Histopathologic features of retinoblastoma and its relation with in vitro drug resistance measured by means of the MTT assay

Background. Retinoblastoma is frequently treated with chemotherapy to facil itate intraocular therapy, as well as to diminish or delay radiotherapy in invasive disease. It is also used more extensively in patients with dissemi nation to the central nervous system and/or the bone marrow. Once the disea se has spread, the prognosis is poor. Radiotherapy is effective in ocular r etinoblastoma, but is associated with facial deformation and a higher chanc e for second primary tumors in the irradiation field. These sequelae emphas ize the need to determine more effective chemotherapy schedules and local t reatment. The aim of this study is to investigate the relation between in v itro drug resistance for ten cytostatic drugs and histopathologic features in primary retinoblastoma. Materials and methods. Forty-four fresh samples of primary retinoblastoma w ere tested for in vitro drug resistance using the 3-[4,5-dimethylthiazol-2y l]-2,5-diphenyl tetrazolium. bromide (MTT) assay. The histopathologic featu res for differentiation, invasion and intra-ocular extension, necrosis, mit osis, and apoptosis were scored. Results. The differentiation of the tumors revealed 24 poorly differentiate d, 14 intermediately differentiated, and 6 well differentiated tumors. Tumo r infiltration showed 3 minimal and 3 massive choroideal invasions, as well as 21 prelaminary and 2 postlaminary optic nerve invasions. The tumor was unifocal in 16 eyes and multifocal in 28 eyes, with extensive retinal invol vement in 10 eyes and tumor seeding in 21 eyes. The MTT assay was successfu l in 82% of the samples after enzymatic handling of the tumor cells was omi tted. Undifferentiated tumors were more sensitive to carboplatin (p=0.034) and doxorubicin (p=0.025), thiotepa (p=0.051) and ifosfamide (p=0.075) in c omparison to differentiated tumors. Type of retinal involvement, invasion, focality, and seeding did not show a relationship with drug resistance. Cal cified tumors were more resistant to actinomycin D and ifosfamide and more sensitive to vincristine; conversely, apoptotic tumors were more sensitive to ifosfamide and more resistant to vincristine (p=0.027). Necrotic tumors were more sensitive to actinomycin D (p=0.004), and mitotic tumors were mor e sensitive to idarubicin (p=0.026). In 90% of the tumors extreme drug resi stance to cytarabin was present. Conclusions. In retinoblastoma many histopathologic features are related to in vitro drug resistance. Undifferentiated tumors are more sensitive to se veral cytostatic drugs. Calcification and apoptosis show an inverse relatio n with invitro drug resistance to ifosfamide and vincristine. Extreme drug resistance to cytarabin is observed, this drug should not be used in retino blastoma treatment. Cancel, 2001; 92:2933-40. (C) 2001 American Cancer Soci ety.