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Purpose: The purpose of this study was to provide guidance on quality management for electronic brachytherapy., Materials and Methods: The task group used the risk-assessment approach of Task Group 100 of the American Association of Physicists in Medicine. Because the quality management program for a device is intimately tied to the procedure in which it is used, the task group first designed quality interventions for intracavitary brachytherapy for both commercial electronic brachytherapy units in the setting of accelerated partial-breast irradiation. To demonstrate the methodology to extend an existing risk analysis for a different application, the task group modified the analysis for the case of post-hysterectomy, vaginal cuff irradiation for one of the devices., Results: The analysis illustrated how the TG-100 methodology can lead to interventions to reduce risks and improve quality for each unit and procedure addressed., Conclusion: This report provides a model to guide facilities establishing a quality management program for electronic brachytherapy., (© 2019 American Association of Physicists in Medicine.)

The issue of neutron leakage has recently been raised in connection with dedicated electron-only linear accelerators used for intraoperative radiation therapy (IORT). In particular, concern has been expressed about the degree of neutron production at energies of 10 MeV and higher due to the need for additional, perhaps permanent, shielding in the room in which the device is operated. In particular, three mobile linear accelerators available commercially offer electron energies at or above the neutron threshold, one at 9 MeV, one at 10 MeV, and the third at 12 MeV. To investigate this problem, neutron leakage has been measured around the head of two types of electron accelerators at a distance of 1 m from the target at azimuthal angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees, and 180 degrees. The first is a dedicated electron-only (nonmobile) machine with electron energies of 6 (not used here), 9, 12, 15, and 18 MeV and the second a conventional machine with electron energies of 6 (also not used here), 9, 12, 16, and 20 MeV. Measurements were made using neutron bubble detectors and track-etch detectors. For electron beams from a conventional accelerator, the neutron leakage in the forward direction in Sv/Gy is 2.1 x 10(-5) at 12 MeV, 1.3 x 10(-4) at 16 MeV, and 4.2 x 10(-4) at 20 MeV, assuming a quality factor (RBE) of 10. For azimuthal angles > 0 degrees, the leakage is almost angle independent [2 x 10(-6) at 12 MeV; (0.7-1.6) x 10(-5) at 16 MeV, and (1.6-2.9) x 10(-5) at 20 MeV]. For the electron-only machine, the neutron leakage was lower than for the conventional linac, but also independent of azimuthal angle for angles > 0 degrees: {[0 degrees: 7.7 x 10(-6) at 12 MeV; 3.0 x 10(-5) at 15 MeV; 1.0 x 10(-4) at 18 MeV]; [other angles: (2.6-5.9) x 10(-7) at 12 MeV; (1.4-2.2) x 10(-6) at 15 MeV; (2.7-4.7) x 10(-6) at 18 MeV]}. Using the upper limit of 6 x 10(-7) Sv/Gy at 12 MeV for the IORT machine for azimuthal angles > 0 degrees and assuming a workload of 200 Gy/wk and an inverse square factor of 10, the neutron dose equivalent is calculated to be 0.012 mSv/wk. For the primary beam at 12 MeV (0 degrees), the 10 x higher dose would be compensated by the attenuation of a primary beam stopper in a mobile linear accelerator. These neutron radiation levels are below regulatory values (National Council on Radiation Protection and Measurements, "Limitation of exposure to ionizing radiation," NCRP Report No. 116, NCRP Bethesda, MD, 1993).

Intraoperative radiation therapy (IORT) has been customarily performed either in a shielded operating suite located in the operating room (OR) or in a shielded treatment room located within the Department of Radiation Oncology. In both cases, this cancer treatment modality uses stationary linear accelerators. With the development of new technology, mobile linear accelerators have recently become available for IORT. Mobility offers flexibility in treatment location and is leading to a renewed interest in IORT. These mobile accelerator units, which can be transported any day of use to almost any location within a hospital setting, are assembled in a nondedicated environment and used to deliver IORT. Numerous aspects of the design of these new units differ from that of conventional linear accelerators. The scope of this Task Group (TG-72) will focus on items that particularly apply to mobile IORT electron systems. More specifically, the charges to this Task Group are to (i) identify the key differences between stationary and mobile electron linear accelerators used for IORT, (ii) describe and recommend the implementation of an IORT program within the OR environment, (iii) present and discuss radiation protection issues and consequences of working within a nondedicated radiotherapy environment, (iv) describe and recommend the acceptance and machine commissioning of items that are specific to mobile electron linear accelerators, and (v) design and recommend an efficient quality assurance program for mobile systems.

The calibration and monthly QA of an electron-only linear accelerator dedicated to intra-operative radiation therapy has been reviewed. Since this machine is calibrated prior to every procedure, there was no necessity to adjust the output calibration at any time except, perhaps, when the magnetron is changed, provided the machine output is reasonably stable. This gives a unique opportunity to study the dose output of the machine per monitor unit, variation in the timer error, flatness and symmetry of the beam and the energy check as a function of time. The results show that, although the dose per monitor unit varied within +/- 2%, the timer error within +/- 0.005 MU and the asymmetry within 1-2%, none of these parameters showed any systematic change with time. On the other hand, the energy check showed a linear drift with time for 6, 9, and 12 MeV (2.1, 3.5, and 2.5%, respectively, over 5 years), while at 15 and 18 MeV, the energy check was relatively constant. It is further shown that based on annual calibrations and RPC TLD checks, the energy of each beam is constant and that therefore the energy check is an exquisitely sensitive one. The consistency of the independent checks is demonstrated.

A recently developed miniature x-ray tube operating at 40 kV has been used in a randomized trial for the treatment of small intracranial lesions. The diameter of these lesions ranges from 10 to 30 mm. A thin window parallel-plate ionization chamber was used to calibrate the output of the x-ray tube, modified by the addition of a thin platinum aperture to reduce the charge collecting area of the chamber. The effect of such an aperture on the measurement of dose versus distance from the x-ray tube in a phantom has been examined as a function of aperture diameter. Aperture diameters were varied between 0 and 5 mm and dose measurements were made for distances between the x-ray source and the front surface of the chamber of 5-30 mm in water. The ratio of doses measured with and without an aperture, when normalized to unity at a distance of 10 mm, differs significantly from unity, for distances between 7.5 and 15 mm, for aperture diameters <1.5 mm and differs from unity, but less significantly, for apertures > or =3 mm. For intermediate diameters, however, this dose dependence is minimized, indicating an aperture diameter that provides a similar distance-dose curve as the measurement taken without an aperture over this range of distances. This diameter was found to be between 2 and 2.5 mm with a dose variation of less than +/- 1%. For distances <7.5 mm, measurements made with a 1.5-mm-diam aperture agree better with those taken with a 1.7-mm-diam chamber compared with a 5.2-mm-diam chamber.

The collection efficiency of a 5.7 cm diameter spherical ionization chamber has been measured in 4 MV and 10 MV x-ray beams at various distances from the source. This chamber was found to have a substantial inefficiency due to its large volume and the high dose rate and pulsed nature of the therapy beams. It was also found that the efficiency depended on the dose rate of the machine because the inter-pulse separation time of the linac is significantly less than the ion transit-time for this chamber. Thus, ionization from more than one beam pulse is collected by the chamber at the same time. The efficiency was determined using three techniques (i) the two-voltage technique, (ii) the voltage extrapolation technique and (iii) a method originally devised for determining the collection efficiency of large volume ionization chambers in diagnostic radiology. The results show that methods (ii) and (iii) agree well, but that the two-voltage technique predicts a much lower efficiency. At about 4 m from the source, the collection efficiency for this chamber varied between 98% and 97% for dose rates between 50 and 250 cGy/min for 4 MV and between 97% and 90% for dose rates between 100 and 600 cGy/min for 10 MV. At isocenter, the comparable figures were 78% and 56% respectively for 4 MV and 65% and 38% respectively for 10 MV.

A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between 60Co and 50 MV and electron beams with nominal energies between 4 and 50 MeV. The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, N(60Co)D,w which are traceable to national primary standards, and the equation D(Q)w = MkQN(60Co)D,w where Q is the beam quality of the clinical beam, D(Q)w is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, M is the fully corrected ion chamber reading, and kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a beam of quality Q. Values of kQ are presented as a function of Q for many ion chambers. The value of M is given by M = PionP(TP)PelecPpolMraw, where Mraw is the raw, uncorrected ion chamber reading and Pion corrects for ion recombination, P(TP) for temperature and pressure variations, Pelec for inaccuracy of the electrometer if calibrated separately, and Ppol for chamber polarity effects. Beam quality, Q, is specified (i) for photon beams, by %dd(10)x, the photon component of the percentage depth dose at 10 cm depth for a field size of 10x10 cm2 on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by R50, the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size > or =10x10 cm2 on the surface of the phantom (> or =20x20 cm2 for R50>8.5 cm) at an SSD of 100 cm. R50 is determined directly from the measured value of I50, the depth at which the ionization falls to 50% of its maximum value. All clinical reference dosimetry is performed in a water phantom. The reference depth for calibration purposes is 10 cm for photon beams and 0.6R50-0.1 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a 10x10 cm2 field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup. For electron beams clinical reference dosimetry is performed with a field size of > or =10x10 cm2 (> or =20x20 cm2 for R50>8.5 cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification compared to the AAPM's TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors. Worksheets for various situations are presented along with a list of equipment required.

Wedge transmission factors have been measured for two sets of physical wedges for the 4 MV beam of a new dual energy linear accelerator as a function of field size and depth. The field size dependence of these factors has been compared with the 4 MV beam from another, single-energy linear accelerator and a difference of a factor of approximately 2 has been observed in the relative wedge transmission factors between the two machines.

The Photon Radiosurgery System is a miniature x-ray device developed for the treatment of small intracranial neoplasms. The x-rays are generated at the tip of a 10-cm-long, 3-mm-diam probe with a nearly isotropic distribution. Results from measurements of the two-dimensional dose distribution around the x-ray source are presented using two methods: (1) dose measurement with an ionization chamber and a water phantom system and (2) dose measurement with radiochromic film and a solid water phantom. The shape of the two angular dose distributions in the axial plane agree with each other to with approximately 10% and the dose at 10 mm from the source, orthogonal to the probe axis, was about 20% lower than at the same distance along the axis. The relative dose difference of 20% corresponds to a change in distance from the source of +/- 0.3 mm at 10 mm. It is shown that the anisotropy of radiation distribution in the axial plane can be improved to approximately 10% by adjusting the electron beam with a 12% reduction in the overall radiation output.

A miniature, battery operated 40 kV x-ray device has been developed for the interstitial treatment of small tumors ( < 3 cm diam) in humans. X rays are emitted from the tip of a 10 cm long, 3 mm diameter probe that is stereotactically inserted into the tumor. The beam, characterized by half-value layer (HVL), spectrum analysis, and isodose contours, behaves essentially as a point isotropic source with an effective energy of 20 keV at a depth of 10 mm in water. The absolute output from the device was measured using a parallel plate ionization chamber, modified with a platinum aperture. The dose rate in water determined from these chamber measurements was found to be nominally 150 cGy/min at a distance of 10 mm for a beam current of 40 microA and voltage of 40 kV. The dose in water falls off approximately as the third power of the distance. To date, 14 patients have been treated with this device in a phase I clinical trial.

Ensuring safe operation for a medical accelerator is a difficult task. Users must assume more responsibility in using contemporary equipment. Additionally, users must work closely with manufacturers in promoting the safe and effective use of such complex equipment. Complex treatment techniques and treatment modality changeover procedures merit detailed, unambiguous written procedural instruction at the control console. A thorough "hands on" training period after receiving instructions, and before assuming treatment responsibilities, is essential for all technologists. Unambiguous written instructions must also be provided to guide technologists in safe response when equipment malfunctions or exhibits unexpected behavior or after any component has been changed or readjusted. Technologists should be given a written list of the appropriate individuals to consult when unexpected machine behavior occurs. They should be assisted in identifying aberrant behavior of equipment. Many centers already provide this instruction, but others may not. Practiced response and discussion with technologists should be a part of an ongoing quality assurance program. An important aspect of a safety program is the need for continuous vigilance. Table III gives a summary of a comprehensive safety program for medical accelerators. Table IV gives a list of summary recommendations as an example of how one might mitigate the consequences of an equipment failure and improve procedures and operator response in the context of the environment described. Most of these recommendations can be implemented almost immediately at any individual treatment center.

A detailed Monte Carlo approach has been employed to investigate the sources of electron contamination for the 25-MV photon beam generated by Varian's Clinac-35. Three sources of contamination were examined: (a) the flattening filter and beam monitor chamber, (b) the fixed primary collimators downstream from the monitor chamber and the adjustable photon jaws, and (c) the air volume separating the treatment head from the observation point. Five source-to-surface distances (SSDs) were considered for a single field size, 28 cm in diameter at 80 cm SSD. It was found that for small SSDs (80-100 cm), the dominant sources of electron contamination were the flattening filter and the beam monitor chamber which accounted for 70% of the unwanted electrons. Thirteen percent of the remaining electrons originated in the downstream primary collimators and the photon jaws, and 17% were produced in air. At larger SSDs, the fraction of unwanted electrons originating in air increased. At 400 cm SSD, 61% of the contaminating electrons present in the beam were produced in air, 34% originated in the flattening filter and beam monitor chamber, and 5% were due to interactions in the fixed collimators downstream from the monitor chamber and the adjustable photon jaws. These calculated results are substantiated by recent experiments.

Tissue phantom ratios (TPR), based on a normalization depth of 5 cm, have been measured in water for field sizes from 5 x 5 cm2 to approximately 40 x 40 cm2 and for depths from 1 to 40 cm for a Varian Clinac 4/100. These TPR's have been compared with those calculated from percent depth doses measured at the same time, and the two sets of data generally agree to better than 1%, with an average ratio of measured to calculated TPR of 0.999 +/- 0.013. Beam profiles have been measured for open and wedged fields, with particular concern for the often observed "horns," or the increase in dose at the corners of the field. The maximum dose at a depth of 1 cm, along the diagonal of the field for this machine, is approximately 5% higher than at the same depth on the central axis, whereas along the principal plane the maximum dose is only about 3% higher.

For megavoltage x-ray beams, it is well known that the percent depth-dose increases considerably with field size in the buildup region, resulting in a significant shift in the apparent position of maximum dose, dmax. The nature of this increase has been investigated using a sweeping magnet placed just below the treatment head of a 25-MV linac. Measurements show that for increasing magnetic fields the dose in the buildup region is continually reduced, until a point is reached beyond which no additional reduction is observed. Here the buildup curve is essentially field size independent. These results clearly show that electrons are the primary cause of dose increase with field size in the buildup region, in contrast to a recent publication claiming that scattered photons are the cause. Further measurements were made by blocking out the primary beam at the level of the jaws and measuring the depth dose of the scattered electrons originating from the jaws. The results show that a thickness of approximately 1 gcm-2, of either polystyrene or lead, reduces the dose by a factor of two, providing further evidence that the scattered component of the beam consists of low energy electrons.

The attributes of high energy photon beams, i.e., low surface dose, large dmax and improved %DD, are compromised with increase in field size. This is due to the relative increase with field size of the electron component in the beam, as shown by recent experiments done here using a sweeeping magnet. The present study shows that the advantages can be partially regained with the use of foils to remove electrons. Various thicknesses of Al, Cu, Sn, and Pb were placed in a 25-MV linac x-ray beam at several sites in the treatment head. Buildup curves were measured with a "pancake" chamber for various SSD and field sizes. The magnitude of improvement achieved is dependent upon field size, SSD, the atomic number of the foil material, and foil thickness. Pb foil (0.55 gcm-2) provided the best overall improvement. Surface dose reduction of 10%-20% can be achieved along with significant increase in dmax and %DD. These findings suggest a new design feature for the next generation of high energy linacs.

A computer program has been written to reconstruct the three-dimensional distribution of radioactive seed implants. The technique requires the use of three rotated films with random input of the seeds. This algorithm differs from previous ones in the constraints that are applied, since no correlation is made between distances on the three films from the plane of rotation through the isocenter nor is there a correlation between the distances from the axis of rotation. Instead, projections from the x-ray source to the seed images on each of the three films are compared to see which combinations of seeds intersect at a point and therefore correspond to real seeds. This program has been written in Fortran IV for the PDP VAX 11/780 and 11/34 computers. Changes have been made to the TP11 program "QSEED" on the PDP 11/34 to accommodate the coordinate data from this program to produce the requisite isodose curves.

The forward production of high-energy electrons from materials with various atomic numbers from carbon to lead has been measured for megavoltage photon beams from 4- to 25-MV peak bremsstrahlung energy by placing a thin-window parallel-plate ionization chamber directly behind foils of the various materials. The relative forward production of electrons decreases with atomic number for energies less than or equal to 10 MV until about Z = 50, after which it rises. For photon energies greater than or equal to 15 MV, forward production increases with atomic number with a break point at Z approximately 50, beyond which the curve becomes steeper.

The use of solid state detectors in the coincidence sum peak technique of 125I activity assay has been studied. The improved energy resolution of these detectors allows distinct resolution of the K a, K beta and gamma peaks, and four sum peaks resulting from the coincident detection of the different X and gamma photons. Theoretical considerations yield three separate equations, each of which can be used, in conjunction with the experimental sum peak spectrum to give the activity of the 125I sample in question. This method is suitable for calibration of 125I activities in the range of 0.01-0.5 muCi with precisions of 2%-3%.

This study concerns the accuracy and precision of the IAEA/WHO LiF TLD system used in intercomparison by mail of absorbed doses from 60Co gamma-radiation and 4-25 MV x rays. The system employs 160 mg LiF powder in polystyrene capsules, which are placed at 5 or 7 cm depth in water and irradiated to doses close to 200 rad (2.00 Gy). The dosimeters are mailed to the IAEA Dosimetry Laboratory and read out under conditions to minimize variatons in instrument sensitivity. The precision of the readout technique, using 3 capsules per irradiation and the readout of 5 aliquots per capsule, is characterized by 0.2% standard deviation of the resulting mean. Since random errors during the irradiation are added, the detectable systematic descrepancy in dose delivery, at the 95% confidence level, is +/- 2% for 60Co gamma and +/- 3% for high-energy x rays.

A method is described for computing the effective center of an x-ray spoke shot pattern and determining its separation from a given reference point, such as the mechanical isocenter. This method is as quick as, but more accurate than, the standard technique of using a ruler.