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.

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.

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Objectives To our knowledge this is the largest report analyzing outcomes for reirradiation (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.

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.

Purpose: Low temporal latency between a gating on/off signal and a linac beam on/off during respiratory gating is critical for patient safety. Although, a measurement of temporal lag is recommended by AAPM Task Group 142 for commissioning and annual quality assurance, there currently exists no published method. Here we describe a simple, inexpensive, and reliable method to precisely measure gating lag at millisecond resolutions. Methods: A Varian Real‐time Position Management™ (RPM) gating simulator with rotating disk was modified with a resistive flex sensor (Spectra Symbol) attached to the gating box platform. A photon diode was placed at machine isocenter. Output signals of the flex sensor and diode were monitored with a multichannel oscilloscope (Tektronix™ DPO3014). Qualitative inspection of the gating window/beam on synchronicity were made by setting the linac to beam on/off at end‐expiration, and the oscilloscope's temporal window to 100 ms to visually examine if the on/off timing was within the recommended 100‐ms tolerance. Quantitative measurements were made by saving the signal traces and analyzing in MatLab™. The on and off of the beam signal were located and compared to the expected gating window (e.g. 40% to 60%). Four gating cycles were measured and compared. Results: On a Varian TrueBeam™ STx linac with RPM gating software, the average difference in synchronicity at beam on and off for four cycles was 14 ms (3 to 30 ms) and 11 ms (2 to 32 ms), respectively. For a Varian Clinac™ 21EX the average difference at beam on and off was 127 ms (122 to 133 ms) and 46 ms (42 to 49 ms), respectively. The uncertainty in the synchrony difference was estimated at ±6 ms. Conclusion: This new gating QA method is easy to implement and allows for fast qualitative inspection and quantitative measurements for commissioning and TG‐142 annual QA measurements.

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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.