The impact of different treatment protocols on achieved radiation doses of high-dose-rate brachytherapy for locally advanced cervical cancer: a comparison between 2 fractions in 1 application and separate applications for each fraction

Purpose
To compare radiation doses achieved by image-guided brachytherapy for locally advanced cervical carcinoma implemented with two different protocols.

Material and methods
Medical records of 117 patients with locally advanced cervical carcinoma treated with brachytherapy from 2009 to 2018 at our institution were retrospectively reviewed. All patients had received previous external beam radio/chemotherapy. We performed magnetic resonance image-guided adaptive high-dose-rate brachytherapy delivered by intra-cavitary/interstitial applicators. Dose prescription was 7 Gy for four fractions within two weeks. Original schedule of brachytherapy was two fractions delivered on consecutive days with one applicator insertion; this process was repeated one week later (group 1, 54 patients). From 2015 onwards, another protocol of brachytherapy was mainly used, separately performing applicator insertions for each of the four administered fractions (group 2, 63 patients).

Results
The high-risk clinical target volume (HR-CTV) D90 planning aim (PA) of ≥ 85 Gy (hard constraint) was not achieved in 9 cases out of 54 (17%) in group 1 compared with only 2 out of 63 cases (3%) in group 2 (p = 0.022). A difference between the two groups was also found in the fulfillment of PA 90 Gy (soft constraint) (p = 0.027). We conducted a sub-group analysis of target volume groups and observed that the differences were most pronounced with very large tumors (> 50 cm3). In these patients, PA 85 Gy was only fulfilled in 67% cases when treatment involved two applications, but in all cases with four separate applicator insertions (p = 0.010).

Conclusions
In our experience, by performing an applicator insertion for each of the fractions, it is possible to correct the non-optimal position of the applicator immediately, and to deliver better doses for consecutive fractions. As a result, the planning aim is more often achieved, especially for large tumors.