swelling in his knees/ that evening both of his knees began to swell; This is a spontaneous report received from a contactable consumer (patient's mother) via Pfizer sponsored program. A 43-year-old male patient received first dose of BNT162B2 (PFIZER-BIONTECH COVID-19 mRNA VACCINE, solution for injection) via an unspecified route of administration, administered in arm on 28May2021 (09:15 or 10:15 AM, before noon) (at the age of 43-year-old) as single dose for COVID-19 immunization. Medical history included ongoing stage 4 metastatic lung cancer from 2019, son went into remission. Concomitant medications were not reported. Patient did not have any history of all previous immunization with the Pfizer vaccine and not received any additional vaccines administered on same date of the Pfizer suspect. The patient did not receive any other vaccines within 4 weeks prior to the vaccination (he never had the flu or pneumonia shot since he has been grown). Patient did not have family medical history relevant to adverse events. She states that her patient went into remission but then the cancer came back, but the cancer is not as bad this time as it was before. She states that they cannot get rid of it in the two spots that it is in this time and that is why he was getting radiation. She was not sure about the exact dates but states that he went for his first treatments in May2019 and thinks that he might have gotten diagnosed in January, by the time they did biopsies. She states that patient got the vaccine on 28May2021. After he received the vaccine, that same night he began to have swelling in his knees. She states that she believes that her son received the vaccine on the 28May2021, in the morning. That evening both of his knees began to swell (28May2021). She states that he has stage 4 metastatic lung cancer. The year before he went through chemotherapy and everything was fine and then the cancer came back and he was going through treatment and the chemotherapy was not working. She states that her son received the shot that morning and then that evening his knees started to swell. He is still experiencing the swollen knees as of today, but he has never had arthritis or anything like that. When asked for the indication, she stated that her son was at his consultation for radiation because the chemotherapy was not working and the cancer was beginning to come back, so he was taking chemotherapy. His Oncologist saw that it was not working so they referred him to a Radiologist. States, her son received the vaccine in his arm, but she does not know which arm. She states that this was the reason his knee should not have swollen up, his arm should have swollen up. Stated patient was supposed to receive his second dose of the vaccine on 16Jun2021. Caller stated that patient had Stage 4 metastatic cancer but he does not have arthritis. She stated that in patient's ultrasound result, there were no blood clots in his knees but they are still swollen. Stated received his first radiation treatment yesterday as well as the ultrasound, and that he saw his Primary care physician yesterday. His doctor sent him to the hospital to have the ultrasound of his knee done to check for blood clotting. The caller confirms that there was no hospital admission, and she clarifies that no blood clots were present in his knees. The caller clarified that he got the ultrasound done at a hospital. Stated that the only thing her son had were the side effects from chemotherapy. The patient did not do anything to treat his swollen knees because he was not a pill taker. Patient not visited emergency room and visited physician office due to the event. Investigation assessment was done. The patient underwent lab tests and procedures which included Ultrasound: no blood clots in his knees but they are still swollen on 02Jun2021. The outcome of event was not recovered.   Information on the lot/batch number has been requested.