ESTRO 2022 - Abstract Book

S527

Abstract book

ESTRO 2022

Purpose or Objective Aim: stereotactic ablative radiotherapy (SABR) has been shown to increase survival in oligometastatic patients. Few studies actually depicted the oligometastatic disease (OMD) evolution after SABR. There is few evidence on which patient will remain disease-free after SABR and which will develop rapidly a polymetastatic disease (PMD) therefore apart from the number of active metastases, there are no clues on which proven factor should be taken into account for prescribing local treatment in OMD. The aim of the present preliminary study based on a large retrospective database is to identify predictive factors of polymetastatic evolution in lung oligometastatic colorectal cancer patients to tailor SABR prescription. Materials and Methods Methods: the study involved 23 centers, and was approved by the Ethical Committee (Prot. Negrar 2019-ZT). The data of 367 lung oligometastatic patients were reported. Primary end-point was the time to the polymetastatic conversion (tPMC), defined as the first occurrence of >5 simultaneous new metastases after SABR. Additionally, oligometastases number and cumulative gross tumor volume (GTV) were used as combined predictive factors of tPMC. Oligometastases number was stratified as 1, 2-3, and 4-5; cumulative GTV was dichotomized to the median value of 15.6 cc. The 6 groups are reported in table 1.

Table 1. group stratification for the analyzed patients Median tPMC (months)

1 metastasis cumGTV <15.6 cc 35.8 1 metastasis cumGTV >15.6 cc 19.3 2-3 metastases cumGTV <15.6 cc 34.1 2-3 metastases cumGTV >15.6 cc 12 4-5 metastases cumGTV <15.6 cc 5.6 4-5 metastases cumGTV >15.6 cc 3.7

Results Results: the median tPMC in the overall population was 26.1 months. The median tPMC stratified for the 6 groups is reported in table 1 (p=0.00). After data checking we were able to classify patients according to their median tPMC in 4 risk classes: low risk (group 1+3), favorable intermediate (group 2), unfavorable intermediate (group 4), high risk (group 5+6). The median tPMC for the 4 risk classes was: 34.6, 19.3, 12, and 6.7 months, respectively (figure 1; p=0.00).

Figure 1. tPMC stratified for risk classes

Conclusion Conclusion: the present study identified predictive factors of polymetastatic evolution after SABR in lung oligometastatic colorectal cancer. The results demonstrated that the sole numerical number is not enough to define the OMD since patients identified as oligometastatic from a numerical point of view might rapidly progress to the PMD when the cumulative tumor volume is high. A tailored approach in SABR prescription should be pursued considering the expected disease evolution after SABR, with the aim to avoid unnecessary treatment and side effect in those at high risk of polymetastatic spread, and maximize local treatment in those with a favorable disease evolution.