IMPROVEMENT OF CT-BASED TREATMENT-PLANNING MODELS OF ABDOMINAL TARGETS USING STATIC EXHALE IMAGING

Purpose: CT-based models of the patient that do not account for the mo tion of ventilation may not accurately predict the shape and position of critical abdominal structures. Respiratory gating technology for im aging and treatment is not Set widely available. The purpose of the cu rrent study is to explore an intermediate step to improve the veracity of the patient model and reduce the treated volume by acquiring the C T data with the patients holding their breath at normal exhale. Method s and Materials: The ventilatory time courses of diaphragm movement fo r 15 patients (with no special breathing instructions) were measured u sing digitized movies from the fluoroscope during simulation. A subseq uent clinical protocol was developed for treatment based on exhale CT models. CT scans (typically 3.5-mm slice thickness) were acquired at n ormal exhale using a spiral scanner. The scan volume was divided into two to three segments, to allow the patient to breathe in between. Mar gins were placed about intrahepatic target volumes based on the ventil atory excursion inferior to the target, and on only the reproducibilit y of exhale position superior to the target. Results: The average pati ent's diaphragm remained within 25% of the range of ventilatory excurs ion from the average exhale position for 42% of the typical breathing cycle, and within 25% of the range from the average inhale position fo r 15% of the cycle. The reproducibility of exhale position over multip le breathing cycles was 0.9 mm (2 sigma), as opposed to 2.6 mm for inh ale. Combining the variation of exhale position and the uncertainty in diaphragm position from CT slices led to typical margins of 10 mm sup erior to the target, and 19 mm inferior to the target, compared to mar gins of 19 mm in both directions under our prior protocol of margins b ased on free-breathing CT studies, For a typical intrahepatic target, these smaller volumes resulted in a 3.6% reduction in V-eff for the li ver. Analysis of portal films shows proper target coverage for patient s treated based on exhale modeled plans. Conclusions: Modeling abdomin al treatments at exhale, while not realizing all the gains of gated tr eatments, provides an immediate reduction in the volume of normal tiss ue treated, and improved reliability of patient data for NTCP modeling , when compared to current ''free breathing'' CT models of patients. ( C) 1998 Elsevier Science Inc.