Whole Body Activity Retentions in the Peptide Receptor Radionuclide Therapy with Lu-177

Whole Body Activity Retentions in the Peptide Receptor Radionuclide Therapy with Lu-177

Liu, B.

Wolterbeek, H.T. (promotor)

Applied Sciences

Radiation, Radionuclides & Reactors

2013-12-16

The patients with the neuroendocrine tumours (liver, spleen, etc.) often need treatment by the Peptide Receptor Radionuclide Therapy with 177Lu. The amount of 177Lu activity in the body of patients has to be known accurately for assessment of the dosimetry and for evaluation of the effectiveness of the therapy. Whole-body activities at the given time after administration can be derived from measurements of the activity of a sub-sample from the excreted urine, collected by the patient him/herself. This approach has to be abandoned because it increases the radiation dose of analysts and nurses involved. Moreover, the collection is patient unfriendly; there is a high risk of contamination by spills, and therefore, a risk of incomplete collection. As such, it is not unlikely that the urine collection method results in an underestimated indication of the total amount of activity actually excreted. As a consequence, the urine collection method may render an overestimated value of the whole-body activity. On the other hand, it is an intrinsic problem that the measurement uncertainty of the whole-body activity derived from the urine collection method increases when the whole-body activity decreases with time. As an example, the expanded uncertainties of the whole-body activity were about ±4% (the coverage factor k=2) shortly after administration and up to >±20% at 24 h; however all expanded uncertainties of the activity of the collected urine were about ±3.4%. The objective of the research described in this thesis was to develop a patient friendly technique for an accurate estimate of the whole-body activity of 177Lu as an alternative for the collection of the excreted urine by patient themselves. Two methods, (i) direct measurement of the activity of the excreted urine in the toilet pot and (ii) paired whole-body measurements, have been developed to overcome the concerns. In the first method, the activity of the excreted urine was measured in the toilet pot itself. A small CeBr3 detector is positioned on the side wall of the toilet room, aligned to centre of the water in the toilet pot. The patient uses the toilet as normal but does not need to flush it. The only modification for the current toilet system is to replace the mechanical flushing by a delayed flushing. The activity of the excreted urine is measured directly in the toilet pot after the patient has left the room. This approach is friendly and comfortable for patients and does not lead to an enhanced radiation dose to the staff that otherwise need handling the collected urine. The effects of urine volume and the voiding flow rate were investigated with 177Lu mock-up urine solutions. The expanded uncertainty varied from ±5 to ±10 %, dependent on the occurrence of diarrhea or extreme voiding styles. Therefore measurement uncertainties of the derived whole-body activity were equivalent to the values derived from the urine collection method or even a little bit worse. As an alternative, a radiation measurement system was designed by which the activity in the patient’s body can be measured directly by positioning the patient between two CeBr3 detectors for simultaneous measurements. Whole-body activities during subsequent measurements are commonly normalized to the administered activity before the first voiding. The detector responses of the measurement system are affected by the fully filled bladder during this measurement before the first voiding and by activity redistributions in between subsequent measurements. This was confirmed by measurements with patients, in which the geometric mean value of the count rates of the two detectors after voiding differed from that before the next voiding and by comparison with the results from the urine collection method. This problem could be largely overcome by a series of paired measurements before and after each voiding consecutively, from which time-dependent detector responses were derived. The whole-body activities were then determined accurately using these time-dependent responses, with measurement uncertainties of about ±7%. The results around 1 h after administration have been validated by measurements of the collected first excreted urine from 5 patients. The uncertainty of the whole-body dose is an essential quantity in the individual dosimetry of radionuclide therapy and is mainly dependent on the time-integrated activity coefficient, which is calculated by the bi-exponential regression of the time-activity curve. Using the trapezoidal area under the measurement curve as a reference, the uncertainty of the time-integrated activity coefficient was estimated from the three components, e.g. the uncertainty of the trapezoidal area under the measurement curve, the sum of squares of fitting residuals and the bias between the fitting and trapezoidal areas. The expanded uncertainty in measurement is about ±4% for the new paired measurements method. The toilet measurement set-up is in principle ready for implementation but requires additional modifications of the logistics in the use of the toilet, such as a delayed flushing and blocking of the access during the measurements. The whole-body measurement system is ready for implementation. Handling of radioactive urine is not needed any more. The approaches meet the objective: a reduction of the burden to radiotherapy patients and analysts in the assays of the amount of activity in the patient body.

PRRT
whole-body dose
activity in the excreted urine
activity in the whole-body
whole-body measurements
uncertainty in measurement

https://doi.org/10.4233/uuid:3db90695-a6cb-475a-aa87-6a33d874996c

IOS Press

9781614993643

Institutional Repository

doctoral thesis

(c) 2013 Liu, B.