A compound of the formula wherein: R 1, R 2, R 3 and R 4 are selected from an electron withdrawing group, -NO 2, -NH 2, halogen or a substituent represented by the following formula wherein Y can be on the ortho, meta or para position on the phenyl rings, and is selected from hydrogen, hydrocarbyl, non-aromatic carbocyclic, non-aromatic heterocyclic, aryl, alkylaryl, arylalkyl; or a hydrocarbyl, non-aromatic carbocyclic, non-aromatic heterocyclic, aryl, alkylaryl or an arylalkyl group substituted with 1 to 4 hydrophilic groups selected from hydroxy, alkoxy, -C OR 5, -SOR 6, -SO 2 R 6, nitro, amido, ureido, carbamato, -SR 7, -NR 8 R 9 or poly-alkyleneoxide; or a substituent represented by formula -X-(CR 10 R 11) r -Z; provided that at least one of R 1, R 2, R 3 and R 4 is the substituent represented by formula wherein Y represents formula; wherein: X is selected from oxygen or sulfur; R 5, R 6, R 7, R 8, R 9, R 10 and R 11 are selected from hydrogen and C 1 to C 4 hydrocarbyl; Z is a carborane cluster comprising at least two carbon atoms and at least three boron atoms, or at least one carbon atom and at least five boron atoms, within a cage structure; r is 0 or an integer from 1 to 20; a represents an integer from 1 to 4; and provided also that at least one of R 1, R 2, R 3 and R 4 is an electron withdrawing group, -NO 2, -NH 2 or halogen; and M is selected from two hydrogen ions, a single monovalent metal ion, two monovalent metal ions, a divalent metal ion, a trivalent metal ion, a tetravalent metal ion, a pentavalent metal ion or a hexavalent metal ion, wherein the porphyrin-metal complex derived from a single monovalent metal ion is charge-balanced by a counter cation, and the porphyrin-metal complex derived from a trivalent, tetravalent, pentavalent or hexavalent metal ion is charge-balanced by an appropriate number of counter anions, dianions or trianions for use in treatment of cancer by X-ray therapy by administration of a single dose of the compound not more often than once every 2 weeks. R 1, R 2, R 3 and R 4 are selected from an electron withdrawing group, -NO 2, -NH 2, halogen or a substituent represented by the following formula wherein Y can be on the ortho, meta or para position on the phenyl rings, and is selected from hydrogen, hydrocarbyl, non-aromatic carbocyclic, non-aromatic heterocyclic, aryl, alkylaryl, arylalkyl; or a hydrocarbyl, non-aromatic carbocyclic, non-aromatic heterocyclic, aryl, alkylaryl or an arylalkyl group substituted with 1 to 4 hydrophilic groups selected from hydroxy, alkoxy, -C OR 5, -SOR 6, -SO 2 R 6, nitro, amido, ureido, carbamato, -SR 7, -NR 8 R 9 or poly-alkyleneoxide; or a substituent represented by formula -X-(CR 10 R 11) r -Z; provided that at least one of R 1, R 2, R 3 and R 4 is the substituent represented by formula wherein Y represents formula; wherein: X is selected from oxygen or sulfur; R 5, R 6, R 7, R 8, R 9, R 10 and R 11 are selected from hydrogen and C 1 to C 4 hydrocarbyl; Z is a carborane cluster comprising at least two carbon atoms and at least three boron atoms, or at least one carbon atom and at least five boron atoms, within a cage structure; r is 0 or an integer from 1 to 20; a represents an integer from 1 to 4; and provided also that at least one of R 1, R 2, R 3 and R 4 is an electron withdrawing group, -NO 2, -NH 2 or halogen; and wherein Y can be on the ortho, meta or para position on the phenyl rings, and is selected from hydrogen, hydrocarbyl, non-aromatic carbocyclic, non-aromatic heterocyclic, aryl, alkylaryl, arylalkyl; or a hydrocarbyl, non-aromatic carbocyclic, non-aromatic heterocyclic, aryl, alkylaryl or an arylalkyl group substituted with 1 to 4 hydrophilic groups selected from hydroxy, alkoxy, -C OR 5, -SOR 6, -SO 2 R 6, nitro, amido, ureido, carbamato, -SR 7, -NR 8 R 9 or poly-alkyleneoxide; or a substituent represented by formula -X-(CR 10 R 11) r -Z; provided that at least one of R 1, R 2, R 3 and R 4 is the substituent represented by formula wherein Y represents formula; wherein: X is selected from oxygen or sulfur; R 5, R 6, R 7, R 8, R 9, R 10 and R 11 are selected from hydrogen and C 1 to C 4 hydrocarbyl; Z is a carborane cluster comprising at least two carbon atoms and at least three boron atoms, or at least one carbon atom and at least five boron atoms, within a cage structure; r is 0 or an integer from 1 to 20; a represents an integer from 1 to 4; and provided also that at least one of R 1, R 2, R 3 and R 4 is an electron withdrawing group, -NO 2, -NH 2 or halogen; and X is selected from oxygen or sulfur; R 5, R 6, R 7, R 8, R 9, R 10 and R 11 are selected from hydrogen and C 1 to C 4 hydrocarbyl; Z is a carborane cluster comprising at least two carbon atoms and at least three boron atoms, or at least one carbon atom and at least five boron atoms, within a cage structure; r is 0 or an integer from 1 to 20; a represents an integer from 1 to 4; and M is selected from two hydrogen ions, a single monovalent metal ion, two monovalent metal ions, a divalent metal ion, a trivalent metal ion, a tetravalent metal ion, a pentavalent metal ion or a hexavalent metal ion, wherein the porphyrin-metal complex derived from a single monovalent metal ion is charge-balanced by a counter cation, and the porphyrin-metal complex derived from a trivalent, tetravalent, pentavalent or hexavalent metal ion is charge-balanced by an appropriate number of counter anions, dianions or trianions for use in treatment of cancer by X-ray therapy by administration of a single dose of the compound not more often than once every 2 weeks.