phys. 26, 163-169 (1993)] technique, to interactively determine physical seed locations from the post-implant CT images. Although less automated than described elsewhere, this approach was found acceptable for clinical practice at our institution.

Matching electron beams without secondary collimation for treatment of extensive recurrent chest-wall carcinoma.

Matching electron beams without secondary collimators (applicators) were used for treatment of extensive, recurrent chest-wall carcinoma. Due to the wide penumbra of such beams, the homogeneity of the dose distribution at and around the junction point is clinically acceptable and relatively insensitive to positional errors. Specifically, dose around the junction point is homogeneous to within +/- 4% as calculated from beam profiles, while the positional error of 1 cm leaves this number essentially unchanged. The experimental isodose distribution in an anthropomorphic phantom supports this conclusion. Two electron beams with wide penumbra were used to cover the desired treatment area with satisfactory dose homogeneity. The technique is relatively simple yet clinically useful and can be considered a viable alternative for treatment of extensive chest-wall disease. The steps are suggested to make this technique more universal.

Evaluation of electron-beam uniformity during commissioning of a linear accelerator.

Electron-beam profiles measured during acceptance testing and commissioning of a linear accelerator were analyzed according to manufacturer's specifications and more stringent AAPM TG-25 recommendations. The difference between specifications of the AAPM report and those experienced commercially is discussed. Although all the electron beams met the manufacturer's specifications, the uniformity parameters for some of them were outside the limits suggested by the AAPM report. Profiles acquired at depths larger than dmax were extremely valuable for evaluation and adjustment of the beams. The following suggestions for practical machine commissioning are made: Profiles taken deeper than at dmax are extremely useful indicators of the beam nonuniformity; acquisition of these profiles should be a part of the routine acceptance testing procedure; and beam uniformity should be evaluated against AAPM recommendations, and this should be included in the purchase specifications.

Effective dose equivalents to patients undergoing cerebral angiography.

PURPOSE: To determine values of the effective dose equivalent, HE, for patients undergoing diagnostic cerebral angiography and compare these values with radiation doses received by patients undergoing other diagnostic examinations of the head. METHODS: The radiographic techniques for ten patients undergoing cerebral angiography were recorded and used to obtain the product of the entrance skin dose and the x-ray beam cross-sectional area. These measured dose-area product data were converted into effective dose equivalents employing published conversion factors which take into account the part of the patient anatomy irradiated and the radiographic technique factors employed. RESULTS: The average patient HE value was 10.6 mSv, with a range of 2.7-23.4 mSv. Fluoroscopy contributed approximately 67% of the total HE, with cut films and digital subtraction angiography contributing 26% and 7%, respectively. CONCLUSIONS: The radiation doses (HE) to patients undergoing diagnostic cerebral angiography are comparable to the patient doses in nuclear medicine brain studies where the typical HE is approximately 10 mSv. In CT, the patient dose is approximately 2 mSv, whereas in plain skull x-ray examinations, the patient dose is much lower at approximately 0.15 mSv.