Background <p>This study aimed to evaluate radiographic changes in cementless stemmed tumor prostheses used for distal femoral reconstruction due to neoplastic conditions, assess the impact of resection magnitude on these changes, and investigate their associations with mechanical complications.</p> Methods <p>This retrospective study included 31 patients who underwent distal femoral tumor prosthesis reconstruction (2010–2023). Demographic data, tumor characteristics, resection length, stem dimensions, and radiographic parameters (canal filling ratio, bone/stem ratio, femoral cortical index, distal lysis, bone ongrowth) were recorded. Radiographic measurements were averaged from independent evaluations by three orthopedic oncologists. Femoral stems were analyzed across three anatomical zones (zones 1–3).</p> Results <p>In the &lt; 30% resection group, a significant bone/stem ratio reduction was observed in AP Zone 1 (<i>p</i> = 0.039), and the CFR and FCI were reduced in lateral Zones 1 and 2 (<i>p</i> = 0.016, <i>p</i> = 0.013). Distal lysis was significantly greater (<i>p</i> = 0.008), whereas ongrowth was lower (<i>p</i> = 0.011). In the &gt; 30% resection group, significant reductions in the bone/stem ratio were noted in AP Zones 1–3 (<i>p</i> = 0.001) and lateral Zones 1–3 (<i>p</i> = 0.002), with CFR reductions in AP Zones 1–2 (<i>p</i> &lt; 0.05) and lateral Zones 1–2 (<i>p</i> = 0.001). The FCI increased significantly in AP Zone 3 (<i>p</i> = 0.040), and ongrowth was greater (<i>p</i> = 0.011). Revision causes included aseptic loosening in the &lt; 30% group and stem/insert fractures in the &gt; 30% group.</p> Conclusions <p>Distal femoral resection &lt; 30% is associated with reduced bone-prosthesis adaptation, increasing the risk of aseptic loosening, whereas &gt; 30% resection leads to increased mechanical stress, increasing the risk of stem fracture. These findings highlight the need for resection-specific prosthesis designs to optimize long-term outcomes.</p>