Early clinical outcome of coverage probability based treatment planning for simultaneous integrated boost of nodes in locally advanced cervical cancer

Introduction: More than 50% of patients with locally advanced cervical cancer (LACC) have pathological nodes. Coverage probability (CovP) is a new planning technique allowing for relaxed dose at the boost periphery minimising collateral irradiation. The aim was to report the first early clinical outcome data for CovP based simultaneous integrated boost (SIB) in LACC. Material and methods: Twenty-three consecutive node positive patients were analysed. FIGO stage IB2/IIB/IIIB/IVA/IVB was 1/14/3/1/4. Treatment was radio(chemo) therapy (RT) delivering 45 Gy/25 fx whole pelvis +/- para-aortic region (PAN) using volumetric arc therapy (VMAT) followed by magnetic resonance imaging (MRI) guided brachytherapy. PAN RT (13 pts) was given if > 2 nodes or if node(s) were present at the common iliac vessels or PAN. Nodal gross tumour volumes (GTV-N) were contoured on both PET-CT and MRI. Clinical target volume (CTV-N) was formed by fusion of GTV-NCT and GTV-NMRI. A 5-mm isotropic margin was used for planning target volume (PTV-N). Nodes in the small pelvis were boosted to 55.0 Gy/25 fx. Common iliac and para-aortic nodes received 57.5 Gy/25 fx. Planning aims for CovP were PTV-N D-98 >= 90%, CTV-N D-98 >= 100% and CTV-N D50 >= 101.5%. Results: Seventy-four nodes were boosted. A consistent 5.0 +/- 0.7 Gy dose reduction from CTV-N D-98 to PTV-N D-98 was obtained. In total, 73/74 nodes were in complete remission at 3 months PET-CT and MRI. Pelvic control was obtained in 21/23 patients. One patient (IB2, clear cell) had salvageable local disease, while another (IIB) failed in a boosted node. Two patients failed in un-irradiated PAN. One patient age 88 (IIIB) did not receive PAN RT, despite a common iliac node. The other (IIB) recurred above L1. Two further patients (IVB) failed systemically. Conclusion: Since complete remission at 3 months is predictive for favourable long-term nodal control, our study indicates that CovP for SIB is promising.