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To construct a 3D-printed phantom insert designed to mimic the variable PET tracer uptake seen in lung tumor volumes and a matching dosimetric insert to be used in simultaneous integrated boost (SIB) phantom studies, and to evaluate the design through end-to-end tests.A set of phantom inserts was designed and manufactured for a realistic representation of gated radiotherapy steps from 4D PET/CT scanning to dose delivery. A cylindrical phantom (Φ80 × 120 mm) holds inserts for PET/CT scanning. The novel 3D printed insert dedicated to 4D PET/CT mimics high PET tracer uptake in the core and low uptake in the periphery. This insert is a variable density porous cylinder (Φ44.5 × 70.0 mm), ABS-P430 thermoplastic, 3D printed by fused deposition modeling an inner (Φ11 × 42 mm) cylindrical void. The square pores (1.8 × 1.8 mm2 each) fill 50% of outer volume, resulting in a 2:1 PET tracer concentration ratio in the void volume with respect to porous volume. A matching cylindrical phantom insert is dedicated to validate gated radiotherapy. It contains eight peripheral holes and one central hole, matching the location of the porous part and the void part of the 3D printed insert, respectively. These holes accommodate adaptors for Farmer-type ion chamber and cells vials. End-to-end tests were designed for imaging, planning, and dose measurements.End-to-end test were performed from 4D PET/CT scanning to transferring data to the planning system, target volume delineation, and dose measurements. 4D PET/CT scans were acquired of the phantom at different respiratory motion patterns and gating windows. A measured 2:1 18F-FDG concentration ratio between inner void and outer porous volume matched the 3D printed design. Measured dose in the dosimetric insert agreed well with planned dose on the imaging insert, within 3% for the static phantom and within 5% for most breathing patterns.The novel 3D printed phantom insert mimics variable PET tracer uptake typical of tumors. Obtained 4D PET/CT scans are suitable for segmentation and treatment planning and delivery in SIB gated treatments. Our experiments demonstrate the feasibility of this set of phantom inserts serving as end-to-end quality-assurance phantoms of SIB radiotherapy.