Background <p>Tumour dosimetry after radionuclide therapy using ¹⁷⁷Lu-labelled radiopharmaceuticals requires determination of the ¹⁷⁷Lu mean concentration, but this is challenging as tumours are often small, or ¹⁷⁷Lu activity is present in nearby organs or other tumours. Here we present a comparison of methods for determination of ¹⁷⁷Lu mean concentration, and in turn absorbed tumour dose, applied to a small number of patients with prostate cancer treated by [¹⁷⁷Lu]Lu-PSMA I&amp;T. For application of each method, specific criteria on tumour diameter and tumour-background ratio must be fulfilled.</p> Results <p>Eighteen tumours in 9 patients were analyzed. Several methods, the so-called Small Volume of Interest (VOI) with a 20&#xa0;mm diameter sphere, Large VOI and Isocontour methods, were found to be in good agreement. Relative to the chosen reference method (Isocontour method with partial volume correction), the relative percentage differences of ¹⁷⁷Lu concentration using either of these methods were in the range (-23)–26%. The relative differences of absorbed doses were in the range (-16)–19%.</p> Conclusions <p>The agreement between the methods permits a comparison between dosimetry studies, where some of these methods are applied. As the application criteria are complementary, it is possible to include both small (&gt; 15&#xa0;mm diameter) solitary tumours and larger (&gt; 30&#xa0;mm diameter), possibly non-solitary, tumours in a dosimetry study.</p>