Purpose: To investigate the use of a system using EM tracking, postprocessing and error‐detection algorithms for measuring brachytherapy catheter locations and for detecting errors and resolving uncertainties in treatment‐planning catheter digitization. Methods: An EM tracker was used to localize 13 catheters in a clinical surface applicator (A) and 15 catheters inserted into a phantom (B). Two pairs of catheters in (B) crossed paths at a distance <2 mm, producing an undistinguishable catheter artifact in that location. EM data was post‐processed for noise reduction and reformatted to provide the dwell location configuration. CT‐based digitization was automatically extracted from the brachytherapy plan DICOM files (CT). EM dwell digitization error was characterized in terms of the average and maximum distance between corresponding EM and CT dwells per catheter. The error detection rate (detected errors / all errors) was calculated for 3 types of errors: swap of two catheter numbers; incorrect catheter number identification superior to the closest position between two catheters (mix); and catheter‐tip shift. Results: The averages ± 1 standard deviation of the average and maximum registration error per catheter were 1.9±0.7 mm and 3.0±1.1 mm for (A) and 1.6±0.6 mm and 2.7±0.8 mm for (B). The error detection rate was 100% (A and B) for swap errors, mix errors, and shift >4.5 mm (A) and >5.5 mm (B); errors were detected for shifts on average >2.0 mm (A) and >2.4 mm (B). Both mix errors associated with undistinguishable catheter artifacts were detected and at least one of the involved catheters was identified. Conclusion: We demonstrated the use of an EM tracking system for localization of brachytherapy catheters, detection of digitization errors and resolution of undistinguishable catheter artifacts. Automatic digitization may be possible with a registration between the imaging and the EM frame of reference. Research funded by the Kaye Family Award 2012

Aims Candidates achieving low scores in the MRCPCH Clinical are deferred from re-sitting for a specified period and advised to seek further training and educational support. The exam is scored out of 120, with a pass mark of 100. Candidates scoring 71 to 80 are deferred from sitting the next exam; those scoring ≤70, from sitting the next two exams. To explore whether deferment has any positive effect, candidates' past exam scores from first attempt to re-sit were analysed. Methods Candidate performance was analysed over the period since 2004 when deferment was instituted (21 exam diets, UK and Overseas). 2156 candidates who had failed at their first attempt were identified. These were divided into those deferred for two diets (n=245), one diet (n=557) and those who had received no deferment (n=1354). Results The mean change in marks from first attempt to re-sit (table 1) was compared for deferred and non-deferred candidate groups. A significant difference (t (56)=2.721, p<0.01) was found for candidates sitting the exam in Overseas centres. Abstract G363 Table 1Mean change in marks from first attempt to re-sit* Exam location Candidate group Mean change in marks n UK Deferred 1 diet (score 72-80) +12.19 261 Not deferred (score 82 or 84) +13.00 52 Overseas Deferred 1 diet (score 72-80) +12.63 38 Not deferred (score 82 or 84) +5.00 20 *all candidates re-sat after a gap of one diet (eight months) Further analysis of candidate re-sit performance suggests that deferment is effective in excluding candidates who are unlikely to pass in the following exam diet. Based on the distribution of re-sit scores of candidates scoring 82 or 84 it is estimated that fewer than 14% of those deferred for one diet would achieve the increase in marks required to pass in the next exam. Conclusion The existing deferment process effectively safeguards Clinical exam places for those candidates with the best chance of passing and potentially protects the children used in the exam from inappropriate physical examination. The effects of deferment on re-sit performance vary according to exam location, and may depend on the quality of educational supervision and support, and the availability of preparatory courses.

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