EGFR L858R lung adenocarcinoma presenting as rectal and peritoneal metastases

What happened, in summary

This 79-year-old Han Chinese man had no smoking or alcohol history. In late October 2024, constipation began and gradually worsened. By late December, colonoscopy at another hospital showed a circumferential irregular rectal lesion above the dentate line with luminal narrowing and contact bleeding, but biopsy showed only chronic inflammation. In early January 2025, he developed worsening constipation plus dyspnea, cough, sputum, lower-limb weakness, and poor appetite. Chest CT showed a suspicious mixed ground-glass nodule in the right upper lung lobe, about 17 x 13 x 11 mm, with obstructed adjacent bronchus and mediastinal lymph-node enlargement. Because of his age, his family declined percutaneous lung biopsy. Repeat rectal evaluation showed a hard circumferential mass near the anal verge, a 7.3 cm mid-to-distal rectal mass on MRI, and severe narrowing. Ultrasound-guided rectal puncture biopsy finally showed poorly differentiated adenocarcinoma. The immunoprofile, including CK7 positivity, TTF-1 positivity, Napsin A positivity, and CK20/CDX2 negativity, favored lung origin rather than primary rectal cancer. On January 22, 2025, he underwent laparoscopic transverse colostomy to relieve bowel dysfunction and peritoneal biopsy. Surgeons saw numerous small nodules over the colonic wall, peritoneum, pelvic wall, omentum, and mesentery. Biopsy confirmed metastatic adenocarcinoma consistent with lung origin. Molecular testing of the rectal metastasis found EGFR L858R and TP53 G245D/R213X, with no actionable ALK, ROS1, BRAF, KRAS, RET, MET, HER2, or NTRK alteration. He started aumolertinib on February 4, 2025, and later received palliative radiotherapy to the rectal lesion on March 24. Clinical stabilization was described after targeted therapy, with longer follow-up still limited.