Raj K. Mitra

Accelerator beam data commissioning equipment and procedures: Report of the TG-106 of the Therapy Physics Committee of the AAPM

For commissioning a linear accelerator for clinical use, medical physicists are faced with many challenges including the need for precision, a variety of testing methods, data validation, the lack of standards, and time constraints. Since commissioning beam data are treated as a reference and ultimately used by treatment planning systems, it is vitally important that the collected data are of the highest quality to avoid dosimetric and patient treatment errors that may subsequently lead to a poor radiation outcome. Beam data commissioning should be performed with appropriate knowledge and proper tools and should be independent of the person collecting the data. To achieve this goal, Task Group 106 (TG-106) of the Therapy Physics Committee of the American Association of Physicists in Medicine was formed to review the practical aspects as well as the physics of linear accelerator commissioning. The report provides guidelines and recommendations on the proper selection of phantoms and detectors, setting up of a phantom for data acquisition (both scanning and no-scanning data), procedures for acquiring specific photon and electron beam parameters and methods to reduce measurement errors (<1%), beam data processing and detector size convolution for accurate profiles. The TG-106 also provides a brief discussion on the emerging trend in Monte Carlo simulation techniques in photon and electron beam commissioning. The procedures described in this report should assist a qualified medical physicist in either measuring a complete set of beam data, or in verifying a subset of data before initial use or for periodic quality assurance measurements. By combining practical experience with theoretical discussion, this document sets a new standard for beam data commissioning.

Measurement of intrafractional prostate motion using magnetic resonance imaging

PURPOSE To quantify the three-dimensional intrafractional prostate motion over typical treatment time intervals with cine-magnetic resonance imaging (cine MRI) studies. METHODS AND MATERIALS Forty-two patients with prostate cancer were scanned supine in an alpha cradle cast using cine MRI. Twenty sequential slices were acquired in the sagittal and axial planes through the center of the prostate. Each scan took approximately 9 min. The posterior, lateral, and superior edges of the prostate were tracked on each frame relative to the initial prostate position, and the size and duration of each displacement was recorded. RESULTS The prostate displacements were (mean +/- SD): 0.2 +/- 2.9 mm, 0.0 +/- 3.4 mm, and 0.0 +/- 1.5 mm in the anterior-posterior, superior-inferior, and medial-lateral dimensions respectively. The prostate motion appeared to have been driven by peristalsis in the rectum. Large displacements of the prostate (up to 1.2 cm) moved the prostate both anteriorly and superiorly and in some cases compressed the organ. For such motions, the prostate did not stay displaced, but moved back to its original position. To account for the dosimetric consequences of the motion, we also calculated the time-averaged displacement to be approximately 1 mm. CONCLUSIONS Cine MRI can be used to measure intrafractional prostate motion. Although intrafractional prostate motions occur, their effects are negligible compared to interfractional motion and setup error. No adjustment in margin is necessary for three-dimensional conformal or intensity-modulated radiation therapy.

LATE MORBIDITY PROFILES IN PROSTATE CANCER PATIENTS TREATED TO 79 - 84 GY BY A SIMPLE FOUR-FIELD COPLANAR BEAM ARRANGEMENT

PURPOSE To describe the frequency and magnitude of late GI and GU morbidity in prostate cancer patients treated to high dose levels with a simple three-dimensional conformal technique. METHODS AND MATERIALS A total of 156 intermediate- and high-risk patients were treated between January 1, 1992 and February 28, 1999 with a simple four-field three-dimensional conformal technique to 79-84 Gy. All patients were treated with a four-field conformal technique; the prostate received 82 Gy and the seminal vesicles and periprostatic tissue 46 Gy. GI and GU toxicity was scored according to the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer Late Morbidity Grading Scale and compared using Kaplan-Meier estimates. RESULTS The late Grade 2 GI complication rate was 9% and 38% at 3 years for patients treated with and without rectal blocking, respectively (p = 0.0004). No Grade 3 late GI complications developed. The rate of Grade 2 late GU complications was 5%, 8%, and 12% at 12, 24, and 36 months, respectively. The Grade 3 late GU complication rate was 2% at 36 months. These differences were not statistically significant. CONCLUSION The treatment method described is a simple four-field conformal technique that can be easily implemented in the general radiation community. A dose of 79-84 Gy can be safely delivered to the prostate, with a 9% rate of late Grade 2 GI, 12% rate of late Grade 2 GU, and 2% rate of late Grade 3 GU complications.

Characteristics and quality assurance of a dedicated open 0.23 T MRI for radiation therapy simulation.

A commercially available open MRI unit is under routine use for radiation therapy simulation. The effects of a gradient distortion correction (GDC) program used to post process the images were assessed by comparison with the known geometry of a phantom. The GDC reduced the magnitude of the distortions at the periphery of the axial images from 12 mm to 2 mm horizontally along the central axis and distortions exceeding 20 mm were reduced to as little as 2 mm at the image periphery. Coronal and sagittal scans produced similar results. Coalescing these data into distortion as a function of radial distance, we found that for radial distances of <10 cm, the distortion after GDC was <2 mm and for radial distances up to 20 cm, the distortion was <5 mm. The dosimetric errors resulting from homogeneous dose calculations with this level of distortion of the external contour is <2%. A set of triangulation lasers has been added to establish a virtual isocenter for convenient setup and marking of patients and phantoms. Repeated measurements of geometric phantoms over several months showed variations in position between the virtual isocenter and the magnetic isocenter were constrained to <2 mm. Additionally, the interscan variations of 12 randomly selected points in space defined by a rectangular grid phantom was found to be within the intraobserver error of approximately 1 mm in the coronal, sagittal, and transverse planes. Thus, the open MRI has sufficient geometric accuracy for most radiation therapy planning and is temporally stable.

MRI simulation: effect of gradient distortions on three-dimensional prostate cancer plans

PURPOSE To quantify the dosimetric consequences of external patient contour distortions produced on low-field and high-field MRIs for external beam radiation of prostate cancer. METHODS AND MATERIALS A linearity phantom consisting of a grid filled with contrast material was scanned on a spiral CT, a 0.23 T open MRI, and a 1.5 T closed bore system. Subsequently, 12 patients with prostate cancer were scanned on CT and the open MRI. A gradient distortion correction (GDC) program was used to postprocess the MRI images. Eight of the patients were also scanned on the 1.5 T MRI with integrated GDC correction. All data sets were fused according to their bony landmarks using a chamfer-matching algorithm. The prostate volume was contoured on an MRI image, irrespective of the apparent prostate location in those sets. Thus, the same target volume was planned and used for calculating the anterior-posterior (AP) and lateral separations. The number of monitor units required for treatment using a four-field conformal technique was compared. Because there are also setup variations in patient outer contours, two different CT scans from 20 different patients were fused, and the differences in AP and lateral separations were measured to obtain an estimate of the mean interfractional separation variation. RESULTS All AP separations measured on MRI were statistically indistinguishable from those on CT within the interfractional separation variations. The mean differences between CT and low-field MRI and CT and high-field MRI lateral separations were 1.6 cm and 0.7 cm, respectively, and were statistically significantly different from zero. However, after the GDC was applied to the low-field images, the difference became 0.4 +/- 0.4 mm (mean +/- standard deviation), which was statistically insignificant from the CT-to-CT variations. The mean variations in the lateral separations from the low-field images with GDC would result in a dosimetric difference of <1%, assuming an equally weighted four-field 18-MV technique for patient separations up to approximately 40 cm. CONCLUSIONS For patients with lateral separations <40 cm, a homogeneous calculation simulated using a 1.5 T MRI or a 0.23 T MRI with a gradient distortion correction will yield a monitor unit calculation indistinguishable from that generated using CT simulation.

Dose perturbations due to contrast medium and air in MammoSite ® treatment: An experimental and Monte Carlo study

In the management of early breast cancer, a partial breast irradiation technique called MammoSite® (Proxima Therapeutic Inc., Alpharetta, GA) has been advocated in recent years. In MammoSite, a balloon implanted at the surgical cavity during tumor excision is filled with a radio-opaque solution, and radiation is delivered via a high dose rate brachytherapy source situated at the center of the balloon. Frequently air may be introduced during placement of the balloon and/or injection of the contrast solution into the balloon. The purpose of this work is to quantify as well as to understand dose perturbations due to the presence of a high-Z contrast medium and/or an air bubble with measurements and Monte Carlo calculations. In addition, the measured dose distribution is compared with that obtained from a commercial treatment planning system (Nucletron PLATO system). For a balloon diameter of 42 mm, the dose variation as a function of distance from the balloon surface is measured for various concentrations of a radio-opaque solution (in the range 5%-25% by volume) with a small volume parallel plate ion chamber and a micro-diode detector placed perpendicular to the balloon axis. Monte Carlo simulations are performed to provide a basic understanding of the interaction mechanism and the magnitude of dose perturbation at the interface near balloon surface. Our results show that the radio-opaque concentration produces dose perturbation up to 6%. The dose perturbation occurs mostly within the distances <1mm from the balloon surface. The Plato system that does not include heterogeneity correction may be sufficient for dose planning at distances ⩾10mm from the balloon surface for the iodine concentrations used in the MammoSite procedures. The dose enhancement effect near the balloon surface (<1mm) due to the higher iodine concentration is not correctly predicted by the Plato system. The dose near the balloon surface may be increased by 0.5%percm3 of air. Monte Carlo simulation suggests that the interface effect (enhanced dose near surface) is primarily due to Compton electrons of short range (<0.5mm). For more accurate dosimetry in MammoSite delivery, the dose perturbation due to the presence of a radio-opaque contrast medium and air bubbles should be considered in a brachytherapy planning system.

RELATIONSHIP BETWEEN PROSTATE VOLUME, PROSTATE-SPECIFIC ANTIGEN NADIR, AND BIOCHEMICAL CONTROL

PURPOSE In patients treated with definitive three-dimensional conformal radiotherapy (3D-CRT) for localized prostatic adenocarcinoma, we sought to evaluate the relationship between pretreatment prostate gland volume and posttreatment prostate-specific antigen (PSA) nadir, as well as the relationship of prostate volume and PSA nadir with biochemical control (bNED). Two subgroups were studied: favorable (PSA <10 ng/mL, Gleason score 2-6, and T1-T2A) and unfavorable (one or more: PSA >/=10 ng/mL, Gleason score 7-10, T2B-T3). MATERIALS AND METHODS A total of 655 men (n = 271 favorable and 384 unfavorable) were treated with 3D-CRT alone between May 1989 and November 1997. All patients had information on prostate volume and a minimum follow-up of 24 months (median 56, range 24-126). Of the 655 men, 481 (n = 230 favorable and 251 unfavorable) remained bNED at time of analysis, with biochemical failure defined in accordance with the American Society for Therapeutic Radiology and Oncology consensus definition. Factors analyzed for predictors of bNED included pretreatment prostate volume, posttreatment PSA nadir, pretreatment PSA, palpation T stage, Gleason score, center of the prostate dose, and perineural invasion (PNI). We also analyzed pretreatment prostate volume and its correlation to prognostic factors. For bNED patients, the relationship between PSA nadir and prostate volume was evaluated. RESULTS On multivariate analysis, prostate volume (p = 0.04) and palpation T stage (p = 0.02) were the only predictors of biochemical failure in the favorable group. On multivariate analysis of the unfavorable group, pretreatment PSA (p <0.0001), Gleason score (p = 0.02), palpation T stage (p = 0.009), and radiation dose (p <0.0001) correlated with biochemical failure, and prostate volume and PNI did not. For all 481 bNED patients, a positive correlation between pretreatment volume and PSA nadir was demonstrated (p <0.0001). Subgroup analysis of the favorable and unfavorable patients also demonstrated a positive correlation between prostate volume and PSA nadir (p = 0.003 and p = 0.0002, respectively). Using multiple regression analysis, the following were found to be predictive of PSA nadir in all bNED patients: prostate volume (p <0.0001), pretreatment PSA (p <0.0001), palpation T stage (p = 0.0002), and radiation dose (p = 0.0034). Gleason score and PNI were not predictive. For the favorable group, palpation T stage (p = 0.0006), pretreatment PSA (p = 0.0083), prostate volume (p = 0.0186), and Gleason score (p = 0.0592) were predictive of PSA nadir, and PNI and radiation dose were not predictive. In the unfavorable group, prostate volume (p = 0.0024), radiation dose (p = 0.0039), pretreatment PSA (p = 0.0182), and palpation T stage (p = 0.0296) were predictive of PSA nadir, and Gleason score and PNI were not predictive. CONCLUSION This report is the first demonstration that prostate volume is predictive of PSA nadir for patients who are bNED in both favorable and unfavorable subgroups. PSA nadir did not correlate with bNED status in the favorable patients, but it was strongly predictive in the unfavorable patients. Prostate gland volume was also predictive of bNED failure in the favorable but not the unfavorable group.

Transmission and dose perturbations with high-Z materials in clinical electron beams

High density and atomic number (Z) materials used in various prostheses, eye shielding, and beam modifiers produce dose enhancements on the backscatter side in electron beams and is well documented. However, on the transmission side the dose perturbation is given very little clinical importance, which is investigated in this study. A simple and accurate method for dose perturbation at metallic interfaces with soft tissues and transmission through these materials is required for all clinical electron beams. Measurements were taken with thin-window parallel plate ion chambers for various high-Z materials (Al, Ti, Cu, and Pb) on a Varian and a Siemens accelerator in the energy range of 5-20 MeV. The dose enhancement on both sides of the metallic sheet is due to increased electron fluence that is dependent on the beam energy and Z. On the transmission side, the magnitude of dose enhancement depends on the thickness of the high-Z material. With increasing thickness, dose perturbation reduces to the electron transmission. The thickness of material to reduce 100% (range of dose perturbation), 50% and 10% transmission is linear with the beam energy. The slope (mm/MeV) of the transmission curve varies exponentially with Z. A nonlinear regression expression (t=E[alpha+beta exp(-0.1Z)]) is derived to calculate the thickness at a given transmission, namely 100%, 50%, and 10% for electron energy, E, which is simple, accurate and well suited for a quick estimation in clinical use. Caution should be given to clinicians for the selection of thickness of high-Z materials when used to shield critical structures as small thickness increases dose significantly at interfaces.

Impact of target volume coverage with Radiation Therapy Oncology Group (RTOG) 98-05 guidelines for transrectal ultrasound guided permanent Iodine-125 prostate implants

PURPOSE Despite the wide use of permanent prostate implants for the treatment of early stage prostate cancer, there is no consensus for optimal pre-implant planning guidelines that results in maximal post-implant target coverage. The purpose of this study was to compare post-implant target volume coverage and dosimetry between patients treated before and after Radiation Therapy Oncology Group (RTOG) 98-05 guidelines were adopted using several dosimetric endpoints. MATERIALS AND METHODS Ten consecutively treated patients before the adoption of the RTOG 98-05 planning guidelines were compared with ten consecutively treated patients after implementation of the guidelines. Pre-implant planning for patients treated pre-RTOG was based on the clinical target volume (CTV) defined by the pre-implant TRUS definition of the prostate. The CTV was expanded in each dimension according to RTOG 98-05 and defined as the planning target volume. The evaluation target volume was defined as the post-implant computed tomography definition of the prostate based on RTOG 98-05 protocol recommendations. Implant quality indicators included V(100), V(90), V(100), and Coverage Index (CI). RESULTS The pre-RTOG median V(100), V(90), D(90), and CI values were 82.8, 88.9%, 126.5 Gy, and 17.1, respectively. The median post-RTOG V(100), V(90), D(90), and CI values were 96.0, 97.8%, 169.2 Gy, and 4.0, respectively. These differences were all statistically significant. CONCLUSIONS Implementation of the RTOG 98-05 implant planning guidelines has increased coverage of the prostate by the prescription isodose lines compared with our previous technique, as indicated by post-implant dosimetry indices such as V(100), V(90), D(90). The CI was also improved significantly with the protocol guidelines. Our data confirms the validity of the RTOG 98-05 implant guidelines for pre-implant planning as it relates to enlargement of the CTV to ensure adequate margin between the CTV and the prescription isodose lines.

Primary small cell carcinoma of the vagina.

BACKGROUND Primary vaginal small cell carcinoma is extremely rare, with a total number reported in English-language journals to date of 23. Most patients die of the disease within 2 years of diagnosis from metastatic disease. CASE A 69-year-old woman presented with vaginal spotting while on Premarin. She was subsequently diagnosed with Stage I (T1N0M0) small cell carcinoma of the vagina. She underwent concurrent chemoradiation and then brachytherapy for persistent disease. Due to residual disease after the brachytherapy, surgical resection was planned but aborted because of metastatic disease. CONCLUSIONS Of the three reported cases treated with concurrent chemoradiation, ours is the first case reported with persistent local disease after therapy. Extrapolating from the available clinical trials from lung carcinoma, concurrent chemoradiation as a primary treatment approach should still be considered.