ESTRO 2021 Abstract Book

S275

ESTRO 2021

Materials and Methods Six patients with LACC were treated with elective EBRT (45Gy/25 fractions) and a SIB (57.5Gy/25 fractions) to the LNs on the CBCT-linac, followed by BT. Planning-CT and MRI before start of EBRT (week 0) and MRI before start of BT (week 4) were conducted. Study plans were generated for each case. This included an elective plan (45Gy/25 fractions) on the CBCT-linac, using the week 0 scans and original delineations with GTV-to-CTV and CTV-to-PTV margins of 3 and 5mm. Additionally, SSB MRL plans (15Gy/5 fractions) were made, using the week 4 MRI to adjust the LN and OAR delineations within 2cm around the LN, taking regression and anatomy change into account. In 8 LNs regression could not be established, because they were out of the field of view of the week 4 MRI. CBCT and MRL plans were summated (disregarding BT contribution), using an EQD 2 approach, and the OAR and target dose levels were compared with the clinical SIB plan. Results All measured LNs showed regression on week 4 MRI (Table 1). PTV D98% was higher in the study plans for all patients. On average, D2cc was lower, but D2cc outside 15 mm around the LN was higher in the study plans for most OARs (Table 1, Figure 1). In the clinical plans no BT contribution was seen in 5 LNs, all located around the aortic bifurcation. In 1 LN a sequential boost (4Gy/2 fractions) was given in the clinical plan. Despite the sequential boost, PTV D98% was higher in the study plan (58Gy versus 59.5Gy EQD2 10 ). In 4 LNs which did not receive a 4Gy boost in the clinical plan, PTV D98% was even higher in the study plans (median PTV D98% 54.1Gy EQD2 10 versus 59.8 Gy EQD2 10 ).

Conclusion Dose escalation to the LNs with a SSB on the MRL facilitates LN dose escalation at a small cost of the OAR dose in the region around the LN. However, due to loss of dose build-up from a SIB, the OAR dose at distance from a LN is higher and needs further research. Especially in patients with larger or higher located LNs, the gain of dose escalation might outweigh the increase in OAR dose. In addition, a SSB gives the opportunity to take the BT contribution into account and adjust the SSB dose if necessary.