To investigate potential uncertainty and difference on planning of an HDR treatment using images from Acuity cone-beam CT and from conventional CT-simulator CT respectively. A 7-pound chicken bought from a grocery store was used as a phantom to mimic a patient for planning a tandem and ovoid HDR treatment. To start the experiment, a tandem and ovoid applicator was inserted firmly inside the chicken phantom. In order to measure the actual delivered dose and compare it with the calculated dose in the treatment plan, we placed NanoDot dosimeters at two reference points inside the phantom before inserting the tandem and ovoid applicator. Then 3-dimensional CT images were obtained by respectively scanning the phantom on the Varian Acuity Cone-Beam CT simulator in our brachytherapy suite and on the conventional CT-simulator that is generally used for external beam and brachytherapy treatment planning. Using these two sets of images with Varian's Brachy Vision treatment planning system we made two tandem and ovoid HDR treatment plans: one for Acuity CBCT based and the other for conventional CT based. A dose of 600 cGy/fraction was prescribed to the traditional defined "A" points in each plan. Source numbers, positions, and dwell times were set exactly the same for both plans. The plan based on the conventional CT images was delivered to treat the phantom. We compared the isodose distributions and the doses at "A" points in two plans as well as the doses between measured and calculated for the two reference points. No significant differences were found in terms of the doses at the "A" points (599.8 cGy to 596.6 cGy) and the isodose distributions between the two plans. The measured doses (1025 cGy, 1044 cGy) and the calculated doses (1015 cGy, 1046 cGy) at two reference points were consistent with each other within the accuracy of NanoDot dosimetry. Planning HDR gynecologic brachytherapy using images from Acuity cone-beam CT and from conventional CT and prescribing to geometrically determined prescription points -results in equivalent results. Using the Acuity cone-beam CT provides a more efficient process, reduces the chance for applicator shifting during movement of the patient, and improves patient comfort. Additional evaluations are ongoing to compare CT and cone-beam CT identification of tumor and normal tissue.

\YHET designing apparatus for the study of skill, it is often difficult to decide how complex the task should be. On the one hand, equipment like the Cambridge Cockpit (Davis, 1946) sets a task which is complex and integrated, overall measures of performance are possible, but may be difficult to interpret in detail. On the other hand, attempts to break the task and its performance into simpler elements, such as are found in the NcDougall Dotter or the Triple ’Tester, restrict the parameters of behaviour that can be studied with confidence. They produce tasks so artificial that it becomes difficult to relate the performance measured to everyday behaviour. There are, however, some daily-life skills where an integrated behaviour is not possible, but in which discrete signals for action occur at irregular-apparently random-intervals of time. The occurrence of these signals can often be predicted with more or less accuracy, but sometimes even this is not possible. Such a task occurs in cotton spinning and winding, the signals then being breakages of the cotton thread. The apparatus which forms the subject of this note was built to enable this type of behaviour to be studied. The apparatus presents a task in which signals occur with an nppyoxiunately random distribution in time. ’There is virtually no repetition of the pattern of signals, the average rate of their occurrence being under the experimenter’s control. The phlsical nature of the sigrials is not fixed, a number of different types being possible, and with some arrangements the subject can predict when they will occur. A recent mathematical discussion by Cox and Smith (1953) has provided the basis for the design. ’The point made in this paper is that, if a number of sources each emit a periodic signal, the combined effect is a series of signals with an approximatel], exponential distribution of the time intervals between them. The limiting factors are that the periods must be slightly different and prime with respect to each other, not too different, and there must be a sufficient number of them. ’tl’hatever the number and length of the periods, the distribution of the time intervals between signals can be calculated, and the divergence from the exponential measured. An apparatus using this principle consists of 16 small dials mounted in a 4 x 4 fashion. Each dial carries a pointer driven through a gear link by a single variable speed motor. This gear link is in each case such that every pointer revolves a t a slightlgr different speed. Behind each dial face and mounted on the pointer spindle, is a cam which actuates a relay once every revolution. Subsequent events, electrical or mechanical, will depend on the type of signal required. For example, the closing of the relay might stop the pointer at an indicated mark until the subject makes a correct response, the time of both events being recorded. Or by responding correctly before the pointer reached the indicated mark, the subject might prevent the pointer from stopping. Again the relevant time relations can be recorded easily. In both cases the beliaviour of the subject would actia2ely affect the nature of the display. Alternatively, closing the cam-actuated relay might leave the pointer unaffected, so that it continued its revolutions, but would