Dosimetric evaluation of systematic source position uncertainty along the source path and robust planning during MRI-only cervical HDR brachytherapy.

BACKGROUND

The knowledge of the dosimetric impact of systematic source position uncertainty and the use of a methodology for robust treatment planning against such uncertainty can help improve the quality of treatment plans, during MR-only cervical brachytherapy treatment.

PURPOSE: To assess the dosimetric impact of systematic source position uncertainty along the source path and evaluate strategies to minimize it during MRI-only cervical high-dose-rate (HDR) brachytherapy treatment planning.

METHODS

Treatment plans from 22 cervical cancer patients (FIGO stages IB2-IVA; 11 intracavitary brachytherapy (ICBT), 11 interstitial brachytherapy (ISBT)) were retrospectively analyzed. Positional uncertainty was modeled by systematically shifting the source position (± 1 to ± 5 mm with 0 mm as the reference position) within applicators along the source path in the original plans. Dosimetric parameters, HRCTV_D90 and D2cc for organs at risk (OARs), were computed for each source position. Statistical significance was assessed via paired t-test and Wilcoxon signed-rank test. Targeted plans exceeding dosimetric limits for OARs were re-optimized by modifying dwell times or gradients within 1 cm of the targeted OAR to improve the variability and overall OAR dosimetry.

RESULTS

Significant dosimetric changes were observed for bladder, sigmoid (all shift positions; p ≤ 0.001) and rectum (1 to 5 mm; p ≤ 0.04) in ICBT, and for all OARs (all shift positions; p ≤ 0.003) in ISBT. HRCTV_D90 was significantly affected in ICBT from -2 to -5 mm (p ≤ 0.042) and at all shifts (p ≤ 0.033) except at -1 and -2 mm in ISBT. Within ± 5 mm shift range, the mean (maximum) drop in HRCTV_D90 and increase in OARs (bladder, bowel, rectum and sigmoid) D2cc were up to -2.0 ± 1.9(-5.4)% and (6.0 ± 3.4(13.6)%, 0.5 ± 2.5(5.5)%, 2.2 ± 2.7(7.4)% and 1.8 ± 1.2(4.2)%) for ICBT, and -6.2 ± 1.4(-11.9)% and (5.2 ± 4.6(17.2)%, 1.3 ± 0.9(3.0)%, 4.6 ± 2.5(7.6)% and 6.6 ± 3.6(13.8)%) for ISBT, respectively. Re-optimized plans showed improved robustness of dosimetry against source positional uncertainty for the targeted OAR with the same or better dosimetry for other OARs and HRCTV. The strategies to obtain this robustness were patient and applicator-type dependent.

CONCLUSION

Systematic source position uncertainty significantly influences dosimetry in MRI-only cervical HDR brachytherapy. Adjusting dwell times and gradients near the targeted OAR could minimize this effect.