Purpose <p>This study aims to quantify and to correct for partial volume effect (PVE) in lymph node metastases of prostate cancer (PC) on Lu-177 SPECT images using 3D-printed phantoms of these structures to establish a ground truth.</p> Procedures <p>Ten individuals with clearly delineated, SPECT-positive lymph node metastases were retrospectively selected from a cohort of PC patients undergoing radioligand therapy (RLT). Manual segmentation of the metastases was performed on the CT component. The segmented lymph nodes were 3D-printed as patient-specific lymph node phantoms with volumes ranging from 0.18 to 23.7&#xa0;ml using a high-resolution 3D printer. These phantoms were filled with Lu-177 and analyzed in a SPECT/CT system to quantify PVE. An exponential curve fit of the recovery coefficient (RC) versus the surface-area to volume ratio (SA:V) was derived from the spheres of a NEMA ICE body phantom and used to correct for PVE in the lymph node phantoms. This method was compared with other post-reconstruction partial volume corrections (PVC).</p> Results <p>For a tumor to background ratio (TBR) of 10:1 the RCs varied widely, from 82% to 10.4% depending on phantom size. Using the NEMA IEC body phantom, an exponential correlation was established between SA:V and RC, with R² values exceeding 0.97 across measurements at four different TBRs. Using this curve for PVE correction, the RCs of the lymph node phantoms had an average deviation from the ground truth of 1.15 ± 10.15% (average ± standard error of the mean). This method had a higher accuracy than the other PVCs studied.</p> Conclusions <p>The low RCs obtained for lymph node metastases suggest that Lu-177-dosimetry is in these structures grossly inaccurate without PVE correction. Applying the SA:V/RC curve established using the NEMA IEC body phantom for this purpose reduces PVE-related errors considerably.</p>