Medical Physics Residency Consortium: collaborative endeavors to meet the ABR 2014 certification requirements.

In 2009, Mary Bird Perkins Cancer Center (MBPCC) established a Radiation Oncology Physics Residency Program to provide opportunities for medical physics residency training to MS and PhD graduates of the CAMPEP-accredited Louisiana State University (LSU)-MBPCC Medical Physics Graduate Program. The LSU-MBPCC Program graduates approximately six students yearly, which equates to a need for up to twelve residency positions in a two-year program. To address this need for residency positions, MBPCC has expanded its Program by developing a Consortium consisting of partnerships with medical physics groups located at other nearby clinical institutions. The consortium model offers the residents exposure to a broader range of procedures, technology, and faculty than available at the individual institutions. The Consortium institutions have shown a great deal of support from their medical physics groups and administrations in developing these partnerships. Details of these partnerships are specified within affiliation agreements between MBPCC and each participating institution. All partner sites began resident training in 2011. The Consortium is a network of for-profit, nonprofit, academic, community, and private entities. We feel that these types of collaborative endeavors will be required nationally to reach the number of residency positions needed to meet the 2014 ABR certification requirements and to maintain graduate medical physics training programs.

Comparison of action levels for patient-specific quality assurance of intensity modulated radiation therapy and volumetric modulated arc therapy treatments.

PURPOSE: To perform a comprehensive and systematic comparison of fixed-beam IMRT and volumetric modulated arc therapy (VMAT) patient-specific QA measurements for a common set of geometries using typical measurement methods. METHODS: Fixed-beam IMRT and VMAT plans were constructed for structure set geometries provided by AAPM Task Group 119. The plans were repeatedly delivered across multiple measurement sessions, and the resulting dose distributions were measured with (1) radiochromic film and ionization chamber and (2) a commercial two-dimensional diode array. The resulting QA measurements from each delivery technique were then analyzed, compared, and tested for statistically significant differences. RESULTS: Although differences were noted between QA results for some plans, neither modality showed consistently better agreement of measured and planned doses: of the 22 comparisons, IMRT showed better QA results in 11 cases, and VMAT showed better QA results in 11 cases. No statistically significant differences (p < 0.05) between IMRT and VMAT QA results were found for point doses measured with an ionization chamber, planar doses measured with radiochromic film, or planar doses measured with a two-dimensional diode array. CONCLUSIONS: These results suggest that it is appropriate to apply patient-specific QA action levels derived from fixed-beam IMRT to VMAT.

Potential of discrete Gaussian edge feathering method for improving abutment dosimetry in eMLC-delivered segmented-field electron conformal therapy.

PURPOSE: The purpose of this work was to investigate the potential of discrete Gaussian edge feathering of the higher energy electron fields for improving abutment dosimetry in the planning volume when using an electron multileaf collimator (eMLC) to deliver segmented-field electron conformal therapy (ECT). METHODS: A discrete (five-step) Gaussian edge spread function was used to match dose penumbras of differing beam energies (6-20 MeV) at a specified depth in a water phantom. Software was developed to define the leaf eMLC positions of an eMLC that most closely fit each electron field shape. The effect of 1D edge feathering of the higher energy field on dose homogeneity was computed and measured for segmented-field ECT treatment plans for three 2D PTVs in a water phantom, i.e., depth from the water surface to the distal PTV surface varied as a function of the x-axis (parallel to leaf motion) and remained constant along the y-axis (perpendicular to leaf motion). Additionally, the effect of 2D edge feathering was computed and measured for one radially symmetric, 3D PTV in a water phantom, i.e., depth from the water surface to the distal PTV surface varied as a function of both axes. For the 3D PTV, the feathering scheme was evaluated for 0.1-1.0-cm leaf widths. Dose calculations were performed using the pencil beam dose algorithm in the Pinnacle(3) treatment planning system. Dose verification measurements were made using a prototype eMLC (1-cm leaf width). RESULTS: 1D discrete Gaussian edge feathering reduced the standard deviation of dose in the 2D PTVs by 34, 34, and 39%. In the 3D PTV, the broad leaf width (1 cm) of the eMLC hindered the 2D application of the feathering solution to the 3D PTV, and the standard deviation of dose increased by 10%. However, 2D discrete Gaussian edge feathering with simulated eMLC leaf widths of 0.1-0.5 cm reduced the standard deviation of dose in the 3D PTV by 33-28%, respectively. CONCLUSIONS: A five-step discrete Gaussian edge spread function applied in 2D improves the abutment dosimetry but requires an eMLC leaf resolution better than 1 cm.

Target localization accuracy in a respiratory phantom using BrainLAB ExacTrac and 4DCT imaging.

This study evaluated the accuracy of measuring the motion of an internal target using four-dimensional computed tomography (4DCT) scanning and the BrainLAB ExacTrac X-ray imaging system. Displacements of a metal coil implanted in a commercial respiratory phantom were measured in each system and compared to the known motion. A commercial respiratory motion phantom containing a metal coil as a surrogate target was used. Phantom longitudinal motions were sinusoidal with a 4.0 second period and amplitudes ranging from 5-25 mm. We acquired 4DCT and ExacTrac images of the coil at specified respiratory phases and recorded the coordinates of the coil ends. Coil displacement relative to the 0% phase (full-inhale) position were computed for the ExacTrac and 4DCT imaging systems. Coil displacements were compared to known displacements based on the phantom's sinusoidal motion. Coil length distortion due to 4DCT phase binning was compared to the known physical length of the coil (31 mm). The maximum localization error for both coil endpoints for all motion settings was 3.5 mm for the 4DCT and 0.8 mm for the ExacTrac gating system. Coil length errors measured on the 4DCT were less than 0.8 mm at end inhale/exhale phases, but up to 8.3 mm at mid-inhalation phases at the largest motion amplitude (25 mm). Due to the fast image acquisition time (100 ms), no coil distortion was observable in the ExacTrac system. 4DCT showed problems imaging the coil during mid-respiratory phases of higher velocity (phases 20%-30% and 70%-80%) due to distortion caused by residual motion within the 4DCT phase bin. The ExacTrac imaging system was able to accurately localize the coil in the respiratory phantom over all phases of respiration. For our clinic, where end-respiration phases from 4DCT may be used for treatment planning calculations, the ExacTrac system is used to measure internal target motion. With the ExacTrac system, planning target size and motion uncertainties are minimized, potentially reducing internal target volume margins in gated radiotherapy.

Evaluation of a commercial flatbed document scanner and radiographic film scanner for radiochromic EBT film dosimetry.

The purpose of this study was to quantify the performance and assess the utility of two different types of scanners for radiochromic EBT film dosimetry: a commercial flatbed document scanner and a widely used radiographic film scanner. We evaluated the Epson Perfection V700 Photo flatbed scanner and the Vidar VXR Dosimetry Pro Advantage scanner as measurement devices for radiochromic EBT film. Measurements were made of scan orientation effects, response uniformity, and scanner noise. Scanners were tested using films irradiated with eight separate 3x3cm2 fields to doses ranging from 0.115-5.119 Gy. ImageJ and RIT software was used for analyzing the Epson and Vidar scans, respectively. For repeated scans of a single film, the measurements in each dose region were reproducible to within +/- 0.3% standard deviation (SD) with both scanners. Film-to-film variations for corresponding doses were measured to be within +/- 0.4% SD for both Epson scanner and Vidar scanners. Overall, the Epson scanner showed a 10% smaller range of pixel value compared to the Vidar scanner. Scanner noise was small: +/- 0.3% SD for the Epson and +/- 0.2% for the Vidar. Overall measurement uniformity for blank film in both systems was better than +/- 2%, provided that the leading and trailing 2 cm film edges were neglected in the Vidar system. In this region artifacts are attributed to the film rollers. Neither system demonstrated a clear measurement advantage. The Epson scanner is a relatively inexpensive method for analyzing radiochromic film, but there is a lack of commercially available software. For a clinic already using a Vidar scanner, applying it to radiochromic film is attractive because commercial software is available. However, care must be taken to avoid using the leading and trailing film edges.

Dosimetric predictors of diarrhea during radiotherapy for prostate cancer.

PURPOSE: To investigate dosimetric predictors of diarrhea during radiotherapy (RT) for prostate cancer. PATIENTS AND METHODS: All patients who underwent external-beam radiotherapy as part of treatment for localized prostate cancer at the University of Texas Medical Branch, Galveston, TX, USA, from May 2002 to November 2006 were extracted from the own database. From the cumulative dose-volume histogram (DVH), the absolute volumes (V-value) of intestinal cavity (IC) receiving 15, 30, and 45 Gy were extracted for each patient. Acute gastrointestinal toxicity was prospectively scored at each weekly treatment visit according to CTC (Common Toxicity Criteria) v2.0. The endpoint was the development of peak grade >or= 2 diarrhea during RT. Various patient, tumor, and treatment characteristics were evaluated using logistic regression. RESULTS: 149 patients were included in the analysis, 112 (75.2%) treated with whole-pelvis intensity-modulated radiotherapy (WP-IMRT) and 37 (24.8%) with prostate-only RT, including or not including, the seminal vesicles (PORT +/- SV). 45 patients (30.2%) developed peak grade >or= 2 diarrhea during treatment. At univariate analysis, IC-V(15) and IC-V(30), but not IC-V(45), were correlated to the endpoint; at multivariate analysis, only IC-V(15) (p = 0.047) along with peak acute proctitis (p = 0.041) was independently correlated with the endpoint. CONCLUSION: These data provide a novel and prostate treatment-specific "upper limit" DVH for IC.

Comparison of three strategies to delineate the bowel for whole pelvis IMRT of prostate cancer.

PURPOSE: To compare three different contouring approaches of the bowel before and during whole pelvis IMRT of localized prostate cancer. MATERIALS: Nine patients were randomly selected among those treated for localized prostate cancer at UTMB from March 2004 to August 2006. On the planning CT, besides the usual organs at risk (OAR), for each patient we contoured the bowel according to three different definitions: each bowel segment ('BS'); 'BS+1', BS uniformly expanded by 1cm; intestinal cavity ('IC') or the 'container' of the bowel loops up to the pelvic/abdominal walls. For each patient we generated three rival plans each considering a different bowel definition, otherwise identical. Provided that the same target coverage and other OAR spare had been achieved, plans were compared for their ability to minimize bowel dose at planning. Furthermore, after co-registering 6 weekly CT to the initial planning CT for each patient, we investigated which of the three definitions would allow the best bowel protection also during treatment. RESULTS: All definitions provided a very similar average bowel DVH at planning. During treatment BS allowed an average approximately 20 cc more of bowel to receive at least 45 Gy over BS+1 and IC (p=0.008 and 0.029, respectively); on the contrary bowel V45 between IC and BS+1 were not significantly different (p=0.65). CONCLUSION: A definition that takes into account internal organ motion is warranted to maximize bowel protection during treatment.

PTV margin determination in conformal SRT of intracranial lesions.

The planning target volume (PTV) includes the clinical target volume (CTV) to be irradiated and a margin to account for uncertainties in the treatment process. Uncertainties in miniature multileaf collimator (mMLC) leaf positioning, CT scanner spatial localization, CT-MRI image fusion spatial localization, and Gill-Thomas-Cosman (GTC) relocatable head frame repositioning were quantified for the purpose of determining a minimum PTV margin that still delivers a satisfactory CTV dose. The measured uncertainties were then incorporated into a simple Monte Carlo calculation for evaluation of various margin and fraction combinations. Satisfactory CTV dosimetric criteria were selected to be a minimum CTV dose of 95% of the PTV dose and at least 95% of the CTV receiving 100% of the PTV dose. The measured uncertainties were assumed to be Gaussian distributions. Systematic errors were added linearly and random errors were added in quadrature assuming no correlation to arrive at the total combined error. The Monte Carlo simulation written for this work examined the distribution of cumulative dose volume histograms for a large patient population using various margin and fraction combinations to determine the smallest margin required to meet the established criteria. The program examined 5 and 30 fraction treatments, since those are the only fractionation schemes currently used at our institution. The fractionation schemes were evaluated using no margin, a margin of just the systematic component of the total uncertainty, and a margin of the systematic component plus one standard deviation of the total uncertainty. It was concluded that (i) a margin of the systematic error plus one standard deviation of the total uncertainty is the smallest PTV margin necessary to achieve the established CTV dose criteria, and (ii) it is necessary to determine the uncertainties introduced by the specific equipment and procedures used at each institution since the uncertainties may vary among locations.

Accuracy and precision of cone-beam computed tomography guided intensity modulated radiation therapy.

PURPOSE: To assess the accuracy and precision of cone-beam computed tomography (CBCT)-guided intensity modulated radiation therapy (IMRT). METHODS AND MATERIALS: A 7-field intensity modulated radiation therapy plan was constructed for an anthropomorphic head phantom loaded with a custom cassette containing radiochromic film. The phantom was positioned on the treatment table at 9 locations: 1 "correct" position and 8 "misaligned" positions along 3 orthogonal axes. A commercial kilovoltage cone-beam computed tomography (kV-CBCT) system (VolumeView, Elekta AB, Stockholm, Sweden) was then used to align the phantom prior to plan delivery. The treatment plan was delivered using the radiation therapy delivery system (Infinity; Elekta AB) 3 times for each of the 9 positions, allowing film measurement of the delivered dose distribution in 3 orthogonal planes. Comparison of the planned and delivered dose profiles along the major axes provided an estimate of the accuracy and precision of CBCT-guided IMRT. RESULTS: On average, targeting accuracy was found to be within 1 mm in all 3 major anatomic planes. Over all 54 measured dose profiles, the means and standard errors of the displacement of the center of the field between the measured and calculated profiles for each of the right-left, anterior-posterior, and superior-inferior axes were +0.08 ± 0.07 mm, +0.60 ± 0.08 mm, and +0.78 ± 0.16 mm, respectively. Agreement between planned and measured 80% profiles was less than 0.4 mm on either side along the right-left axis. A systematic shift of the measured profile of slightly less than 1 mm in anterior and superior directions was noted along the anterior-posterior and superior-inferior axes, respectively. CONCLUSIONS: Submillimeter targeting accuracy can be achieved using a commercial kV-CBCT IGRT system.

Weekly dose-volume parameters of mucosa and constrictor muscles predict the use of percutaneous endoscopic gastrostomy during exclusive intensity-modulated radiotherapy for oropharyngeal cancer.

PURPOSE: To define predictors of percutaneous endoscopic gastrostomy (PEG) use during intensity-modulated radiotherapy (IMRT) for oropharyngeal cancer. METHODS AND MATERIALS: Data for 59 consecutive patients treated with exclusive IMRT at a single institution were recovered. Of 59 patients, 25 were treated with hyperfractionation (78 Gy, 1.3 Gy per fraction, twice daily; "HYPER"); and 34 of 59 were treated with a once-daily fractionation schedule (66 Gy, 2.2 Gy per fraction, or 70 Gy, 2 Gy per fraction; "no-HYPER"). On the basis of symptoms during treatment, a PEG tube could have been placed as appropriate. A number of clinical/dosimetric factors, including the weekly dose-volume histogram of oral mucosa (OM DVHw) and weekly mean dose to constrictors and larynx, were considered. The OM DVHw of patients with and without PEG were compared to assess the most predictive dose-volume combinations. RESULTS: Of 59 patients, 22 needed a PEG tube during treatment (for 15 of 22, ≥3 months). The best cutoff values for OM DVHw were V9.5 Gy/week <64 cm(3) and V10 Gy/week <54 cm(3). At univariate analysis, fractionation, mean weekly dose to OM and superior and middle constrictors, and OM DVHw were strongly correlated with the risk of PEG use. In a stepwise multivariate logistic analysis, OM V9.5 Gy/week (≥64 vs. <64 cm(3)) was the most predictive parameter (odds ratio 30.8, 95% confidence interval 3.7-254.2, p = 0.0015), confirmed even in the no-HYPER subgroup (odds ratio 21, 95% CI 2.1 confidence interval 210.1, p = 0.01). CONCLUSIONS: The risk of PEG use is drastically reduced when OM V9.5-V10 Gy/week is <50-60 cm(3). These data warrant prospective validation.

Verification of calculated skin doses in postmastectomy helical tomotherapy.

PURPOSE: To verify the accuracy of calculated skin doses in helical tomotherapy for postmastectomy radiation therapy (PMRT). METHODS AND MATERIALS: In vivo thermoluminescent dosimeters (TLDs) were used to measure the skin dose at multiple points in each of 14 patients throughout the course of treatment on a TomoTherapy Hi·Art II system, for a total of 420 TLD measurements. Five patients were evaluated near the location of the mastectomy scar, whereas 9 patients were evaluated throughout the treatment volume. The measured dose at each location was compared with calculations from the treatment planning system. RESULTS: The mean difference and standard error of the mean difference between measurement and calculation for the scar measurements was -1.8% ± 0.2% (standard deviation [SD], 4.3%; range, -11.1% to 10.6%). The mean difference and standard error of the mean difference between measurement and calculation for measurements throughout the treatment volume was -3.0% ± 0.4% (SD, 4.7%; range, -18.4% to 12.6%). The mean difference and standard error of the mean difference between measurement and calculation for all measurements was -2.1% ± 0.2% (standard deviation, 4.5%: range, -18.4% to 12.6%). The mean difference between measured and calculated TLD doses was statistically significant at two standard deviations of the mean, but was not clinically significant (i.e., was <5%). However, 23% of the measured TLD doses differed from the calculated TLD doses by more than 5%. CONCLUSIONS: The mean of the measured TLD doses agreed with TomoTherapy calculated TLD doses within our clinical criterion of 5%.

Pretreatment verification of IMSRT using electronic portal imaging and Monte Carlo calculations.

The use of an amorphous silicon electronic portal imaging device (EPID) and Monte Carlo calculations were investigated for pretreatment fluence verification in intensity modulated stereotactic radiotherapy (IMSRT). Monte Carlo calculations were performed using BEAM, a general purpose Monte Carlo code to simulate radiation beams from radiotherapy units. The dose distribution to the EPID phosphor was calculated by BEAM and then converted to pixel value using a pixel calibration curve. The calibration correlated calculated pixel dose to the measured pixel value for a range of open fields. Points within the region bounded by the photon jaws were extracted for comparison. Criteria for successful verification were 5% local percent difference in high dose regions, 1 mm distance to agreement in high gradient regions, or 2% of the Monte Carlo calculated central axis pixel value in low dose regions. Software was written to quantitatively compare the measured and calculated EPID images. Successful verification of the modulated field required that >or=95% of compared points fall within the comparison criteria. Dose response of the EPID was found to be linear with Monte Carlo calculated doses over the dose ranges examined in this work Comparison of the measured and calculated EPID dose distributions showed good agreement with 97% of the points passing criteria. The sensitivity of the methodology to detect field shaping errors was tested by introducing positioning errors in segments of the modulated field. These sensitivity tests indicate that the comparison software designed for this work can detect a 1 mm positioning error in a single segment of the composite IMSRT field. It should be noted, however, that the work presented here is a proof of concept and currently not a clinically viable QA tool. It represents a limited evaluation using a single IMSRT field, and verification of additional fields will be required for a comprehensive evaluation of the described methods before broad conclusions can be drawn. Additionally, the results of this work are subject to the comparison criteria that were used. Clinical implementation of the proposed technique should be evaluated for the specific institutional criteria where it will be employed.

Acute toxicity of whole-pelvis IMRT in 87 patients with localized prostate cancer.

PURPOSE: To assess the acute toxicity profile of whole pelvis IMRT (WP-IMRT) for localized prostate cancer. MATERIALS: Eighty seven patients treated with definitive WP-IMRT at UTMB from May 2002 to November 2006 were retrospectively reviewed. Treatment consisted of two sequential phases, WP-IMRT to 54 Gy at 1.8 Gy per fraction to the pelvic nodes and seminal vesicles and 60 Gy at 2 Gy to the prostate, and a separate external beam boost, 3DCRT or IMRT, to bring the dose to the prostate to 76 Gy. Acute toxicity was prospectively scored weekly during treatment and at 3 month follow-up according to CTC v2.0 for 10 genitourinary (GU) and gastrointestinal (GI) domains. The proportion of patients experiencing a given level of peak acute toxicity at a given point is reported. RESULTS: Treatment was feasible with delivered doses to PTVs not significantly lower than planned ones and with only two patients experiencing treatment gaps longer than 5 days. About 2/3 and 1/10 of the patients experienced peak grade 2 and grade 3 reactions at least once during RT, respectively. Frequency/urgency (Grade 2+: 37.9%) and diarrhea (36.7%) were the most prevalent symptoms followed by proctitis (21.8%) and dysuria (16.1%). GI reactions were generally shorter lasting compared to GU ones which accumulated progressively during treatment. At 3 months, almost half of the patients were asymptomatic and most of observed reactions (89.2%) were mild, with GI ones more likely to be fully resolved (92.5%) than GU ones (68.7%, chi(2), p=0.001). CONCLUSION: Our approach is dosimetrically and clinically feasible with intense, but transient, acute toxicity.

Does treatment of the pelvic nodes with IMRT increase late rectal toxicity over conformal prostate-only radiotherapy to 76 Gy?

PURPOSE: To compare late rectal toxicity rates after three-dimensional conformal radiotherapy to the prostate alone (P-3D-CRT) and whole-pelvis intensity-modulated radiotherapy along with a prostate boost (WP-IMRT/PB) to the same nominal total dose to the prostate. PATIENTS AND METHODS: 68 patients treated with conformal radiotherapy to the prostate only to 76 Gy at the National Institute for Cancer Research, Genoa, Italy, represented the first group (P-3D-CRT). The second group consisted of 45 patients treated at the University of Texas Medical Branch (UTMB), Galveston, TX, USA, with IMRT covering the pelvic nodes and seminal vesicles to 54 Gy at 1.8 Gy per fraction and the prostate to 60 Gy in the same 30 fractions. A separate phase boosted the prostate to 76 Gy (WP-IMRT/PB). Major aspects of planning were remarkably similar at both institutions leaving the inclusion or not of pelvic nodes as the main treatment-related difference between the two groups. Late rectal toxicity was prospectively scored according to the RTOG scale. All patients have a 12-month minimum follow-up, and mean follow-up, similar in both groups, is 25.9 months (SD [standard deviation]: 8.4 months). RESULTS: At 2 years, the estimated cumulative incidence of grade 2 late rectal toxicity is 6%+/-4% for WP-IMRT/PB and 21.2%+/-6% for P-3D-CRT (p=0.06). The difference became significant (HR [hazard ratio]=0.1, 95% CI [confidence interval]: 0.0-0.6; p=0.01) at multivariate analysis. None of the patients developed grade 3+ toxicity. CONCLUSION: Despite the larger treated volume, WP-IMRT/PB allows more rectal sparing than P-3D-CRT.