ESTRO 2021 Abstract Book

S1367

ESTRO 2021

number or SPR. The largest difference was observed around the tumor where the dose was highest (Figure 1). The average dose difference for the target was highest for the liver plan (Table 1). The amount of lung tissue traversed by the beams in the liver plan was substantially higher than for HN and prostate plan; this could mean that there is a larger difference between the two range uncertainty strategies in lung tissue.

Conclusion There is a difference in dose distribution when adding range uncertainty to the CT number or SPR. Even though the difference is minor, considerations must be made when choosing the range uncertainty strategy, and values cannot simply be taken from the literature without context. PO-1647 linac activation of radioisotopes and underground gammaspectroscopic analyses P. Weber 1 , J. Vuilleumier 2 , G. Guibert 1 1 RHNe, Radio-oncology, la Chaux-de-Fonds, Switzerland; 2 Bern University, Physics Department, Bern, Switzerland Purpose or Objective After irradiating various medical linac parts with photon beams, underground gamma-ray spectroscopy (GRS) measurements of the samples were performed a few minutes after the irradiations, in order to observe activated short-lived radioisotopes. Materials and Methods Three samples have been used: an Elekta Synergy flattening filter never irradiated before, an old flattening filter, unmounted from a linac in 2012 and a tungsten leaf, unmounted from the multileaf collimator (MLC) of the same linac in 2012. All three samples were first measured in the underground laboratory of la Vue-des- Alpes. This underground location of the detector is providing a natural shielding against cosmic rays corresponding to 600 meters water-equivalent, reducing the cosmic muon flux by a factor 1000, and completely suppressing the neutrons. The laboratory is equipped with an ultralow noise germanium detector, protected against the rock radioactivity with an ultraclean lead and copper shielding. Moreover, the detector is enclosed in an airtight box, flushed out with nitrogen, in order to avoid radon to enter the detector. Results A residual activity of 236.9 Bq was measured in the old filter with long-lived radioisotopes; with this work, it was possible to identify them: 57Co (T1/2=271.8 days), 54Mn (T1/2=312.3 days) and 60Co (T1/2=5.27 years). Prior to any irradiation, the tungsten leaf and the new filter were gamma counted, and showed only tiny amounts of trace impurities of 238U, 57Co, 54Mn and 60Co, with a total activity of 0.51 and 0.02 Bq, respectively. The new flattening filter and the leaf were then irradiated with ~200 Gy, under 15MV and 6MV