Applicator reconstruction and applicator shifts in 3D MR-based PDR brachytherapy of cervical cancer.

PURPOSE: To evaluate the methods of applicator reconstruction in 3D MR-based planning for brachytherapy of cervical cancer, and to investigate applicator shifts and changes in DVH parameters during PDR treatment.
METHODS: For each application MR scans with applicator in situ were made: three T2-weighted (4.5 mm slices) Turbo Spin Echo (TSE) scans and a balanced Steady State Free Precession scan (1.5 mm). Three observers tested two applicator reconstruction methods: (A) directly on the bSSFP scan and (B) on a resampled combination of the three T2-weighted scans. For 10 patients MR imaging was repeated on the second day of each PDR fraction to determine applicator shifts and changes in DVH parameters.
RESULTS: For both applicator reconstruction methods the interobserver variation for the DVH parameters was comparable (average <1.5% in dose). Differences between the two methods were larger (up to 6.4% for target) and were related to position differences during MR scanning. The average applicator shift relative to the pelvic structures was 5-6 mm into the ventral direction and 3-4 mm cranially. For a single PDR fraction, the average D90 (HR-CTV) on 'day 2' was 0.2 (SD 2.0) Gy lower than that for day 1. The average increase in D(2 cc) (bladder) was 1.0 (SD 3.0) Gy(alphabeta3) for a single PDR fraction. If the effect of both fractions was combined, for 1 patient a total decrease of D90 of 7 Gy(alphabeta10) was found, whereas for another patient the total increase in bladder dose was 12 Gy(alphabeta3).
CONCLUSIONS: Applicator reconstruction on MR data is feasible. In the overall accuracy during PDR brachytherapy the reconstruction uncertainty is of minor importance. Applicator and/or organ movement during the course of the PDR fraction produce larger uncertainties.