The present invention provides a kit for immobilization of a patient body part for radiotherapy applications comprising two moldable thermoplastic sheets, a lower sheet for covering the anatomical contours of a first area of said body part and an upper sheet for covering the anatomical contours of a second area of said body part which is not covered by the lower sheet thereby forming a double shell mask enclosing said body part, the sheets are suitable to be connected and retained by a fixation device； wherein said sheets have different physical and visual properties.
