Water calorimetry measurements in a 85 MeV clinical proton beam.

The importance of water calorimetry in clinical proton beam dosimetry has increased for various reasons: there has been an increasing interest in proton therapy from the radiotherapy world; existing dosimetry protocols for clinical proton beams recommend calorimetry as the primary dosimetry method (ie, AAPM and ECHED); water has recently become the reference material for dose specification. For the water calorimetric dose evaluation, the water calorimeter developed in Ghent was used. Ionometry was performed following the ECHED protocol. The study resulted in a calorimetric to ionometric dose ratio of 0.974 +/- 0.009. The discrepancy should in our opinion be attributed to the (Wle)p,air value of 35.2 J/C implemented in the protocol, possibly next to ion chamber dependent effects for which indications are found in the ionometry measurements.