The usefulness of an independent patient-specific treatment planning verification method using a benchmark plan in high-dose-rate intracavitary brachytherapy for carcinoma of the uterine cervix

摘要

In this study, Fletcher-type (Fletcher-Williamson Asian-Pacific) tandem-ovoid and tandem-cylinder metal applicators (Nucletron International B.V., Veendaal, the Netherlands) were used. Various sizes and combinations of these applicators were constructed by one radiation oncologist with the ‘ideal' applicator geometry outside the patient (Fig.?1a and b) and then reviewed by a medical physicist. We constructed eight kinds of tandem-ovoid and six kinds of benchmark plans (Table?1). Dose was calculated with the following formula introduced in AAPM TG43-U1 [22]. ICRU report 38 [23] recommends reporting the reference volume as well as total reference air kerma strength and absorbed dose at reference points. The reference volume is the volume encompassed by the reference isodose surface, which is represented by the major dimensions of the following: We retrospectively analyzed 168 and 29 clinical cases from 2009 through 2010 with a tandem-ovoid and tandem-cylinder, respectively. The difference in total dwell time between a benchmark plan and an individual treatment plan was calculated using the following formula: Figure?3 shows a histogram of differences in total dwell time between benchmark and individual treatment plans. Differences averaged –0.2%?±?3.8% (range, –13.3–9.6%), and exceeded 5% in 23 of 169 clinical cases. Figure?4 shows a histogram of differences in dose distribution shapes between the benchmark and individual treatment plans in tandem-ovoid applications. The differences in dh, dt and dw between the benchmark and individual plans averaged –1.0?mm?±?2.6?mm (range, –8.6?mm to +6.5?mm), 0.8?mm?±?1.3?mm (range, –1.4?mm to +5.2?mm) and –0.1?mm?±?1.5?mm (range, –5.1?mm to +4.0?mm), respectively. We verified that all tandem-ovoid treatment plans were appropriately created without any planning errors, including wrong source position, wrong decay correction of source strength and inappropriate optimization or use of an unplanned size or combination of applicators. However, 24 of 169 cases had a 5% difference in total dwell time. To explain these differences, we investigated the correlation between differences in total dwell times and dh, dt and dw between the benchmark and individual plans. Figure?5a shows the relationship of differences in total dwell time (vertical axis) with those in dh (horizontal axis). Spearman's rank correlation coefficient (r_s?=?0.836, P???0.01) showed a strong relationship between the discrepancy in total dwell time and those in dh. In contrast, no correlations were found between the discrepancy in total dwell time and those in dt (r_s?=?0.371, P??=?0.075) or dw (r_s?=?0.290, P??=?0.149) (Fig.?5b and c). Figure?6a shows differences in total dwell time between the benchmark and individual treatment plans. Differences averaged –1.5%?±?3.1% (range, –13.0% to +0.4%), with 2 of 29 cases exceeding 11%. For tandem-ovoid cases, the tolerance level of total dwell time, dh, dt, and dw were –7.5% to +7.2, –6.0?mm to +4.1?mm, –1.8?mm to +3.4?mm and –3.0?mm to +2.8?mm, respectively (Fig.?7a).