Estou tentando buscar dados específicos de um documento XML usando a instrução sql. Este documento é o seguinte:

<?xml version="1.0" encoding="Shift-JIS" ?>
<Ophthalmology xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:nsCommon="http://www.joia.or.jp/standardized/namespaces/Common" xmlns:nsREF="http://www.joia.or.jp/standardized/namespaces/REF" xsi:schemaLocation="http://www.joia.or.jp/standardized/namespaces/Common Common_schema.xsd http://www.joia.or.jp/standardized/namespaces/REF REF_schema.xsd">
	<nsCommon:Common>
		<nsCommon:Company>TOPCON</nsCommon:Company>
		<nsCommon:ModelName>KR-800</nsCommon:ModelName>
		<nsCommon:MachineNo>01</nsCommon:MachineNo>
		<nsCommon:ROMVersion>1.05.08</nsCommon:ROMVersion>
		<nsCommon:Version>1.2</nsCommon:Version>
		<nsCommon:Date>2016-09-17</nsCommon:Date>
		<nsCommon:Time>01:17:35</nsCommon:Time>
		<nsCommon:Patient>
			<nsCommon:No.>0006</nsCommon:No.>
			<nsCommon:ID>0006</nsCommon:ID>
			<nsCommon:FirstName></nsCommon:FirstName>
			<nsCommon:MiddleName></nsCommon:MiddleName>
			<nsCommon:LastName></nsCommon:LastName>
			<nsCommon:Sex></nsCommon:Sex>
			<nsCommon:Age></nsCommon:Age>
			<nsCommon:DOB></nsCommon:DOB>
			<nsCommon:NameJ1></nsCommon:NameJ1>
			<nsCommon:NameJ2></nsCommon:NameJ2>
		</nsCommon:Patient>
		<nsCommon:Operator>
			<nsCommon:No.></nsCommon:No.>
			<nsCommon:ID></nsCommon:ID>
		</nsCommon:Operator>
	</nsCommon:Common>
	<nsREF:Measure type="REF">
		<nsREF:VD unit="mm">13.75</nsREF:VD>
		<nsREF:DiopterStep unit="D">0.25</nsREF:DiopterStep>
		<nsREF:AxisStep unit="deg">1</nsREF:AxisStep>
		<nsREF:CylinderMode>-</nsREF:CylinderMode>
		<nsREF:REF>
			<nsREF:R>
				<nsREF:List No="1">
					<nsREF:Sphere unit="D">0.50</nsREF:Sphere>
					<nsREF:Cylinder unit="D">-1.25</nsREF:Cylinder>
					<nsREF:Axis unit="deg">178</nsREF:Axis>
					<nsREF:SE unit="D">-0.25</nsREF:SE>
					<nsREF:CataractMode></nsREF:CataractMode>
					<nsREF:IOLMode></nsREF:IOLMode>
					<nsREF:ConfidenceIndex></nsREF:ConfidenceIndex>
				</nsREF:List>
				<nsREF:List No="2">
					<nsREF:Sphere unit="D">0.50</nsREF:Sphere>
					<nsREF:Cylinder unit="D">-1.00</nsREF:Cylinder>
					<nsREF:Axis unit="deg">177</nsREF:Axis>
					<nsREF:SE unit="D">0.00</nsREF:SE>
					<nsREF:CataractMode></nsREF:CataractMode>
					<nsREF:IOLMode></nsREF:IOLMode>
					<nsREF:ConfidenceIndex></nsREF:ConfidenceIndex>
				</nsREF:List>
				<nsREF:Median>
					<nsREF:Sphere unit="D">0.50</nsREF:Sphere>
					<nsREF:Cylinder unit="D">-1.00</nsREF:Cylinder>
					<nsREF:Axis unit="deg">177</nsREF:Axis>
					<nsREF:SE unit="D">0.00</nsREF:SE>
				</nsREF:Median>
			</nsREF:R>
			<nsREF:L>
				<nsREF:List No="1">
					<nsREF:Sphere unit="D">0.50</nsREF:Sphere>
					<nsREF:Cylinder unit="D">-0.50</nsREF:Cylinder>
					<nsREF:Axis unit="deg">179</nsREF:Axis>
					<nsREF:SE unit="D">0.25</nsREF:SE>
					<nsREF:CataractMode></nsREF:CataractMode>
					<nsREF:IOLMode></nsREF:IOLMode>
					<nsREF:ConfidenceIndex></nsREF:ConfidenceIndex>
				</nsREF:List>
				<nsREF:List No="2">
					<nsREF:Sphere unit="D">0.50</nsREF:Sphere>
					<nsREF:Cylinder unit="D">-0.50</nsREF:Cylinder>
					<nsREF:Axis unit="deg">179</nsREF:Axis>
					<nsREF:SE unit="D">0.25</nsREF:SE>
					<nsREF:CataractMode></nsREF:CataractMode>
					<nsREF:IOLMode></nsREF:IOLMode>
					<nsREF:ConfidenceIndex></nsREF:ConfidenceIndex>
				</nsREF:List>
				<nsREF:List No="3">
					<nsREF:Sphere unit="D">0.50</nsREF:Sphere>
					<nsREF:Cylinder unit="D">-0.50</nsREF:Cylinder>
					<nsREF:Axis unit="deg">179</nsREF:Axis>
					<nsREF:SE unit="D">0.25</nsREF:SE>
					<nsREF:CataractMode></nsREF:CataractMode>
					<nsREF:IOLMode></nsREF:IOLMode>
					<nsREF:ConfidenceIndex></nsREF:ConfidenceIndex>
				</nsREF:List>
				<nsREF:Median>
					<nsREF:Sphere unit="D">0.50</nsREF:Sphere>
					<nsREF:Cylinder unit="D">-0.50</nsREF:Cylinder>
					<nsREF:Axis unit="deg">179</nsREF:Axis>
					<nsREF:SE unit="D">0.25</nsREF:SE>
				</nsREF:Median>
			</nsREF:L>
		</nsREF:REF>
		<nsREF:PD>
			<nsREF:WorkingDistance unit="cm"></nsREF:WorkingDistance>
			<nsREF:Distance unit="mm">68.50</nsREF:Distance>
			<nsREF:Near unit="mm">68.50</nsREF:Near>
		</nsREF:PD>
	</nsREF:Measure>
</Ophthalmology>

Digitei minha consulta sql para buscar o valor da esfera mediana da seguinte maneira:

SELECT RSphere
              FROM XMLTABLE (
                      '/nsREF:Measure/nsREF:REF/nsREF:R/nsREF:Median/nsREF:Sphere'
                      PASSING XMLTYPE (BFILENAME ('MY_DIR', 'aaa.xml'),
                                       NLS_CHARSET_ID ('Shift-JIS'))
                      COLUMNS RSphere VARCHAR2 (255) PATH 'text()')

Mas estou recebendo erro

[Erro] Execução (4: 17): ORA-19228: XPST0008 - identificador não declarado: prefixo 'nsREF' nome local 'nsREF: Medida'

Você poderia me ajudar a obter o valor correto