In the case that you have lymphoid tumors in your pheasants the following information may be of interest:
ALV subgroup F was found in pheasants including ring-necked pheasants (see also DOP 14th ed, vol I, page 600). I checked PubMed using Pheasants and lymphoid leukosis as search terms and found 30 references (e.g., Simon MC, Neckameyer WS, Hayward WS, Smith RE. Genetic determinants of neoplastic diseases induced by a subgroup F avian leukosis virus. J Virol. 1987 Apr;61(4):1203-12). There is also an ALV subgroup G described in other pheasant species then ring-necked pheasants (see PubMed).
More interesting is the report By Dr. Dren from Hungary where he described an outbreak of tumors in pen-reared pheasants. He linked that to Reticuloendothelios virus (Dren CN, Saghy E, Glavits R, Ratz F, Ping J, Sztojkov V. Lymphoreticular tumour in pen-raised pheasants associated with a reticuloendotheliosis like virus infection. Avian Pathol. 1983;12(1):55-71). From my perspective REV would be high on my list to investigate. REV is known to cause tumors in many bird species
Finally Marek’s disease virus. DOP 14th and 13th ed refer to the 12th edition which I do not have at home, nor do I have the old typescript of the 12th or 11th editions. I do remember that Julius Fabricant tried to infect pheasants with MDV but he did not publish the results and I do not feel comfortable to state with certainty the results. I believe that he did not see tumor development but there may have been virus re-isolation. It is possible that the results were mentioned in earlier editions of DOP. PubMed showed one publication: Lesník F, Pauer T, Vrtiak OJ, Danihel M, Gdovinová A, Gergely M. Prenos Markovej choroby na vol'ne zijúcu pernatú zver [Transmission of Marek's disease to wild feathered game]. Vet Med (Praha). 1981 Oct;26(10):623-30. Slovak. The abstract is in English.
May not be a tumor, difficult to say from the pic. Collect samples ann send to any oncology lab for histopathological confirmation, try collecting sample from live birds for virus isolation and identification.