Physician review of image registration and normal structure delineation.

INTRODUCTION: Image registration and delineation of organs at risk (OARs) are key components of three-dimensional conformal (3DCRT) and intensity-modulated radiotherapy (IMRT) treatment planning. This study hypothesized that image registration and OAR delineation are often performed by medical physicists and/or dosimetrists and are not routinely reviewed by treating physicians. METHODS: An anonymous, internet-based survey of medical physicists and dosimetrists was distributed via the MEDPHYS and MEDDOS listserv groups. Participants were asked to characterize standard practices for completion and review of OAR contouring, target volume contouring, and image registration at their institution along with their personal training in these areas and level of comfort performing these tasks. Likert-type scales are reported as Median [Interquartile range] with scores ranging from 1 = "Extremely/All of the time" to 5 = "Not at all/Never." RESULTS: Two hundred and ninety-seven individuals responded to the survey. Overall, respondents indicated significantly less frequent physician review (3 [2-4] vs 2 [1-3]), and less confidence in the thoroughness of physician review (3 [2-4] vs 2 [1-3], P < 0.01) of OAR contours compared to image registration. Only 19% (95% CI 14-24%) of respondents reported a formal process by which OAR volumes are reviewed by physicians in their clinic. The presence of a formal review process was also associated with significantly higher perceived thoroughness of review of OAR volumes compared to clinics with no formal review process (2 [2-3] vs 3 [2-4], P < 0.01). CONCLUSION: Despite the critical role of OAR delineation and image registration in the 3DCRT and IMRT treatment planning process, physician review of these tasks is not always optimal. Radiotherapy clinics should consider implementation of formal processes to promote adequate physician review of OARs and image registrations to ensure the quality and safety of radiotherapy treatment plans.

A feasibility study of 2-mm bolus for postmastectomy radiation therapy.

PURPOSE: The purpose of this study was to prospectively evaluate the use of daily 2-mm bolus in patients undergoing postmastectomy radiation without reconstruction using optically stimulated luminescence dosimetry and weekly assessment of skin toxicity. METHODS AND MATERIALS: We prospectively collected data from the first 49 women treated with a daily 2-mm Superflab bolus during their postmastectomy radiation therapy from 2013 to 2016 at The University of Chicago Comprehensive Cancer Center at Silver Cross. Within the first 3 days of starting radiation therapy, we measured the surface dose in vivo at 5 anatomical locations under the 2-mm bolus on the chest wall. We assessed weekly the acute skin toxicity during radiation using the National Cancer Institute Common Toxicity Criteria. Patients with reconstruction before radiation therapy were excluded. RESULTS: Forty-nine women with a mean age of 54.3 years were treated with daily 2-mm bolus to the chest wall following mastectomy. Median follow-up was 32.7 weeks. The mean percentages of prescribed dose (standard deviation) for the median, central, lateral, superior, and inferior optically stimulated luminescence dosimeters were 100.1% (5.6%), 108.1% (6.7%), 98.1% (6.5%), 102.6% (8.9%), and 106.3% (6.6%), respectively. The majority (71.4%) of women experienced a maximum acute National Cancer Institute Common Toxicity Criteria skin toxicity score of 2, with only 12.2% experiencing a score of 3. There were no grade 4 toxicities. There were no local recurrences during our follow-up period. CONCLUSIONS: A daily 2-mm bolus is a feasible regimen for chest wall bolus during postmastectomy radiation therapy with acceptable dose buildup and skin toxicity.

Technical Note: High temporal resolution characterization of gating response time.

PURPOSE: Low temporal latency between a gating ON/OFF signal and the LINAC beam ON/OFF during respiratory gating is critical for patient safety. Here the authors describe a novel method to precisely measure gating lag times at high temporal resolutions. METHODS: A respiratory gating simulator with an oscillating platform was modified to include a linear potentiometer for position measurement. A photon diode was placed at linear accelerator isocenter for beam output measurement. The output signals of the potentiometer and diode were recorded simultaneously at 2500 Hz with an analog to digital converter for four different commercial respiratory gating systems. The ON and OFF of the beam signal were located and compared to the expected gating window for both phase and position based gating and the temporal lag times extracted. RESULTS: For phase based gating, a real-time position management (RPM) infrared marker tracking system with a single camera and a RPM system with a stereoscopic camera were measured to have mean gate ON/OFF lag times of 98/90 and 86/44 ms, respectively. For position based gating, an AlignRT 3D surface system and a Calypso magnetic fiducial tracking system were measured to have mean gate ON/OFF lag times of 356/529 and 209/60 ms, respectively. CONCLUSIONS: Temporal resolution of the method was high enough to allow characterization of individual gate cycles and was primary limited by the sampling speed of the data recording device. Significant variation of mean gate ON/OFF lag time was found between different gating systems. For certain gating devices, individual gating cycle lag times can vary significantly.

Monte Carlo simulation of inverse geometry x-ray fluoroscopy using a modified MC-GPU framework.

Scanning-Beam Digital X-ray (SBDX) is a technology for low-dose fluoroscopy that employs inverse geometry x-ray beam scanning. To assist with rapid modeling of inverse geometry x-ray systems, we have developed a Monte Carlo (MC) simulation tool based on the MC-GPU framework. MC-GPU version 1.3 was modified to implement a 2D array of focal spot positions on a plane, with individually adjustable x-ray outputs, each producing a narrow x-ray beam directed toward a stationary photon-counting detector array. Geometric accuracy and blurring behavior in tomosynthesis reconstructions were evaluated from simulated images of a 3D arrangement of spheres. The artifact spread function from simulation agreed with experiment to within 1.6% (rRMSD). Detected x-ray scatter fraction was simulated for two SBDX detector geometries and compared to experiments. For the current SBDX prototype (10.6 cm wide by 5.3 cm tall detector), x-ray scatter fraction measured 2.8-6.4% (18.6-31.5 cm acrylic, 100 kV), versus 2.1-4.5% in MC simulation. Experimental trends in scatter versus detector size and phantom thickness were observed in simulation. For dose evaluation, an anthropomorphic phantom was imaged using regular and regional adaptive exposure (RAE) scanning. The reduction in kerma-area-product resulting from RAE scanning was 45% in radiochromic film measurements, versus 46% in simulation. The integral kerma calculated from TLD measurement points within the phantom was 57% lower when using RAE, versus 61% lower in simulation. This MC tool may be used to estimate tomographic blur, detected scatter, and dose distributions when developing inverse geometry x-ray systems.

Reirradiation for locoregionally recurrent lung cancer: outcomes in small cell and non-small cell lung carcinoma.

OBJECTIVES: To our knowledge this is the largest report analyzing outcomes for re-irradiation (reRT) for locoregionally recurrent lung cancer, and the first to assess thoracic reRT outcomes in patients with small cell lung cancer (SCLC). METHODS: Forty-eight patients (11 SCLC, 37 non-small cell lung cancer [NSCLC]) receiving reRT to the thorax were identified; 44 (92%) received reRT by intensity-modulated radiotherapy. Palliative responses, survival outcomes, and prognostic factors were analyzed. RESULTS: NSCLC patients received a median of 30 Gy in a median of 10 fractions, whereas SCLC patients received a median of 37.5 Gy in a median of 15 fractions. Median survival for the entire cohort from reRT was 4.2 months. Median survival for NSCLC patients was 5.1 months, versus 3.1 months for the SCLC patients (P=0.15). In NSCLC patients, multivariate analysis demonstrated that Karnofsky performance status≥80 and higher radiation dose were associated with improved survival following reRT, and 75% of patients with symptoms experienced palliative benefit. In SCLC, 4 patients treated with the intent of life prolongation for radiographic recurrence had a median survival of 11.7 months. However, acute toxicities and new disease symptoms limited the duration of palliative benefit in the 7 symptomatic SCLC patients to 0.5 months. CONCLUSIONS: ReRT to the thorax for locoregionally recurrent NSCLC can provide palliative benefit, and a small subset of patients may experience long-term survival. Select SCLC patients may experience meaningful survival prolongation after reRT, but reRT for patients with symptomatic recurrence and/or extrathoracic disease did not offer meaningful survival or durable symptom benefit.

Calibration of GafChromic XR-RV3 radiochromic film for skin dose measurement using standardized x-ray spectra and a commercial flatbed scanner.

PURPOSE: In this study, newly formulated XR-RV3 GafChromic film was calibrated with National Institute of Standards and Technology (NIST) traceability for measurement of patient skin dose during fluoroscopically guided interventional procedures. METHODS: The film was calibrated free-in-air to air kerma levels between 15 and 1100 cGy using four moderately filtered x-ray beam qualities (60, 80, 100, and 120 kVp). The calibration films were scanned with a commercial flatbed document scanner. Film reflective density-to-air kerma calibration curves were constructed for each beam quality, with both the orange and white sides facing the x-ray source. A method to correct for nonuniformity in scanner response (up to 25% depending on position) was developed to enable dose measurement with large films. The response of XR-RV3 film under patient backscattering conditions was examined using on-phantom film exposures and Monte Carlo simulations. RESULTS: The response of XR-RV3 film to a given air kerma depended on kVp and film orientation. For a 200 cGy air kerma exposure with the orange side of the film facing the source, the film response increased by 20% from 60 to 120 kVp. At 500 cGy, the increase was 12%. When 500 cGy exposures were performed with the white side facing the x-ray source, the film response increased by 4.0% (60 kVp) to 9.9% (120 kVp) compared to the orange-facing orientation. On-phantom film measurements and Monte Carlo simulations show that using a NIST-traceable free-in-air calibration curve to determine air kerma in the presence of backscatter results in an error from 2% up to 8% depending on beam quality. The combined uncertainty in the air kerma measurement from the calibration curves and scanner nonuniformity correction was +/- 7.1% (95% C.I.). The film showed notable stability. Calibrations of film and scanner separated by 1 yr differed by 1.0%. CONCLUSIONS: XR-RV3 radiochromic film response to a given air kerma shows dependence on beam quality and film orientation. The presence of backscatter slightly modifies the x-ray energy spectrum; however, the increase in film response can be attributed primarily to the increase in total photon fluence at the sensitive layer. Film calibration curves created under free-in-air conditions may be used to measure dose from fluoroscopic quality x-ray beams, including patient backscatter with an error less than the uncertainty of the calibration in most cases.