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Purpose: Interplay between organ motion and leaf motion has been shown to generally have a small dosimetric impact for most clinical IMRT treatments. However, it has also been shown that for some MLC sequences there can be large daily variations in the delivered dose, depending on details of the patient motion or number of fractions. This study investigates guidelines for dynamic MLC sequences that will keep daily dose variations within 10%. Materials and Methods: Dose distributions for a range of MLC separations (0.2 – 5.0cm) and displacements between adjacent MLCs (0 – 1.5cm) were exported from Eclipse to purpose‐written software which simulated the dose distribution moving across a moving target. Target motion parallel and perpendicular to the MLC motion was investigated for a range of amplitudes (0.5 – 4.0cm), periods (1.5 – 10s), and MLC speeds (0.1 – 3.0 cm/s). Target motion was modeled as sin6. MLC sequences were identified which kept dose variations within 10% compared to the dose delivered with no motion. Results were confirmed experimentally by measuring the dose delivered to MOSFETs in a moving phantom for a range of MLC sequences. Results: The maximum allowable MLC speed when target motion is parallel to the MLC motion can be conservatively summarized as a simple function of target amplitude and MLC separation. When the target motion is perpendicular to MLC motion the maximum allowable MLC speed can be described as a function of MLC slit width and the displacement of adjacent MLCs. The guidelines were successfully applied to two‐dimensional motion. Rules were less restrictive for periods

Purpose The purpose of this study is to determine the effect of headscatter for IMRT fields. Methods and material In air measurements were made for two types of fields. For the first type of fields, a series of offset fields with field sizes ranging between 10 and 21.5cm were used. The offset changed between 0 and 10cm depending on the field size. The detector was always placed on the central axis (CAX). For the second type of fields, headscatter factors were measured for a series of 10×10cm2 fields composed of slits 0.3, 0.4, 0.6, 0.8 and 1.0cm in width. In‐air output ratio, Sc , for a series of clinical IMRT fields was also measured. Sc is defined as dose per MU measured in a water‐equivalent miniphantom between IMRT field and a 10×10 cm2 open field. The measurements are compared with calculation using a two‐source headscatter model1. Results Sc on CAX for the same open beam with different offset changed by up to 4% for the Siemens accelerators. For stop‐and‐shoot method, Sc for 10×10cm2 fields composed of slit fields of different widths changes with the slit width to within 8% and 6.4% for 6 and 15MV, respectively. The 8% uncertainty is completely due to delivery error and does not seem to correlate with the slit width. The two‐source model predicts Sc for all cases including IMRT fields to within 1%. Conclusion In‐air output ratio changes with field shaping by up to 4%, even when the point of measurement is within the radiation field. Thus, it is important to model the headscatter in order to predict Sc for IMRT fields. Our two‐source model can accurately predict the headscatter for points within radiation field.

A scanning multiple slit assembly (SMSA) has been constructed for the purpose of reducing scatter in medical radiography. The SMSA consists of a series of long, narrow beam-defining slits above the patient that are aligned and synchronously moved with scatter-eliminating slots beneath the patient during an exposure. Evidence, based on measurements of the ratio of scattered-to-primary radiation imaged and radiographs of patients, is presented indicating that such a device is a practical and efficient method of reducing scatter and improving contrast compared to conventional grids. The design considerations and trade-offs associated with the choice of slit width, slit separation distance, and aft slot depth are discussed along with the effect of these parameters on the SMSA's performance. The various problems encountered in obtaining a uniform scan and the manner in which they were handled are also discussed.

Inherent in the line-spread function approach in modulation transfer function (MTF) determinations is the use of a microdensitometer to scan the density pattern resulting from the line exposure. In such a procedure, loss of fidelity in the distribution being scanned is expected. This loss is due in part to the use of a finite-width scanning slit. In spite of the central role of the scanning slits in the MTF determination, little if anything has appeared in the literature from which one can quantitate the effect of a scanning slit on the MTF determination. This paper analyzes the loss of response attendant with use of scanning slits of differing widths. Specifically, the MTF of a finite scanning slit is derived. A method for correcting for loss of response, given a specific slit width, is indicated on the basis of the chain-multiplication property of MTFs. The current practice of using a 10-mum width for film-screen combinations is shown to be justified. Results can be generalized to any configuration scan slit. In addition, it may also be applied to any generalized sampling or display-type slit.

To determine the modulation transfer function (MTF) of a radiographic film or film-screen system, it is common to expose the system to a line source and scan the result with a microdensitometer. The question arises as to how the MTF determination is affected by such factors as microdensitometer slit width and possible misalignment of the slit with the line pattern being scanned. In any experiment, some degree of misalignment can be expected. This paper analyzes the resulting problem and derives an expression for the MTF of slit misalignment and slit width: MTF = [sin[2pif(L/2)]/2pif(L/2)] X (sin[2pif[H sin(theta/2)[[/2pif[Hsin(theta/2)[[), where L is the slit width, H the slit height, theta the angle of misalignment, and f the spatial frequency variable. The left-hand factor of this product is the MTF of a finite slit width, while the right-hand factor is the MTF of misalignment. It is shown that, for radiographic film-screen combinations, small-angle misalignment up to 0.25 degrees is not serious with 1.5-cm-long slits, while such a misalignment may become serious in highbandwidth systems such as nonscreen films.