Treatment planning with a 2.5 MV photon beam for radiation therapy.

PURPOSE: The shallow depth of maximum dose and higher dose fall-off gradient of a 2.5 MV beam along the central axis that is available for imaging on linear accelerators is investigated for treatment of shallow tumors and sparing the organs at risk (OARs) beyond it. In addition, the 2.5 MV beam has an energy bridging the gap between kilo-voltage (kV) and mega-voltage (MV) beams for applications of dose enhancement with high atomic number (Z) nanoparticles. METHODS: We have commissioned and utilized a MATLAB-based, open-source treatment planning software (TPS), matRad, for intensity-modulated radiation therapy (IMRT) dose calculations. Treatment plans for prostate, liver, and head and neck (H&N), nasal cavity, two orbit cases, and glioblastoma multiforme (GBM) were performed and compared to a conventional 6 MV beam. Additional Monte Carlo calculations were also used for benchmarking the central axis dose. RESULTS: Both beams had similar planning target volume (PTV) dose coverage for all cases. However, the 2.5 MV beam deposited 6%-19% less integral doses to the nasal cavity, orbit, and GBM cases than 6 MV photons. The mean dose to the heart in the liver plan was 10.5% lower for 2.5 MV beam. The difference between the doses to OARs of H&N for two beams was under 3%. Brain mean dose, brainstem, and optic chiasm max doses were, respectively, 7.5%-14.9%, 2.2%-8.1%, and 2.5%-19.0% lower for the 2.5 MV beam in the nasal cavity, orbit, and GBM plans. CONCLUSIONS: This study demonstrates that the 2.5 MV beam can produce clinically relevant treatment plans, motivating future efforts for design of single-energy LINACs. Such a machine will be capable of producing beams at this energy beneficial for low- and middle-income countries, and investigations on dose enhancement from high-Z nanoparticles.

IPEM code of practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service.

The 1990 code of practice (COP), produced by the IPSM (now the Institute of Physics and Engineering in Medicine, IPEM) and the UK National Physical Laboratory (NPL), gave instructions for determining absorbed dose to water for megavoltage photon (MV) radiotherapy beams (Lillicrap et al 1990). The simplicity and clarity of the 1990 COP led to widespread uptake and high levels of consistency in external dosimetry audits. An addendum was published in 2014 to include the non-conventional conditions in Tomotherapy units. However, the 1990 COP lacked detailed recommendations for calibration conditions, and the corresponding nomenclature, to account for modern treatment units with different reference fields, including small fields as described in IAEA TRS483 (International Atomic Energy Agency (IAEA) 2017, Vienna). This updated COP recommends the irradiation geometries, the choice of ionisation chambers, appropriate correction factors and the derivation of absorbed dose to water calibration coefficients, for carrying out reference dosimetry measurements on MV external beam radiotherapy machines. It also includes worked examples of application to different conditions. The strengths of the 1990 COP are retained: recommending the NPL2611 chamber type as secondary standard; the use of tissue phantom ratio (TPR) as the beam quality specifier; and NPL-provided direct calibration coefficients for the user's chamber in a range of beam qualities similar to those in clinical use. In addition, the formalism is now extended to units that cannot achieve the standard reference field size of 10 cm × 10 cm, and recommendations are given for measuring dose in non-reference conditions. This COP is designed around the service that NPL provides and thus it does not require the range of different options presented in TRS483, such as generic correction factors for beam quality. This approach results in a significantly simpler, more concise and easier to follow protocol.

Octavius 4D characterization for flattened and flattening filter free rotational deliveries.

PURPOSE: In this study the Octavius detector 729 ionization chamber (IC) array with the Octavius 4D phantom was characterized for flattening filter (FF) and flattening filter free (FFF) static and rotational beams. The device was assessed for verification with FF and FFF RapidArc treatment plans. METHODS: The response of the detectors to field size, dose linearity, and dose rate were assessed for 6 MV FF beams and also 6 and 10 MV FFF beams. Dosimetric and mechanical accuracy of the detector array within the Octavius 4D rotational phantom was evaluated against measurements made using semiflex and pinpoint ionization chambers, and radiochromic film. Verification FF and FFF RapidArc plans were assessed using a gamma function with 3%∕3 mm tolerances and 2%∕2 mm tolerances and further analysis of these plans was undertaken using film and a second detector array with higher spatial resolution. RESULTS: A warm-up dose of >6 Gy was required for detector stability. Dose-rate measurements were stable across a range from 0.26 to 15 Gy∕min and dose response was linear, although the device overestimated small doses compared with pinpoint ionization chamber measurements. Output factors agreed with ionization chamber measurements to within 0.6% for square fields of side between 3 and 25 cm and within 1.2% for 2 × 2 cm(2) fields. The Octavius 4D phantom was found to be consistent with measurements made with radiochromic film, where the gantry angle was found to be within 0.4° of that expected during rotational deliveries. RapidArc FF and FFF beams were found to have an accuracy of >97.9% and >90% of pixels passing 3%∕3 mm and 2%∕2 mm, respectively. Detector spatial resolution was observed to be a factor in determining the accurate delivery of each plan, particularly at steep dose gradients. This was confirmed using data from a second detector array with higher spatial resolution and with radiochromic film. CONCLUSIONS: The Octavius 4D phantom with associated Octavius detector 729 ionization chamber array is a dosimetrically and mechanically stable device for pretreatment verification of FF and FFF RapidArc treatments. Further improvements may be possible through use of a detector array with higher spatial resolution (detector size and∕or detector spacing).

Mechanical characterization of the varian Exact-arm and R-arm support systems for eight aS500 electronic portal imaging devices.

PURPOSE: The aim of this study is to compare the positioning accuracy at different gantry angles of two electronic portal imaging devices (EPIDs) support arm systems by using EPID difference images as a measure for displacement. This work presents a comparison of the mechanical performance of eight Varian aS500 (Varian Medical Systems, Palo Alto, CA) EPIDs, mounted using either the Varian Exact-arm or R-arm. METHODS: The mechanical performance of the two arm systems was compared by investigating the variation in sensitivity with gantry angle, both before and after the EPID position was adjusted after gantry rotation. Positional errors were investigated by subtracting images from a reference image taken at gantry 0 degrees, and the amplitude of the peaks and troughs at the field edges for longitudinal (radial) and lateral (transverse) profiles across the resulting image was related to the distance of displacement. Calibration curves based on a pixel-by-pixel shift were generated for each EPID and the Varian hand pendant accuracy was compared to the calibration data. RESULTS: The response of the EPIDs was found to change with gantry rotation, with the largest difference at 180 degrees. The Exact-arm was found to correct well for any displacement, while the R-arm tended to overcorrect following repositioning using the hand pendant. The calibration curves were consistent within each set of matched linacs, and the hand pendant accuracy was similar for both arm systems, although generally in different directions. With respect to gantry rotation effects, the mechanical performance of the Exact-arm systems was found to be much better than that of the R-arm systems. At gantry positions 90 degrees, 270 degrees, and 180 degrees the average misalignment in the longitudinal direction was +4.2 +/- 0.2, +1.8 +/- 1.6, and +7.4 +/- 0.5 mm for the R-arms, and +2.9 +/- 0.2, +2.1 +/- 0.8, and +4.9 +/- 0.7 mm for the Exact-arms. In the lateral direction the average positional errors were +2.1 +/- 0.4, -4.7 +/- 0.4, and -2.5 +/- 0.5 mm for the R-arms, and -0.3 +/- 0.3, -0.5 +/- 0.3, and -0.4 +/- 0.2 mm for the Exact-arms. The hand pendant correction had minimal impact in the lateral direction for both arm systems. However in the longitudinal direction the mean errors for the R-arms were +3.4 +/- 0.7, +1.5 +/- 0.6, and +4.6 +/- 0.7 mm at gantry angles 90 degrees, 270 degrees, and 180 degrees, and the equivalent Exact-arm errors were +0.9 +/- 0.3, +1.2 +/- 0.3, and +1.9 +/- 0.9 mm, respectively. CONCLUSIONS: The performance of the EPIDs demonstrate that the Exact-arm system provides a more reproducible position and better agreement with the EPID position as indicated on the EPID pendant at all gantry angles than the R-arm.

Leaflet arrest in St Jude Medical and CarboMedics valves: an experimental study.

OBJECTIVE: Two patients who suffered acute failure of their St Jude Medical Masters aortic valve prostheses due to leaflet arrest that were unrelated to suture material are presented. It was hypothesized that the valves failed because force applied to bear upon the valve annulus caused the hinge mechanism to become restricted or to arrest. METHODS: A study was designed to measure the force that would cause leaflet arrest in three sizes of St Jude Medical Masters valves and CarboMedics mechanical valves. A specially manufactured pushrod device was used to apply a variable force to the sewing cuff, low annulus level, or to the pivot guards of all the valves. RESULTS: For every valve size tested, the St Jude Medical Masters valves required significantly less force applied than did the CarboMedics valves to cause one or both leaflets to arrest (P < 0.0004). CONCLUSIONS: A force applied on the valve ring of mechanical valves can cause leaflet arrest. The force required to arrest the leaflets is within an order of magnitude of that measured in other in vivo animal studies. We conclude that force on the valve rings in patients after aortic valve surgery could cause leaflet malfunction and even arrest in some patients.