Thursday, July 11, 20245:00 PM - 6:00 PMMSOR1 Presentation Time: 5:00 PM

Purpose

HDR brachytherapy is an integral part of radiation therapy and is used jointly with EBRT in the treatment of numerous cancers including cervical cancer. For targets jointly treated with HDR and EBRT, ICRU 89 recommends the use of an EQD2 assessment on the accumulated dose between EBRT and brachytherapy fractions, assuming equivalent RBE between EBRT and HDR. Recent studies have shown that RBE can vary with photon energy, however, whether these differences in RBE translate to different biologic effects in cervical cancer, among others, have yet to be well studied. The goal of this study is to investigate biological endpoints in cervical cancer cell lines for an HDR remote afterloader and a clinical linear accelerator.

Materials and Methods

We developed a portable 3D printed platform allowing for the joint study of clinical HDR brachytherapy (Bravos, Varian Medical Systems, Palo Alto, CA) and clinical EBRT (Edge, Varian Medical Systems, Palo Alto, CA). Gamma-H2AX (gH2AX) foci formation and clonogenic survival assays were performed in an effort to characterize the biological effects of clinical irradiators commonly used to treat cervical cancer in three cervical cancer cell lines: HT3, CasKi, and SW756. Cells were irradiated using the 3D printed platform for both HDR and EBRT irradiations, as the design can accommodate cells plated in a standard sized tissue culture plate which can be irradiated by up to 14 HDR source channels or by clinical EBRT irradiators. GH2AX foci were quantified to estimate DNA double strand breaks at two post irradiation time points: 30 minutes for early damage and 24 hours as a surrogate for DNA repair capacity. Clonogenic survival was determined 10-14 days after RT (n > 50 cells). For HDR irradiations, a dose of 4 Gy (gH2AX) or 6 Gy (clonogenic survival) was delivered at a source-to-cell distance (SCD) of 0.4 cm and 1 cm giving dose rates of 30-50 Gy/min and 3-5 Gy/min, respectively. EBRT irradiations uniformly delivered 4 Gy (gH2AX) or 6 Gy (clonogenic survival) to a 96-well tissue culture plate with a single anterior/posterior 15 × 15 cm² field at a 100 cm source-to-axis distance. Statistical analysis was performed using the one way ANOVA statistical test (p < 0.05) with Tukey's multiple comparisons test and the Mann-Whitney U test (p < 0.05) for the gH2AX assay and the clonogenic survival assay, respectively.

Results

At 30 minutes, all irradiated groups had significantly more gH2AX foci than their respective control groups. There were no statistically significant differences in this cohort between modalities for HT3 at either 30 minutes or 24 hours. For CasKi and SW756, EBRT had significantly fewer gH2AX foci than HDR at 0.4 cm SCD and 1 cm SCD. In terms of clonogenic survival for HT3, the EBRT treated cells had a significantly higher survival fraction than those treated with HDR at 0.4 cm SCD but not 1 cm SCD. CasKi had a significantly higher survival fraction with EBRT compared to both HDR at 0.4 cm SCD and 1 cm SCD conditions. Clonogenic survival data for the SW756 cell line is ongoing.

Conclusion

Using equivalent dose but varying modality, we find that EBRT induced fewer gH2AX foci and resulted in higher clonogenic survival compared to HDR in a cell-line dependent manner. These data suggest that modality-dependent radiobiological differences may be exploited in the treatment of cervical cancer.