ESTRO 2020 Abstract Book

S77 ESTRO 2020

relationship between EQD2 and local control at 2 years, using bounded logistic regression. Finally, we adjusted local control rates for T stage mix, using data from Maas et al (Lancet Oncol 2010; 11: 835–44), and fitted dose- response separately for cT1-2 and cT3-4. Results We screened 745 papers, and identified 15 non- overlapping cohorts for inclusion (Table 1). EQD2 to the primary tumour varied from 40.7Gy to >160Gy, with higher dose levels typically delivered using endorectal brachytherapy or contact X-ray therapy. Nine papers reported on dedicated NOM cohorts, with local control at 2 years at 12%-72%. A significant (p<0.001) dose-response relationship was estimated from these reports (Figure 1), characterised by D 50 = 70.6Gy (95% CI 61.5-79.7Gy) and γ 50 = 1.09 (0.95-1.23). The dose-response appeared to taper off for >100Gy, with upper bound on local control at 72%. The published cohorts had substantial heterogeneity in cT stage distribution, but this alone did not explain the variation in local control (p<0.001). Correcting for stage mix, the EQD2 needed for 50% local control at 2 years differed by nearly 20Gy for cT1-2 versus T3-4 tumours; D 50 = 66Gy for cT1-2 and D 50 = 85Gy for T3-4.

Conclusion This exploratory analysis of the rectal cancer organ preservation literature indicates a dose-response relationship for NOM. Future trials are needed for validation. The difference in dose response for T1-2 versus T2-3 suggests a need for patient stratification in trial design.

Poster Highlights: Poster highlights 6 RTT: Optimising techniques for breast and H&N RT

PH-0163 Harmonization of breast cancer radiotherapy treatment planning in the Netherlands M. Verhoeven 1 , M. Sangen van der 1 , C. Hurkmans 1 , L. Boersma 2 , K. Verhoeven 2 , N. Bijker 3 , K. Crama 3 , T. Nuver 4 1 Catharina Ziekenhuis, radiotherapy, Eindhoven, The Netherlands ; 2 Maastro Clinic, radiotherapy, Maastricht, The Netherlands ; 3 Amsterdam UMC, radiotherapy, Amsterdam, The Netherlands ; 4 Radiotherapiegroep, radiotherapy, Deventer, The Netherlands Purpose or Objective Currently there is no consensus concerning dosimetric and clinical parameters to evaluate breast cancer radiotherapy treatment planning. Although fractionation schedules are harmonized, each institute uses its own plan evaluation criteria including its own OAR nomenclature. A quality improvement project was started to quantify the current variation and to formulate national consensus guidelines on plan evaluation. Material and Methods A Benchmark Case including target and OAR delineations conforming to ESTRO guidelines was sent to all 20 radiotherapy departments. The institutes were requested to submit 4 photon treatment plans based on their own