Combined Intracavitary/Interstitial Brachytherapy to Optimize Dosimetry for Cervical Cancer

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Intracavitary brachytherapy (ICB) is an important means to dose escalate in curative radiation therapy for cervical cancer. With image-guided brachytherapy, higher HR-CTV dose (D90>87 Gy10) increases local control. Cases of large tumor, extensive parametrial residuum, or unfavorable topography can benefit from interstitial brachytherapy (ISB) to improve coverage. Combining ICB and ISB, regardless of clinico-anatomical factors, could also facilitate dosimetric optimization. We reviewed external beam radiation therapy (EBRT) and ICB (pulsed-dose rate) plans for 28 women treated at our institution on a study of MRI-guided brachytherapy from 2012 to 2015. We identified 11 cases (all tandem + ring) where GEC-ESTRO recommendations were met, but HR-CTV D90<90 Gy10 (EBRT+ICB). Cases were categorized by dose level (HR-CTV D90<87 Gy10 (DL1) vs. ≥ 87 Gy10 (DL2)). Ratios of organ at risk (OAR) D2cc (for rectum, bladder, and sigmoid) relative to HR-CTV D90 were calculated. Virtual ISB catheters were retrospectively added to the ICB plans and further optimization undertaken. Catheter (n=2-5) positions and depths were tailored to anatomy, with fixed conformation modelled on our tandem+ring+catheter system. Dosimetry for ICB and ICB+ISB was compared. Median HR-CTV volume for 11 cases was 29.5 cc [13.2-78.2 cc] (46% FIGO stage IB, 18% IIA, 36% IIB). For ICB, median HR-CTV D90 was 86.9 Gy10 [81.0-89.9 Gy10], and D2cc for rectum 58.3 Gy3 [49.2-66.8 Gy3], for bladder 64.3 Gy3 [57.9-90.3 Gy3], and for sigmoid 64.3 Gy3 [52.6-74.9 Gy3]. For ICB+ISB, median HR-CTV D90 was 90.8 Gy10 [88.4-94.9 Gy10], with D2cc for rectum, bladder, and sigmoid of 58.5 Gy3 [49.8-71.6 Gy3], 64.8 Gy3 [58.5-83.0 Gy3], and 67.5 Gy3 [52.6-73.8 Gy3], respectively. For DL1 (n=6), the median OAR/HR-CTV dose ratios for rectum, bladder and sigmoid (ICB | ICB+ISB) were 0.66 | 0.61, 0.96 | 0.83, and 0.82 | 0.79, respectively, showing a relative dose improvement. There was less change with ISB addition in DL2 (n=5), where median OAR/HR-CTV dose ratios for rectum, bladder and sigmoid (ICB | ICB+ISB) were 0.71 | 0.68, 0.70 | 0.68, and 0.64 | 0.61, respectively. Institutional planning aims (HR-CTV D90≥90 Gy10, D2cc rectum<75 Gy3, D2cc bladder<90 Gy3, D2cc sigmoid<75 Gy3) still could not be met in 1 case with ICB+ISB. In this planning exercise, ISB+ICB increased HR-CTV D90 without substantially increasing OAR doses. This was less impactful in those cases with HR-CTV D90 already ≥ 87 Gy10. Bladder and sigmoid constraints were more challenging in plans where HR-CTV D90<87 Gy10, even with ISB, hinting at a subset of patients with clinico-anatomical factors requiring further mitigation strategies.