Session Item

The Venezia Advanced Gyneacological applicator developed by Elekta is an insterstitial-intracavitary hybrid applicator. This work reports on some discrepancies encountered between the expected dwell positions and the actual source positions in the commissioning process of the 26 mm diameter Venezia applicator.

We obtained and compared the most distal source position in relation to the applicator by three methods: i) using the Oncentra applicator library (central path (C) and “real” source path (SP)); ii) by auto-radiography (AR) using Gafchromic EBT3 (a distance of 1300 mm was entered following manufacturer specifications); and iii) X-Ray imaging (RX) with the Varian OBI imaging system. For the last, the source trajectory was reproduced with a dummy source and the applicator was held in horizontal position in order to simulate clinical conditions.

A relevant deviation from the expected was detected for the first position in the semilunar ovoid 2 (Ov2, table 1) when using auto-radiography: 3mm to that predicted by SP. After further investigation, we determined that the source was not able to reach positions beyond the 4^th position, which resulted in an overdosage at that position. This problem was attributed to internal friction, although Flexitron did not warn about any obstruction.

In order to establish possible clinical implications if the defect had not been detected, we calculated differences in dose delivery (shown in table 2) at several points around the applicator (image 1). Calculation was performed using the TG43 algorithm implemented in OncentraBrachy 4.5.3. treatment planning system.

Despite the resulting dosimetric differences not being relevant clinically in principle (points 3 to 10), the Ov2 was replaced with a new one. The commissioning of the new Ov2 resulted in no important geometric discrepancies (less than 1mm).

Brachytherapy entails a lot of uncertainties, so all personnel involved should ensure that uncertainties are minimized. Specifically, the medical physicist should perform the necessary measures to know in detail their applicators and ensure that they are suitable for clinical use.