Session Item

Nowadays curative EBRT is delivered at high dose levels with IMRT and VMAT techniques, frequently combined with online imaging of the target volume. Concerns have been raised that associated increased low-dose volumes and scattering might increase second primary cancer (SPC) risk, especially when treated at a young age. In the current study we investigated excess SPC risk associated with modern EBRT protocols for prostate cancer (PCa) within five radiotherapy (RT) institutes. We hypothesized that differences in excess SPC risk, specifically in the non-pelvic region might exist between the applied RT protocols as a result of different exposures to scatter and low-dose volumes.

We collected modern EBRT protocol information from five Dutch RT institutes (Table 1). All PCa patients (N=5372), who received modern EBRT treatment in one of those institutes, were identified in the Netherlands Cancer Registry (NCR). From the NCR we obtained patient and PCa characteristics, as well as solid SPC information. Standardized incidence ratios (SIR) were calculated for the endpoints non-pelvis and pelvis (adjusted for age and calendar year) to compare SPC rates to the Dutch male general population. A latency period of one year from date of PCa diagnosis was applied. SIRs were compared to reference SIRs obtained from a nationwide PCa patient group treated with EBRT since 2008 (N=14946). This time period acted as a proxy for the modern EBRT era in The Netherlands. 

The reference numbers of the nationwide cohort show significantly increased risks for non-pelvic SPC (age ≤70) and for pelvic SPC (all ages) (Table 2). For the 5 institutes combined, no significant excess risks were observed for non-pelvic SPC, whereas for pelvic SPC a significant excess risk was observed in the combined age group. For the individual institutes we observed for non-pelvic significant lower risks for institute D in the >70 age group, and significant higher risks for institute A in the ≤70 age group. For pelvic SPCs, we observed similar trends in the nationwide group, the combined institutes, and the individual institutes, with elevated risks for all ages. For the age group ≤70 the estimates show some deviating results, with a relatively high SIR for institute A compared to the relatively low SIR estimates of institute B,C, and D. For the age group >70, the SIR for institute A is relatively low compared to the other institutes.

The results from the RT institute analysis show variation in the risk for non-pelvic and pelvic SPC. We could, however, not identify a clear trend with respect to the identified RT protocol characteristics. Results of such analyses should be interpreted with caution because of potential confounding factors like 1) regional variations in lifestyle factors associated with cancer risks, and 2) presence of non-identified differences in local procedures. Further research into the potential associations between EBRT characteristics and SPC incidence is currently ongoing.