Dosimetric effects of patient displacement and collimator and gentry anglemisalignment on intensity modulated radiation therapy

Purpose and objective: The primary goal of this study was to examine system atically the dosimetric effect of small patient movements and linear accele rator angular setting misalignments in the delivery of intensity modulated radiation therapy. We will also provide a method for estimating dosimetric errors for an arbitrary combination of these uncertainties. Materials and methods: Sites in two patients (lumbar-vertebra and nasophary nx) were studied. Optimized intensity modulated radiation therapy treatment plans were computed for each patient using a commercially available invers e planning system (CORVUS, NOMOS Corporation, Sewickley, PA). The plans use d nine coplanar beams. For each patient the dose distributions and relevant dosimetric quantities were calculated, including the maximum, minimum, and average doses in targets and sensitive structures. The corresponding dose volumetric information was recalculated by purposely varying the collimator angle or gantry angle of an incident beam while keeping other beams unchan ged. Similar calculations were carried out by varying the couch indices in either horizontal or vertical directions. The intensity maps of all the bea ms were kept the same as those in the optimized plan. The change of a dosim etric quantity, Q, for a combination of collimator and gantry angle misalig nments and patient displacements was estimated using Delta = Sigma(Delta Q/ Delta x(i))Delta x(i). Here Delta Q is the variation of Q due to Delta x(i) , which is the change of the i-th variable (collimator angle, gantry angle, or couch indices), and Delta Q/Delta x(i) is a quantity equivalent to the partial derivative of the dosimetric quantity Q with respect to x(i). Results: While the change in dosimetric quantities was case dependent, it w as found that the results were much more sensitive to small changes in the couch indices than to changes in the accelerator angular setting. For insta nce, in the first example in the paper, a 3-mm movement of the couch in the anterior-posterior direction can cause a 38% decrease in the minimum targe t dose or a 41% increase in the maximum cord dose, whereas a 5 degrees chan ge in the theta(i) = 20 degrees beam only gave rise to a 1.5% decrease in t he target minimum or 5.1% in the cord maximum. The effect of systematic pos itioning uncertainties of the machine settings was more serious than random uncertainties, which tended to smear out the en ors in dose distributions. Conclusions: The dose distribution of an intensity modulated radiation ther apy (IMRT) plan changes with patient displacement and angular misalignment in a complex way. A method was proposed to estimate dosimetric errors for a n arbitrary combination of uncertainties in these quantities. While it is i mportant to eliminate the angular misalignment, it was found that the couch indices (or patient positioning) played a much more important role. Accura te patient set-up and patient immobilization is crucial in order to take ad vantage fully of the technological advances of IMRT. In practice, a sensiti vity check should be useful to foresee potential IMRT treatment complicatio ns and a warning should be given if the sensitivity exceeds an empirical va lue. Quality assurance action levels for a given plan can be established ou t of the sensitivity calculation. (C) 2000 Elsevier Science Ireland Ltd. Al l rights reserved.