RADIOTHERAPY OF BLADDER-CANCER - RELEVANCE OF BLADDER VOLUME CHANGES IN PLANNING BOOST TREATMENT

Purpose: This study aims to evaluate tumor motion with controlled chan ges of the bladder volume, and to assess the reproducibility of bladde r land tumor) position using a urinary catheter balloon as an immobili zation device. Methods and Materials: First, three patients with tumor growths in three different bladder regions (trigone, left lateral wal l, anterior mall) were evaluated. Three-dimensional CT-based reconstru cted images were used to measure the displacement of the tumors when 1 00 cc were removed from a bladder originally filled with 170 cc of con trast. The 3D calculated boost beam arrangements and field sizes for t he three tumors in the partially emptied bladders were used to simulat e treatment of the same tumors in the maximally filled bladders. Dose- volume histograms were obtained. Second, verification of an ellipsoid model for bladder volume changes was undertaken in 41 patients. Third, in eight additional patients a urinary catheter baboon filled with 80 -cc sterile saline solution was used in an attempt to reproduce the sh ape and spatial coordinates of the bladder during the boost treatment. A pair of orthogonal films with the 80-cc balloon filled with contras t material were taken at simulation and repeated twice at weekly inter vals during radiotherapy. The reproducibility was quantified by sequen tially calculating the common surface of the bladder images in each or thogonal view. Results: Target motion, especially in the craniocaudal axis, appeared to be more relevant for tumors arising in the bladder w alls (5 mm) than in the trigone (5 mm). Underdosage (<95% of the presc ribed dose to the target volume) was observed in 20, 20, and 50% (with 1 cm margins around the tumor) and in 10, 10, and 15% (with 1.5 cm ma rgins around the tumor) of the tumors arising in the trigone, left lat eral wall, and anterior wall, respectively. The ellipsoidal model was validated with a strong correlation coefficient allowing to establish a predictive model for bladder wall displacements as a function of bla dder volume. In the balloon reproducibilty study, mean reproducibility factors of 0.84 (+/-0.06) and 0.82 (+/-0.07) were obtained for both a nteroposterior and lateral views, respectively. Conclusions: Changes i n bladder volume and shape related to bladder filling can result in cl inically significant displacements of the target volume. A minimum of 2-cm margins around the target may compensate for extreme bladder volu me changes during boost treatment. An ellipsoidal model for the bladde r is consistent with these observations. Although an 80-cc urinary cat heter balloon helped to immobilize the bladder, reproducibility was le ss than perfect. (C) 1998 Elsevier Science Inc.