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

Abstract: 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 (4.5mm slices) Turbo Spin Echo (TSE) scans and a balanced Steady State Free Precession scan (1.5mm). 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 5–6mm into the ventral direction and 3–4mm 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 D2cc (bladder) was 1.0 (SD 3.0)Gyαβ3 for a single PDR fraction. If the effect of both fractions was combined, for 1 patient a total decrease of D90 of 7Gyαβ10 was found, whereas for another patient the total increase in bladder dose was 12Gyαβ3. 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. [Copyright &y& Elsevier]