Research Detectors (United States)

Spot size σ (in air at isocenter), interspot spacing d, and spot charge q influence dose delivery efficiency and plan quality in Intensity Modulated Proton Therapy (IMPT) treatment planning. The choice and range of parameters varies among different manufacturers. The goal of this work is to demonstrate the influence of the spot parameters on dose quality and delivery in IMPT treatment plans, to show their interdependence, and to make practitioners aware of the spot parameter values for a certain facility. Our study could help as a guideline to make the trade-off between treatment quality and time in existing PBS centers and in future systems. We created plans for seven patients and a phantom, with different tumor sites and volumes, and compared the effect of small-, medium-, and large-spot widths (σ = 2.5, 5, and 10 mm) and interspot distances (1σ, 1.5σ, and 1.75σ) on dose, spot charge, and treatment time. Moreover, we quantified how postplanning charge threshold cuts affect plan quality and the total number of spots to deliver, for different spot widths and interspot distances. We show the effect of a minimum charge (or MU) cutoff value for a given proton delivery system. Spot size had a strong influence on dose: larger spots resulted in more protons delivered outside the target region. We observed dose differences of 2-13 Gy (RBE) between 2.5 mm and 10 mm spots, where the amount of extra dose was due to dose penumbra around the target region. Interspot distance had little influence on dose quality for our patient group. Both parameters strongly influence spot charge in the plans and thus the possible impact of postplanning charge threshold cuts. If such charge thresholds are not included in the treatment planning system (TPS), it is important that the practitioner validates that a given combination of lower charge threshold, interspot spacing, and spot size does not result in a plan degradation. Low average spot charge occurs for small spots, small interspot distances, many beam directions, and low fractional dose values. The choice of spot parameters values is a trade-off between accelerator and beam line design, plan quality, and treatment efficiency. We recommend the use of small spot sizes for better organ-at-risk sparing and lateral interspot distances of 1.5σ to avoid long treatment times. We note that plan quality is influenced by the charge cutoff. Our results show that the charge cutoff can be sufficiently large (i.e., 106 protons) to accommodate limitations on beam delivery systems. It is, therefore, not necessary per se to include the charge cutoff in the treatment planning optimization such that Pareto navigation (e.g., as practiced at our institution) is not excluded and optimal plans can be obtained without, perhaps, a bias from the charge cutoff. We recommend that the impact of a minimum charge cut impact is carefully verified for the spot sizes and spot distances applied or that it is accommodated in the TPS.

Recent developments in CMOS devices have improved their radiation hardness, response linearity, readout noise and thermal noise, making them suitable for x-ray crystallography detectors. Large (14.8 x 9.4 cm) CMOS sensors with a pixel size of 100 x100 microns are now available that can be butted together on three sides. We have fabricated a 6-tile system in a 2x3 array with a 28.2 x 29.5 cm continuous imaging area. To make an x-ray detector the CMOS sensor is covered with a 3 mm flat fibre-optic plate (for radiation protection) and a Gd2O2S:Tb scintillator screen. A special feature of these systems is that they can be read out continuously at 10 frames/sec with excellent dynamic range without interrupting data collection.We have installed this system at beamline 4.4.2 of the Advanced Light Source synchrotron. Anomalous diffraction data were recorded without an x-ray shutter, rotating the crystal sample continuously with an exposure time of 0.1 sec/frame and a rotation speed of 1°/sec for 180 degrees. The 1,800 frame datasets were processed in D*TREK and XDS data analysis programs and experimental phases were determined in PHENIX. The crystallographic results are typically significantly better than equivalent data recorded on a conventional CCD system, due to the 10X finer angular resolution of the recorded data. Very large systems can now be made that would have an active area of 56 x 59 cm2 with 33 x 106 pixels.