Role of water in radiochromic LiPCDA monomer crystal packing and radiotherapy dose response

Radiochromic films comprised of lithium-10,12-pentacosa diynoate (LiPCDA) crystals, a form of diacetylene, have been developed for in vivo real-time dosimetry. The polymerization of LiPCDA results in a change in optical density that is related to the absorbed dose. The dose response of diacetylene monomers is dependent on their packing, determined by the R groups. LiPCDA used in commercial film can have two distinct dose sensitive forms, 635LiPCDA and 674LiPCDA, with peak absorbances occurring at 635 nm and 674 nm, respectively. While the two forms do not differ in their R groups, they have different dosimetric behaviours. The 674LiPCDA achieved through desiccation of 635LiPCDA was 3-fold less sensitive to dose and had ~7-fold higher dynamic range. This indicates that the dosimetric behaviour of radiochromic crystals is primarily dependent on structure, controlled by more than just the chemical composition of individual monomers as water appears to have a role in the packing.