ESTRO 2024 - Abstract Book

S231

Brachytherapy - Gynaecology

ESTRO 2024

Purpose/Objective:

In cervical HDR brachytherapy in order to achieve the target HRCTV dose it is often required to compromise the dose constraints for the organs at risk (OARs). Multiple plans for a particular fraction can be generated with different compromises dependant on the OARs that already have breached or will breach the established dose constraints. We propose the use of a radar plot to provide a visual display of the dosimetry to help in selection of the most appropriate plan and also use a ratio of the area for the plan generated compared to the area of an ideal plan as a qualitative measure.

Material/Methods:

We reviewed the brachytherapy plans and clinical notes for a consecutive cohort of patients treated with radical chemoradiation and brachytherapy over a period that spanned the introduction of interstitial brachytherapy. The total doses to the d90, RV-RP and d2cc rectum, sigmoid, small bowel and bladder were used to generate a 6 point radar plot. The area of the plot was compared with the area of an ideal plan (one achieving all targets and dose constraints) to generate a ratio (6P-ratio). The cohort was split into 4 groups based on outcomes: group A alive with no toxicities, group B alive with grade 1 toxicities, group C alive with grade ≥2 toxicities and group D deceased due disease; the Kruskall-Wallis test was used to assess for differences in the 6P-ratio amongst the 4 groups. The t-test was used to assess for differences in the 6P-ratio between the pre-interstitial brachytherapy group v.s. post interstitial brachytherapy group. Correlation of the 6P-ratio with HRCTV volume was assessed using a Pearson’s correlation coefficient. For all statistical tests 2-tailed significance testing p≤ 0.05 was used.

Results:

A total of 112 patients with median age of 49 year were included, with a median follow up of 2.93 years (IQR 1.78 – 4.06). Median values HRCTV=24.68cc (IQR 20.3 – 32.2) and 6p-ratio = 0.42 (IQR 0.32 – 0.54). For all toxicities, grade 1= 32.1% and grade ≥2 = 16.1%. There was a statistically significant difference in the 6P-ratio amongst the 4 groups based on clinical outcomes (groups A n= 43, B n=34, C n=18 and D n=16), Kruskall-Wallis H statistic= 8.991 (p=0.0294, df=3). There was a significant difference in the 6P-ratio between the groups pre and post the introduction of interstitial brachytherapy, t-test p = 1.927 x10-5. (with higher d90 and lowers OAR doses in post- interstitial group). The 6P ratio showed only a weak inverse correlation with HRCTV volume (Pearson’s correlation coefficient = -0.383).