AAPM Report 373: The content, structure, and value of the Professional Doctorate in Medical Physics (DMP)

The Professional Doctorate in Medical Physics (DMP) was originally conceived as a solution to the shortage of medical physics residency training positions. While this shortage has now been largely satisfied through conventional residency training positions, the DMP has expanded to multiple institutions and grown into an educational pathway that provides specialized clinical training and extends well beyond the creation of additional training spots. As such, it is important to reevaluate the purpose and the value of the DMP. Additionally, it is important to outline the defining characteristics of the DMP to assure that all existing and future programs provide this anticipated value. Since the formation and subsequent accreditation of the first DMP program in 2009–2010, four additional programs have been created and accredited. However, no guidelines have yet been recommended by the American Association of Physicists in Medicine. CAMPEP accreditation of these programs has thus far been based only on the respective graduate and residency program standards. This allows the development and operation of DMP programs which contain only the requisite Master of Science (MS) coursework and a 2‐year clinical training program. Since the MS plus 2‐year residency pathway already exists, this form of DMP does not provide added value, and one may question why this existing pathway should be considered a doctorate. Not only do we, as a profession, need to outline the defining characteristics of the DMP, we need to carefully evaluate the potential advantages and disadvantages of this pathway within our education and training infrastructure. The aims of this report from the Working Group on the Professional Doctorate Degree for Medical Physicists (WGPDMP) are to (1) describe the current state of the DMP within the profession, (2) make recommendations on the structure and content of the DMP for existing and new DMP programs, and (3) evaluate the value of the DMP to the profession of medical physics.

Impact of inhomogeneity corrections on dose coverage in the treatment of lung cancer using stereotactic body radiation therapy

The purpose of this study is to assess the real target dose coverage when radiation treatments were delivered to lung cancer patients based on treatment planning according to the RTOG-0236 Protocol. We compare calculated dosimetric results between the more accurate anisotropic analytical algorithm (AAA) and the pencil beam algorithm for stereotactic body radiation therapy treatment planning in lung cancer. Ten patients with non-small cell lung cancer were given 60 Gy in three fractions using 6 and 10 MV beams with 8-10 fields. The patients were chosen in accordance with the lung RTOG-0236 protocol. The dose calculations were performed using the pencil beam algorithm with no heterogeneity corrections (PB-NC) and then recalculated with the pencil beam with modified Batho heterogeneity corrections (PB-MB) and the AAA using an identical beam setup and monitor units. The differences in calculated dose to 95% or 99% of the PTV, between using the PB-NC and the AAA, were within 10% of prescribed dose (60 Gy). However, the minimum dose to 95% and 99% of PTV calculated using the PB-MB were consistently overestimated by up to 40% and 36% of the prescribed dose, respectively, compared to that calculated by the AAA. Using the AAA as reference, the calculated maximum doses were underestimated by up to 27% using the PB-NC and overestimated by 19% using the PB-MB. The calculations of dose to lung from PB-NC generally agree with that of AAA except in the small high-dose region where PB-NC underestimates. The calculated dose distributions near the interface using the AAA agree with those from Monte Carlo calculations as well as measured values. This study indicates that the real minimum PTV dose coverage cannot be guaranteed when the PB-NC is used to calculate the monitor unit settings in dose prescriptions.

AAPM/SNMMI Joint Task Force: report on the current state of nuclear medicine physics training

The American Association of Physicists in Medicine (AAPM) and the Society of Nuclear Medicine and Molecular Imaging (SNMMI) recognized the need for a review of the current state of nuclear  medicine physics training and the need to explore pathways for improving nuclear medicine physics training opportunities. For these reasons, the two organizations formed a joint AAPM/SNMMI Ad Hoc Task Force on Nuclear Medicine Physics  Training. The mission of this task force was to assemble a representative group of stakeholders to:• Estimate the demand for board-certified nuclear medicine physicists in the next 5-10 years,• Identify the critical issues related to supplying an adequate number of physicists who have received the appropriate level of training in nuclear medicine physics, and• Identify approaches that may be considered to facilitate the training of nuclear medicine physicists.As a result, a task force was appointed and chaired by an active member of both organizations that included representation from the AAPM, SNMMI, the American Board of Radiology (ABR), the American Board of Science in Nuclear Medicine (ABSNM), and the Commission for the Accreditation of Medical Physics Educational Programs (CAMPEP). The Task Force first met at the AAPM Annual Meeting in Charlotte in July 2012 and has met regularly face-to-face, online, and by conference calls. This manuscript reports the findings of the Task Force, as well as recommendations to achieve the stated mission.

Evidence for alterations in stimulatory G proteins and oxytocin levels in children with autism

The neurotransmitter oxytocin plays an important role in social affiliation. Low oxytocin levels and defects in the oxytocin receptor have been reported in childhood autism. However, little is known about oxytocin's post-receptor signaling pathways in autism. Oxytocin signals via stimulatory and inhibitory G proteins. c-fos mRNA expression has been used as a marker of OT signaling as well as of G protein signaling. Herein, we hypothesized that oxytocin and its signaling pathways would be altered in children with autism. We measured plasma oxytocin levels by ELISA, G-protein and c-fos mRNA by PCR, and G proteins by immunoblot in cultured peripheral blood mononuclear cells (PBMCs) in children with autism and in age-matched controls. Males with autism displayed elevated oxytocin levels compared to controls (p<0.05). Children with autism displayed significantly higher mRNA for stimulatory G proteins compared to controls (p<0.05). Oxytocin levels correlated strongly positively with c-fos mRNA levels, but only in control participants (p<0.01). Oxytocin, G-protein, and c-fos mRNA levels correlated inversely with measures of social and emotional behaviors, but only in control participants. These data suggest that children with autism may exhibit a dysregulation in oxytocin and/or its signaling pathways.

Reliability of adverse symptom event reporting by clinicians

PURPOSE

Adverse symptom event reporting is vital as part of clinical trials and drug labeling to ensure patient safety and inform risk-benefit decision making. The purpose of this study was to assess the reliability of adverse event reporting of different clinicians for the same patient for the same visit.

METHODS

A retrospective reliability analysis was completed for a sample of 393 cancer patients (42.8% men; age 26-91, M = 62.39) from lung (n = 134), prostate (n = 113), and Ob/Gyn (n = 146) clinics. These patients were each seen by two clinicians who independently rated seven Common Terminology Criteria for Adverse Events (CTCAE) symptoms. Twenty-three percent of patients were enrolled in therapeutic clinical trials.

RESULTS

The average time between rater evaluations was 68 min. Intraclass correlation coefficients were moderate for constipation (0.50), diarrhea (0.58), dyspnea (0.69), fatigue (0.50), nausea (0.52), neuropathy (0.71), and vomiting (0.46). These values demonstrated stability over follow-up visits. Two-point differences, which would likely affect treatment decisions, were most frequently seen among symptomatic patients for constipation (18%), vomiting (15%), and nausea (8%).

CONCLUSION

Agreement between different clinicians when reporting adverse symptom events is moderate at best. Modification of approaches to adverse symptom reporting, such as patient self-reporting, should be considered.

Evaluation of the dynamic conformal arc therapy in comparison to intensity-modulated radiation therapy in prostate, brain, head-and-neck and spine tumors

To evaluate dynamic conformal arc therapy (DAT) dose distribution and clinical applicability in comparison to intensity‐modulated radiotherapy (IMRT) in different types of tumors and locations, twelve patients with prostate cancer with no node involvement and three patients with single tumors in the pituitary, in the neck and in the thoracic spinal region treated with IMRT, were retrospectively planned with DAT using Eclipse (V8.1). The prostate cases were also planned with three‐dimensional conformal radiation therapy (3DCRT). Dose distributions were evaluated through comparisons of dose‐volumetric histograms and in‐house IMRT protocol constraints, as well as validated via ion chamber array measurements. DAT plans for prostate showed a statistically comparable achievement of tumor conformity and dose sparing for bladder and rectum when compared to IMRT. Dose on femoral heads were similar to those achieved using 3DCRT. DAT could be planned with similar results to those obtained in IMRT for the dose constraints of the defined structures by using a 360° arc for the brain lesion and several arcs including noncoplanar ones for the head‐and‐neck and spinal tumors. Experimental validation of the calculated dose distributions via gamma analysis of composite distributions for DAT provided that more than 95% of the pixels satisfy the criteria 3 mm–3%, which was similar to that of IMRT. The average number of monitor units was approximately five times lower than IMRT. In conclusion, DAT is capable of providing conformal dose distributions to the targets accomplishing many of the IMRT dose constraints simultaneously. Experimental dose‐validation accuracy, ease of planning and reduced treatment times make DAT both acceptable and attractive for clinical use.

PACS numbers: 87.55.D‐, 87.55.dk, 87.55.Qr, 87.56.bd, 87.56.Fc, 87.53.Kn, 87.55. de, 87.55.kd

Radiation dose from kilovoltage cone beam computed tomography in an image-guided radiotherapy procedure

PURPOSE

Image-guided radiation therapy has emerged as the new paradigm in radiotherapy. This work is to provide detailed information concerning the additional imaging doses to patients' radiosensitive organs from a kilovoltage cone beam computed tomography (kV CBCT) scan procedure.

METHODS AND MATERIALS

The Vanderbilt-Monte-Carlo-Beam-Calibration (VMCBC; Vanderbilt University, Nashville, TN) algorithm was used to calculate radiation dose to organs resulting from a kV CBCT imaging guidance procedure. Eight patients, including 3 pediatric and 5 adult patients, were investigated. The CBCT scans in both full- and half-fan modes were studied.

RESULTS

For a head-and-neck scan in half-fan mode, dose-volume histogram analyses show mean doses of 7 and 8 cGy to the eyes, 5 and 6 cGy to the spinal cord, 5 and 6 cGy to the brain, and 18 and 23 cGy to the cervical vertebrae for an adult and a 29-month-old child, respectively. The dose from a scan in full-fan mode is 10-20% lower than that in half-fan mode. For an abdominal scan, mean doses are 3 and 7 cGy to prostate and 7 and 17 cGy to femoral heads for a large adult patient and a 31-month-old pediatric patient, respectively.

CONCLUSIONS

Doses to radiosensitive organs can total 300 cGy accrued over an entire treatment course if kV CBCT scans are acquired daily. These findings provide needed data for clinicians to make informed decisions concerning additional imaging doses. The dose to bone is two to four times greater than dose to soft tissue for kV x-rays, which should be considered, especially for pediatric patients.

A study on adaptive IMRT treatment planning using kV cone-beam CT

BACKGROUND AND PURPOSE

Changes in tumor size during the course of radiotherapy warrant performing adaptive radiotherapy (ART). This work investigates the feasibility and usefulness of acquiring on-board cone-beam CT (CBCT) for ART for patients with bulky head and neck tumors treated with IMRT and for prostate patients with potentially significant target position variations during the treatment course.

MATERIALS AND METHODS

A phantom designed for CT quality assurance was used to compare the dosimetric and geometric accuracy between conventional CT and CBCT from a linear accelerator's on-board imager. Patient planning CT and CBCT images were acquired before treatment and at mid-course. The IMRT plans made on the CT were applied to the CBCT and dose-volume histograms were calculated.

RESULTS

In both phantom and patient studies, the dose-volume histograms (DVHs) based on CBCT images were in excellent agreement with DVHs based on planning CT images. Minimum, maximum and mean doses agreed very well. In a patient study, doses for targets and normal tissues from the same IMRT plans calculated on CBCT images agreed within 1-3% with those calculated on planning CT images.

CONCLUSIONS

CBCT images can be used to accurately predict dosimetric results. It is feasible to use CBCT to determine dosimetric consequences resulting from tumor shrinkage and patient geometry changes. An additional planning CT may be necessary to perform IMRT re-planning at present in order to accurately delineate tumor and organs. The CBCT has potential to become a very useful tool for on-line ART.

Deep-inspiration breath-hold kilovoltage cone-beam CT for setup of stereotactic body radiation therapy for lung tumors: Initial experience

We report our initial experience with deep-inspiration breath-hold (DIBH) cone-beam CT (CBCT) on the treatment table, using the kilovoltage imager integrated into our linear accelerator, for setting up patients for DIBH stereotactic body radiation therapy (SBRT) for lung tumors. Nine patients with non-small cell lung cancer (seven stage I), were given 60Gy in three fractions. All nine patients could perform a DIBH for 35s. For each patient we used a diagnostic reference CT volume image acquired during a DIBH to design an SBRT plan consisting of 7-10 noncoplanar conformal beams. Four patients were setup by registering DIBH kilovoltage projection radiographs or megavoltage portal images on the treatment table to digitally reconstructed radiographs from the reference CT. Each of the last 14 fractions out of a total of 27 was setup by acquiring a CBCT volume image on the treatment table in three breath-holds. The CBCT and reference CT volume images were directly registered and the shift was calculated from the registration. The CBCT volume images contained excellent detail on soft tissue and bony anatomy for matching to the reference CT. Most importantly, the tumor was always clearly visible in the CBCT images, even when it was difficult or impossible to see in the radiographs or portal images. The accuracy of the CBCT method was confirmed by DIBH megavoltage portal imaging and each treatment beam was delivered during a DIBH. CBCT acquisition typically required five more minutes than radiograph acquisition but the overall setup time was often shorter using CBCT because repeat imaging was minimized. We conclude that for setting up SBRT treatments of lung tumors, DIBH CBCT is feasible, fast and may result in less variation among observers than using bony anatomy in orthogonal radiographs.

Characteristics of kilovoltage x-ray beams used for cone-beam computed tomography in radiation therapy

The purpose of this investigation is to characterize the beams produced by a kilovoltage (kV) imager integrated into a linear accelerator (Varian on-board imager integrated into the Trilogy accelerator) for acquiring high resolution volumetric cone-beam computed tomography (CBCT) images of the patient on the treatment table. The x-ray tube is capable of generating photon spectra with kVp values between 40 and 125 kV. The Monte Carlo simulations were used to study the characteristics of kV beams and the properties of imaged target scatters. The Monte Carlo results were benchmarked against measurements, and excellent agreements were obtained. We also studied the effect of including the electron impact ionization (EII), and the simulation showed that the characteristic radiation is increased significantly in the energy spectra when EII is included. Although only slight beam hardening is observed in the spectra of all photons after passing through the phantom target, there is a significant difference in the spectra and angular distributions between scattered and primary photons. The results also show that the photon fluence distributions are significantly altered by adding bow tie filters. The results indicate that a combination of large cone-beam field size and large imaged target significantly increases scatter-to-primary ratios for photons that reach the detector panel. For phantoms 10 cm, 20 cm and 30 cm thick of water placed at the isocentre, the scatter-to-primary ratios are 0.94, 3.0 and 7.6 respectively for an open 125 kVp CBCT beam. The Monte Carlo simulations show that the increase of the scatter is proportional to the increase of the imaged volume, and this also applies to scatter-to-primary ratios. This study shows both the magnitude and the characteristics of scattered x-rays. The knowledge obtained from this investigation may be useful in the future design of the image detector to improve the image quality.

First macro Monte Carlo based commercial dose calculation module for electron beam treatment planning--new issues for clinical consideration

The purpose of this study is to present our experience of commissioning, testing and use of the first commercial macro Monte Carlo based dose calculation algorithm for electron beam treatment planning and to investigate new issues regarding dose reporting (dose-to-water versus dose-to-medium) as well as statistical uncertainties for the calculations arising when Monte Carlo based systems are used in patient dose calculations. All phantoms studied were obtained by CT scan. The calculated dose distributions and monitor units were validated against measurements with film and ionization chambers in phantoms containing two-dimensional (2D) and three-dimensional (3D) type low- and high-density inhomogeneities at different source-to-surface distances. Beam energies ranged from 6 to 18 MeV. New required experimental input data for commissioning are presented. The result of validation shows an excellent agreement between calculated and measured dose distributions. The calculated monitor units were within 2% of measured values except in the case of a 6 MeV beam and small cutout fields at extended SSDs (>110 cm). The investigation on the new issue of dose reporting demonstrates the differences up to 4% for lung and 12% for bone when 'dose-to-medium' is calculated and reported instead of 'dose-to-water' as done in a conventional system. The accuracy of the Monte Carlo calculation is shown to be clinically acceptable even for very complex 3D-type inhomogeneities. As Monte Carlo based treatment planning systems begin to enter clinical practice, new issues, such as dose reporting and statistical variations, may be clinically significant. Therefore it is imperative that a consistent approach to dose reporting is used.

Commissioning stereotactic radiosurgery beams using both experimental and theoretical methods

The purpose of this investigation is to study the feasibility of using an alternative method to commission stereotactic radiosurgery beams shaped by micro multi-leaf collimators by using Monte Carlo simulations to obtain beam characteristics of small photon beams, such as incident beam particle fluence and energy distributions, scatter ratios, depth-dose curves and dose profiles where measurements are impossible or difficult. Ionization chambers and diode detectors with different sensitive volumes were used in the measurements in a water phantom and the Monte Carlo codes BEAMnrc/DOSXYZnrc were used in the simulation. The Monte Carlo calculated data were benchmarked against measured data for photon beams with energies of 6 MV and 10 MV produced from a Varian Trilogy accelerator. The measured scatter ratios and cross-beam dose profiles for very small fields are shown to be not only dependent on the size of the sensitive volume of the detector used but also on the type of detectors. It is known that the response of some detectors changes at small field sizes. Excellent agreement was seen between scatter ratios measured with a small ion chamber and those calculated from Monte Carlo simulations. The values of scatter ratios, for field sizes from 6 x 6 mm2 to 98 x 98 mm2, range from 0.67 to 1.0 and from 0.59 to 1.0 for 6 and 10 MV, respectively. The Monte Carlo calculations predicted that the incident beam particle fluence is strongly affected by the X-Y-jaw openings, especially for small fields due to the finite size of the radiation source. Our measurement confirmed this prediction. This study demonstrates that Monte Carlo calculations not only provide accurate dose distributions for small fields where measurements are difficult but also provide additional beam characteristics that cannot be obtained from experimental methods. Detailed beam characteristics such as incident photon fluence distribution, energy spectra, including composition of primary and scattered photons, can be independently used in dose calculation models and to improve the accuracy of measurements with detectors with an energy-dependent response. Furthermore, when there are discrepancies between results measured with different detectors, the Monte Carlo calculated values can indicate the most correct result. The data set presented in this study can be used as a reference in commissioning stereotactic radiosurgery beams shaped by a BrainLAB m3 on a Varian 2100EX or 600C accelerator.

AAPM protocol for 40-300 kV x-ray beam dosimetry in radiotherapy and radiobiology

The American Association of Physicists in Medicine (AAPM) presents a new protocol, developed by the Radiation Therapy Committee Task Group 61, for reference dosimetry of low- and medium-energy x rays for radiotherapy and radiobiology (40 kV < or = tube potential < or = 300 kV). It is based on ionization chambers calibrated in air in terms of air kerma. If the point of interest is at or close to the surface, one unified approach over the entire energy range shall be used to determine absorbed dose to water at the surface of a water phantom based on an in-air measurement (the "in-air" method). If the point of interest is at a depth, an in-water measurement at a depth of 2 cm shall be used for tube potentials > or = 100 kV (the "in-phantom" method). The in-phantom method is not recommended for tube potentials < 100 kV. Guidelines are provided to determine the dose at other points in water and the dose at the surface of other biological materials of interest. The protocol is based on an up-to-date data set of basic dosimetry parameters, which produce consistent dose values for the two methods recommended. Estimates of uncertainties on the final dose values are also presented.

Small photon field dosimetry for stereotactic radiosurgery

Performing and assuring the quality of the planning and delivery of stereotactic radiosurgery with photon beams requires accurate evaluation of beam parameters, usually including output factors, tissue-phantom ratios and off-axis ratios, and measurement of actual dose distributions from simulated treatments. For the small photon fields used in radiosurgery, these measurements require special equipment and techniques, which are described in this review.

Dose model for a beta-emitting stent in a realistic artery consisting of soft tissue and plaque

A model for the description of the near-field dose deposition from a 32p impregnated stent in an arterial system consisting of soft tissue and dense plaque is presented. The model is based on the scaling property of the dose-point-kernel (DPK) function which is extended to a heterogeneous medium consisting of a series of layers of different materials. It is shown that, for each point source originating from the stent surface, the DPK function for water can be scaled consistently along the path through the different layers of material to predict the dose at a given point in the heterogeneous medium. Radiochromic film dosimetry on actual 32p stents is used to test the new model. The experimental setup consists of a water-equivalent phantom in which a stent is deployed and on which a thin layer of polytetrafluoroethylene (PTFE) is deposited to simulate the presence of plaque. Layers of radiochromic films stacked over the phantom are used to measure the dose at distances varying from approximately 0.1 mm to approximately 3 mm from the stent surface with and without PTFE. It is shown that the proposed new DPK model for a heterogeneous medium agrees very well with the experimental data and that it compares favorably to the usual homogeneous DPK model. These results indicate that the new model can be used with confidence to predict the dose in a realistic artery in the presence of plaque.

Radiochromic film dosimetry of a high dose rate beta source for intravascular brachytherapy

Good clinical physics practice requires that dose rates of brachytherapy sources be checked by the institution using them, as recommended by American Association of Physicists in Medicine Task Group 56 and The American College of Radiology. For intravascular brachytherapy with catheter-based systems, AAPM Task Group 60 recommends that the dose rate be measured at a reference point located at a radial distance of 2 mm from the center of the catheter axis. AAPM Task Group 60 also recommends that the dose rate along the catheter axis at a radial distance of 2 mm should be uniform to within +/- 10% in the center two-thirds of the treated length, and the relative dose rate in the plane perpendicular to the catheter axis through the center of the source should be measured at distances from 0.5 mm to R90 (the distance from a point source within which 90% of the energy is deposited) at intervals of 0.5 mm. Radiochromic film dosimetry has been used to measure the dose distribution in a plane parallel to and at a radial distance of 2 mm from the axis of a novel, catheter-based, beta source for intravascular brachytherapy. The dose rate was averaged along a line parallel to the catheter axis at a radial distance of 2 mm, in the centered 24.5 mm of the treated length. This average dose rate agreed with the dose rate measured with a well ionization chamber by the replacement method using source trains calibrated with an extrapolation chamber at the National Institute of Standards and Technology. All of the dose rates in the centered 24.5 mm of a line parallel to the axis at a distance of 2 mm were within +/-10% of the average.

Dose distribution for a 32P-impregnated coronary stent: comparison of theoretical calculations and measurements with radiochromic film

PURPOSE

Restenosis, caused by proliferation of smooth-muscle cells, limits the efficacy of catheter-based revascularization of coronary arteries. Irradiation has been shown to inhibit growth of smooth-muscle cells in vitro and to prevent restenosis in animal models following stent placement. An intraarterial source of 32P, a pure beta emitter with a half-life of 14.28 days and a 90% range in water of 3.6 mm, is almost ideal for irradiating just arterial wall without exposing any other part of the patient's heart or any other organs, while posing minimal hazards to medical personnel. Two types of previously developed coronary stent impregnated with 32P were investigated. This study aimed to calculate and measure the dose outside of two types of 32P-impregnated beta-emitting coronary stents under conditions closely simulating clinical use.

METHODS AND MATERIALS

The dose distributions in water surrounding these stents were calculated using a convolution method and measured by exposing radiochromic film in a solid-water phantom.

RESULTS

Experimental results were in excellent agreement with theoretical calculations.

CONCLUSIONS

Radiochromic dosimetry can be used to measure the dose distribution around a beta-emitting intraarterial stent at distances as small as 0.1 mm from the stent surface. A simple cylindrical shell model is adequate for calculating the dose at points farther than 0.5 mm from the stent surface.

Radiation dose from a phosphorous-32 impregnated wire mesh vascular stent

The near field dose distribution of a realistic vascular stent impregnated with radioactive 32P is calculated employing the dose-point-kernel (DPK) method in a homogeneous and uniform medium. The cylindrical wire mesh geometry for the Palmaz-Schatz [Palmaz-Schatz is a tradename of Cordis (a Johnson & Johnson company)] stent is incorporated in the model calculation, and the dose distribution generated by the beta particles emitted from the decayed radioactive 32P is computed at distances ranging from 0.1 to 2 mm exterior to the stent surface. Dose measurements were obtained using radiochromic film dosimetry media on an actual Palmaz-Schatz half-stent impregnated with 32P using ion implantation, and compared to the DPK model predictions. The close agreement between the model calculation and the film dosimetry data confirms the validity of the model which can be adapted to a variety of different stent designs.

Use of a micro-ionization chamber and an anthropomorphic head phantom in a quality assurance program for stereotactic radiosurgery

Quality assurance methods used in association with radiosurgery must include all aspects of the radiosurgery process: visualization and localization of the target, treatment and dose planning and dose delivery. Presented here is a quality assurance method that utilizes an anthromorphic head phantom and a micro-ionization chamber to demonstrate precise target localization and accurate dose delivery. This micro-ionization chamber method offers an immediate readout which is both accurate and reproducible. Additionally, this method allows unlimited repetition of the dose measurement process without repeated radiographic localization studies as is necessary with the conventional methods of TLD, film, and Fricke gels. The method and techniques presented can be used in the acceptance testing and routine quality assurance of both linac-based and Gamma Knife radiosurgery units.

Low-dose, beta-particle emission from 'stent' wire results in complete, localized inhibition of smooth muscle cell proliferation

BACKGROUND

Restenosis after catheter-based revascularization has been demonstrated to be primarily caused by medial and/or intimal smooth muscle cell (SMC) proliferation. The objective of this study was investigate the ability of local emission of beta-particles from a 32P-impregnated titanium "stent" wire source to inhibit vascular SMC and endothelial cell proliferation in cell culture and to determine the dose-response characteristics of this inhibition.

METHODS AND RESULTS

A series of experiments were performed using 0.20-mm-diameter titanium wires that were impregnated with varying low concentrations of 32P (activity range, 0.002 to 0.06 microCi/cm wire, n = 47) or 31P (nonradioactive control, n = 28) in cultures of rat and human aortic SMCs and in cultured bovine aortic endothelial cells. The zone of complete cell growth inhibition (in millimeters from stent wire) was measured using light microscopy in the cultures exposed to the radioactive (32P) or control (31P) wires at 6 and 12 days after plating. In both rat and human SMC cultures there was a distinct 5.5- to 10.6-mm zone of complete SMC inhibition at wire activity levels > or = 0.006 microCi/cm. In contrast, there was no zone of inhibition surrounding the control (31P impregnated) wires (P < .001 versus 32P wires at all wire activities > or = 0.006 microCi/cm for human and rat SMCs). Proliferating bovine endothelial cells were more radioresistant than SMCs, with no zone of inhibition observed at wire activity levels up to 0.019 microCi/cm (P < .001 versus SMCs at 0.006 microCi/cm and 0.019 microCi/cm).

CONCLUSIONS

We conclude that very low doses of beta-particle emission from a 32P-impregnated stent wire (activity levels as low as 0.006 microCi/cm of wire) completely inhibit the growth and migration of both rat and human SMCs within a range of 5.5 to 10.6 mm from the wire. Endothelial cells appear to be much more radioresistant than SMCs. These data suggest that an intra-arterial stent impregnated with a low concentration of 32P may have a salutary effect on the restenosis process. Whether this approach can be used successfully and safely to inhibit restenosis in vivo and in the clinical setting is under investigation.

A tissue equivalent phantom for stereotactic radiosurgery localization and dose verification

A tissue equivalent head phantom was utilized in the stereotactic localization and dose verification of radiosurgery procedures with the Leksell Gamma Knife Unit at the University of Kentucky Medical Center. A radiation dose-dependent color-doped gel target was positioned within the head phantom and stereotactically localized using either angiography, CT, or MR techniques. Utilizing standard Gamma Knife treatment procedures, the head phantom was irradiated, which resulted in a color change of the gel tumor at the position of the treatment isocenter and thereby confirmed the localization procedure. Additionally, a radiation dosimeter (thermoluminescent dosimetry--TLD) was positioned within the head phantom and localized using an angiography frame and a standard radiation therapy simulator. The phantom skull measurements and the dosimeter coordinates were entered into the Leksell Gamma Knife dose planning computer (KULA) and an irradiation time for 40 Gy using the 18-mm collimator was determined. The TLD dose evaluations were relatively determined using a cobalt-60 calibration curve. The experimental dose verification results agreed well (+/- 4%) with computer dose estimates.

Radiation induced acute tumor lysis syndrome in the bone marrow transplant setting

Patients with highly chemo-responsive malignancies have for some time been known to be at risk for life threatening acute tumor lysis syndrome (TLS). This report describes two cases involving acute TLS in patients with malignant diseases which had become refractory to therapy and received initial total body irradiation as a part of the preparative regimen for allogeneic bone marrow transplantation.

Dosimetric evaluation of a variable energy superficial X-ray machine with applications for endocavitary radiotherapy techniques

This investigation presents the beam characteristics of a newly-marketed variable energy superficial X ray machine for radiotherapy. The X ray system hardware, including a high voltage generator, and console software allow for nine independent operator-selected X ray beams from 10-150 kVp. Filament current values are also independently variable; 1.0-13.0 mA for 90-150 kVp, and 1.0-30.0 mA for 10-90 kVp. The HVL's, effective energies, and radiation outputs for the nine combinations of kVp and mA are presented. In addition, percentage depth dose and beam uniformity results are presented as a function of cone size. Radiation output stability and reproducibility results are included. Discussion of adaptation of this X ray system to the Papillon technique for the treatment of rectal cancers is presented.

A slice geometry phantom for cross sectional tomographic imagers

This investigation presents the design and fabrication of a magnetic resonance imaging (MRI) test phantom for determining slice thickness, slice adjacency, slice offset, and slice angulation. This test phantom is a three-dimensional conic section of MRI image producing material; proper orientation allows analysis in each of the major imaging planes. The phantom design (geometrical configuration) incorporates both theoretical and quantitative methodologies. The necessary mathematical analyses are both simple and rapid. In addition, this phantom has been successfully used to assess the image slice parameters for computed tomography (CT) scanners and single photon emission computed tomography (SPECT) imaging systems. Slice profile parameter results, full width at half maximum (FWHM), from this phantom design are compared with conventional methods.

Thoracic vertebral photopenia may predict fatty changes of the corresponding bone marrow following irradiation

We present a patient with unresectable bronchogenic carcinoma who underwent irradiation, then had four consecutive bone imaging studies in a 26-month period. Apparent photopenia of the upper thoracic vertebrae developed within six months after irradiation and became more apparent in the images thereafter. At autopsy a section of the corresponding bone marrow showed extensive fatty changes with very few residual blood vessels. Radiation damage of the vascular networks may significantly reduce the blood supply, when integrity of the blood supply is essential for delivery of a normal bone image by the bone-imaging agent. Interruption of blood supply may cause photopenia, and this interruption plus irradiation to marrow elements may also affect the hematopoietic activity of the corresponding bone marrow. The occurrence of radiation-induced photopenia on a bone-imaging study may indicate fatty changes of the corresponding marrow.

Methods for evaluating image quality in magnetic resonance imaging

A quality control program for magnetic resonance imaging is necessary to evaluate the major imaging characteristics and to monitor image performance constancy. The methods used to evaluate and monitor imaging quality characteristics and image quality variables are discussed. These imaging characteristics and methods may be utilized by the trained radiology technologist to assist in the continuation of a quality control program for magnetic resonance imaging.

Five-year cure of cervical cancer treated using californium-252 neutron brachytherapy

Female pelvic carcinoma is one of the common malignancies seen at the University of Kentucky Medical Center and often presents in an advanced stage. In 1976, we began to test californium-252 neutron brachytherapy (NT) for its efficacy for control of primary and recurrent advanced uterine, cervix, and vaginal cancers. The first protocol used was 5000-5500 rad of whole pelvis irradiation followed by 1-2 Cf-252 insertions using a single tandem placed in the utero-cervico-vaginal region. Of 27 patients with primary carcinomas treated, 10 are alive and well 5 years later (37%). Two of two recurrent tumors were locally controlled but failed later. These patients had advanced cervical, vaginal, or endometrial carcinomas. In 1977, a transitional year, treatment of only unfavorable stages and presentations with NT was initiated. Similar results were obtained with NT as compared to conventional photon therapy (PT). Further improvement in treatment results can be anticipated as NT brachytherapy is used for advanced cancer therapy by more effective treatment schedules and radiation doses. Cf-252 can be used as a radium substitute and achieved similar rates of tumor control and 5-year survivals.

A new technique for the calculation of scattered radiation for 10 MV photon beams

An alternative to the Clarkson technique for calculating total scattered radiation is presented for use with 10 MV photon beams. When compared to the same field size and depth parameters, the resultant TMR values are in good agreement with results utilizing the Clarkson technique. Lengthy iterative procedures are avoided in this new scatter technique in favor of a more efficient SMR algorithm, thereby reducing the amount of required computation time. The speed and simplicity of this method make it especially amenable to use with small desk top computers and even programmable pocket calculators.

X-ray beam characteristics of the Varian Clinac 6-100 linear accelerator

Beam characteristics and dosimetry measurements of the 6 MV x-ray beam from a Varian Associates Clinac 6-100 linear accelerator are presented. Percentage depth dose tables are given as a function of field size for square fields. Tissue-maximum ratios and scatter-maximum ratios are tabulated as a function of depth and field size. Investigation of surface dose and depth of maximum dose reveal a dependence on field size. Other beam parameters measured are presented include field flatness, symmetry, SSD dependence, and penumbra. Treatment planning parameters including field size output factors and wedge factors are discussed.

Radiation pneumonitis following electron beam radiotherapy

A case of radiation pneumonitis is reported. The problem presented in a postmastectomy breast patient after completion of 5000 rads of high-energy electron beam radiotherapy. Within 7 months the lungs showed almost complete clearing and clinical symptoms had disappeared.

A new technique for the calculation of scattered radiation from 60Co-teletherapy beams

An alternative to the Clarkson technique for calculating total scattered radiation is presented. Resultant TAR values are compared with those obtained by the Clarkson technique for the same field. The new scatter technique is always more than 2.8 times faster than the Clarkson technique and more than 4 times faster on a large number of fields. The new approach appears to offer a simple and expedient alternative to the more traditional methods for iiregular field dose calculations.

Use of a programmable pocket calculator in radiotherapy treatment planning

Software packages for a programmable pocket calculator have been developed for use in dosimetry. Using a field equation and a mathematical model of the beam profile, one can find the dose delivered to any point within the irradiated volume. Use of these programs for simple field calculations allows the radiologist to concentrate on more complex treatment plans.