Changes in ultrasound findings in posterior uveal melanoma after ruthenium 106 brachytherapy

Abstract

Purpose To analyze the postbrachytherapy ultrasonographic dynamics of uveal melanoma tumor height and internal reflectivity. Design Prospective, comparative, observational case series. Participants One hundred forty-seven patients (147 eyes) with posterior uveal melanoma having a mean age of 61 years (range, 29–97 years) who were treated with Ruthenium 106 brachytherapy. Methods Patients were followed-up with ultrasonography using both A and B modes of standardized echography every 6.7 ± 0.3 months (mean ± standard error of the mean) for a total of 1001 ultrasound examinations. On average, each patient was examined 5.8 times (range, 3–17 times). The echographic parameters included tumor base size, height, internal reflectivity, regularity, vascularity, and extra-scleral extension. To compare the response of tumors of different sizes, each tumor was standardized to its initial size at brachytherapy. Main outcome measures The dynamics of the tumor height and internal reflectivity. Results At the time of brachytherapy, the mean height of the tumors was 5.2 mm (range, 2.2–11.8 mm). After brachytherapy, 142 tumors (96.6%) responded by a decrease in height. The decrease in height was at an initial rate of approximately 3% per month. Most of the tumors did not regress entirely; rather, their height stabilized on a constant value (on average 61% of the initial height) after approximately 18 to 24 months. The decrease in height after brachytherapy was best fitted by the sum of a first order exponential decay and a constant (h = 61 + 35∗e −0.12t, in which t = time in months). The half-life of the decay was 5.8 months. Large tumors (>8 mm) had a faster initial decrease in height, and stabilized on a lower percentage of their initial height (50%) compared with small tumors (70%). Thus, the half-life of the height exponential decay was 5.3 months for small tumors (2–4 mm) and 3.3 months for the large tumors (>8 mm). Internal reflectivity increased from a mean of 40% before therapy to 70% after 2 years. The dynamics of the reflectivity were best fitted with the function: f = 45 + 24(1-e −0.09t). Larger tumors, which initially had lower internal reflectivity, presented with a slower increase in internal reflectivity (t 1 2 = 8.7 months) compared with smaller tumors (t 1/2 = 5 months). Conclusions The postbrachytherapy ultrasonographic dynamics of uveal melanomas resemble a function composed of the sum of a constant and a first order exponent, suggesting the possible existence of two components (cell populations), one unaffected by brachytherapy and the other a radiosensitive population that reacts to brachytherapy in an exponential decay. An exponential decay can imply that the postbrachytherapy death of each tumor cell is a stochastic Markovian process that is independent of the death of other tumor cells.