AN EVALUATION OF SETUP UNCERTAINTIES FOR PATIENTS TREATED TO PELVIC SITES

Purpose: Successful delivery of conformal fields requires stringent im mobilization and treatment verification, as well as knowledge of the s etup reproducibility, The purpose of this study was to compare the thr ee-dimensional distribution of setup variations for patients treated t o pelvic sites with electronic portal imaging devices (EPID) and porta l film. Methods and Materials: Nine patients with genitourinary and gy necological cancers immobilized with custom casts and treated with a f our-field whole-pelvis technique were imaged daily using an EPID and f ilmed once every five to seven treatments, The three-dimensional trans lational and rotational setup errors were determined using a technique that relies on anatomical landmarks identified on simulation and trea tment images, The distributions of the translational and rotational va riations in each dimension as well as the total displacement of the tr eatment isocenter from the simulation isocenter were determined. Resul ts: Grouped analysis of all patients revealed average unidirectional t ranslational deviations of less than 2 mm and a standard deviation of 5.3 mm, The average total undirected distance between the treatment an d simulated isocenters was 8.3 mm with a standard deviation of 5 mm, I ndividual patient analysis revealed eight of nine patients had statist ically significant nonzero mean translational variations (p < 0.05), T ranslational variations measured with film were an average of 1,4 mm l ess than those measured with EPID, but this difference was not statist ically significant. Conclusion: Translational variations measured in t his study are in general agreement with previous studies, The use of t he EPID in this study was less intrusive and may have resulted in less additional attention being given each imaging setup, This may explain the slightly larger average translational variations observed with EP ID vs, film, and suggests that the use of EPIDs is a superior method f or assessing the true extent of setup displacements, Although no stati stically significant translational variations for the patient group ov erall were observed, 90% of patients had significant translational var iations in at least one direction when analyzed separately, The margin to be added to the clinical target volume (CTV) to account for setup uncertainties will depend on whether it is possible to identify patien ts with significant translational variations, and to eliminate these d isplacements from routine treatments, The choice to eliminate these va riations and to use a smaller CTV margin will have to be accompanied b y stringent frequent position verification methods and repositioning.