ESTRO 2020 Abstract Book

S48 ESTRO 2020

smokers” subgroup, the adjusted sHR was 1.9 (0.5-7.6) for SPCs in the pelvis, 4.6 (1.0-20.9) for SPCs in the abdomen, and 2.7 (1.0-7.3) for SPCs in the remaining anatomical regions. Conclusion Our study suggests a complex relationship between IMRT, smoking status, and SPC risks. IMRT in current smokers was significantly associated with increased SPC risks, which potentially means that exposure of healthy tissue to low- dose baths and scatter should be minimized in current smokers needing cancer treatment. This observation is biologically plausible, since nicotine is known to promote the growth of cancer cells. Further validation in a multicenter setting with prolonged follow-up is currently ongoing.

Conclusion We have developed and validated a Bayesian network structure from 14 trial cohort data for predicting local recurrence in locally advanced rectal cancer patients. The causal relationships between the variables in the structure are developed and validated by domain experts from different radiotherapy centers where treatment protocols may differ OC-0101 Second primary cancer risks among prostate cancer radiotherapy survivors: effect of smoking and IMRT M. Jahreiβ 1 , K.K. Aben 2 , M.S. Hoogeman 1 , M.L. Dirkx 1 , H. Reuvekamp 3 , M. Ahmadi 3 , K.C. De Vries 1 , L. Incrocci 1 , W.D. Heemsbergen 1 1 Erasmus MC Cancer Institute, Dept of Radiation Oncology, Rotterdam, The Netherlands ; 2 Netherlands Comprehensive Cancer Organisation, Dept of Research, Utrecht, The Netherlands ; 3 InHolland University of Applied Sciences, Medical Technology Research Group, Haarlem, The Netherlands Purpose or Objective Radiotherapy (RT) and smoking are known as potential modifying and interactive factors affecting the risk of developing second primary cancer (SPC) after RT. Modern intensity-modulated RT (IMRT), with increased low-dose baths and out-of-field scattering, may increase such risks even more. Here we report on the impact of IMRT and smoking status in a single-center retrospective cohort study, using an internal reference group of 3-dimensional conformal RT (3DCRT) patients. Material and Methods The study cohort comprised 1,468 prostate cancer survivors (mean age 70.0 years, 6.7 1SD) treated with either 3DCRT or IMRT in the period 2006-2012 ( Table 1 ). IMRT was gradually introduced during the period 2007- 2010. All patients received standard treatment at that time (72-78 Gy in 2Gy fractions) and had no previous or simultaneous RT to other pelvic areas. Data on SPC incidences (solid, non-skin) were retrieved from the Netherlands Cancer Registry. Only first SPCs were included in the analysis. The Fine and Gray model was applied with death and non-solid SPC as competing risk, to estimate relative risks (Subhazard Ratios [sHRs]) for IMRT vs 3DCRT within smoking categories. Models were adjusted for age at RT, and calendar year of RT. Time was calculated from start RT and was maximized at 10 years to adjust for differences between both groups in maximum follow-up. A latency period of 6 months was considered, resulting in the exclusion of 3 SPCs. Results Median follow-up was 7.5 (IMRT) and 9.5 years (3DCRT). We observed 236 SPCs in the period 0.5-10 years. Most frequent SPCs were bladder (n=44), lung (n=41), colon (n=35), and rectum (n=23). Vital status at the end of follow-up was: 62% alive, 37% died (13% after SPC), 1% emigrated. Distribution of smoking status was: 17% current, 20% previous, 31% never, 32% unknown. For the total cohort (regardless smoking status), the adjusted sHR with 95% CI to develop SPC (IMRT vs 3DCRT) was 1.4 (1.0- 2.0). For never smokers, 1.0 (0.5-2.0), for previous smokers 1.4 (0.6-3.0), for current smokers 2.8 (1.3-5.8), Figure 1 , and for unknown smoker status 1.2 (0.5-2.6). Interaction between smoking status (current y/n) and technique was significant ( p <0.01). Within the “current

Proffered Papers: Proffered papers 6: Novel treatment planning strategies

OC-0102 Reducing secondary lung cancer risk by optimized planning of accelerated partial breast irradiation N. Hoekstra 1 , S. Habraken 1 , A. Swaak - Kragten 1 , S. Breedveld 1 , J. Pignol 2 , M. Hoogeman 1 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands ; 2 Dalhousie University, Radiotherapy, Halifax NS, Canada Purpose or Objective The survival benefit of adjuvant radiotherapy for low-risk breast cancer patients might be partially offset by the risk