Session Item

Online adaptive radiotherapy (oART) in cervical cancer has the potential to reduce dose to organs at risk (OAR) whilst improving clinical target volume (CTV) coverage. However, uncertainty remains over the optimal application. The Varian Ethos emulator allows simulation of oART treatments using previously acquired CBCT images. This simulation study evaluates the dosimetric impact of oART in cervical cancer using the Ethos Therapy system.

Ten historic patients treated with 50.4 Gy/28 fractions for cervical cancer were selected (5 with CBCTs from Truebeam, 5 with CBCTs from Ethos). For each patient, an initial (scheduled) Ethos 12-field IMRT plan was auto-optimised using a custom template. Ten treatment sessions (sampled throughout the course) were simulated. For each fraction, the Ethos system contoured “influencer” structures (bladder, bowel, rectum and uterus) and propagated target structures on the CBCT, these were edited by either a radiation oncologist or radiation therapist. The Ethos system then recalculated the scheduled plan (SP) and a newly-optimised ART plan (AP) based on the new contours. Margins followed local protocols throughout. Each patient course was simulated twice, once selecting the SP and once the AP each time.

Ethos provides dose accumulation over the course of a treatment based on propagation of the delivered dose during each delivered fraction. A comparison of dose delivered using the SP (representing current practice) against the AP was performed using Wilcoxon signed rank test. Reported dose values have been scaled up from the simulated 10 deliveries to 28 fractions for direct comparison to clinical protocols. The time taken for each simulated session was noted. 

The mean CTV d_min was increased by 6% (range -0.8 to 24.6) using oART (p<0.01). The mean CTV d_max was reduced by 0.7% (p=0.04). Overall, CTV coverage was improved with oART, though not statistically significant at all levels, e.g. D_99% increased by 0.5 Gy using oART (p=0.06), whereas the D_99.5% increased by 1.4 Gy (p=0.02). As shown in Table 1, all OAR differences which were statistically significant were in favour of oART. The average time taken for each simulated session was 21 minutes (range 12-34). 

Whilst overall there was some small benefit seen for oART (increased CTV coverage and reduced OAR doses) there was large inter-patient variation in the benefit of oART. This indicates that even for a traditionally mobile target such as the cervix-uterus complex, criteria for patient selection is required to determine if oART will provide a significant benefit. Further work investigating the dosimetric impact of CBCT-guided oART in cervical cancer in the live setting is ongoing. Improved accuracy of oART should allow margin reduction, leading to further reductions in OAR dose.