Technical Note: Improving proton stopping power ratio determination for a deformable silicone-based 3D dosimeter using dual energy CT.

PURPOSE: The aim of this study was to investigate whether the stopping power ratio (SPR) of a deformable, silicone-based 3D dosimeter could be determined more accurately using dual energy (DE) CT compared to using conventional methods based on single energy (SE) CT. The use of SECT combined with the stoichiometric calibration method was therefore compared to DECT-based determination. METHODS: The SPR of the dosimeter was estimated based on its Hounsfield units (HUs) in both a SECT image and a DECT image set. The stoichiometric calibration method was used for converting the HU in the SECT image to a SPR value for the dosimeter while two published SPR calibration methods for dual energy were applied on the DECT images. Finally, the SPR of the dosimeter was measured in a 60 MeV proton by quantifying the range difference with and without the dosimeter in the beam path. RESULTS: The SPR determined from SECT and the stoichiometric method was 1.10, compared to 1.01 with both DECT calibration methods. The measured SPR for the dosimeter material was 0.97. CONCLUSIONS: The SPR of the dosimeter was overestimated by 13% using the stoichiometric method and by 3% when using DECT. If the stoichiometric method should be applied for the dosimeter, the HU of the dosimeter must be manually changed in the treatment planning system in order to give a correct SPR estimate. Using a wrong SPR value will cause differences between the calculated and the delivered treatment plans.

Patient assessed symptoms are poor predictors of objective findings. Results from a cross sectional study in patients treated with radiotherapy for pharyngeal cancer.

INTRODUCTION: The aim of the study was to assess the value of the EORTC questionnaires C30 and H&N35, as an instrument for the study of side effects. PATIENTS AND METHODS: We invited all recurrence free patients, treated with radical radiotherapy for pharyngeal cancer between 1998 and 2002 at our institution, to participate in the study. Data was retrieved using questionnaires, functional endoscopic evaluation of swallowing, dental examination with orthopantomography and saliva flow measurements. Thirty-five (55% of invited) participated. RESULTS: Side effects were omnipresent and often severe. The value of patient assessed symptom scores to predict equivalent objective changes was varied: We found a sensitivity of 0.59-0.93, specificity of 0.40-0.81, positive predictive value of 0.28-0.81 and a negative predictive value of 0.46-0.94. CONCLUSION: The questionnaire should be used to retrieve information on subjective side effects and objective examination should be used for objective changes. One cannot conclude from one to the other.