Abstract
Practical problems influence the accuracy with which computed distributions are found to agree with in vivo point dose measurements. Dose calculation with Cunningham's program ISODOS has been done routinely for two years over 80 patients with intracavitary implants. Doses in a group of patients were measured at several points with Harshaw TLD rods spaced 2 cm apart in a bladder catheter inserted and left for 6-8 h during irradiation. In some cases TLD values have agreed with computed doses to better than 5%; in others they differed by up to 60%. Five factors leading to discrepancies are examined: (1) slippage of TLD's in the catheter; (2) shift of applicator in the patient; (3) incorrect allowance for attenuation by applicator walls; (4) position distortion of orthogonal radiographs locating sources and TLDs; (5) amount of attenuation in tissue. The first factor was measured in vivo and found to produce the greatest differences between calculated and measured doses. Other factors were studied in a phantom with applicator, TLDs and sources in known positions.
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