VALIDATION OF WHOLE BRAIN IRRADIATION WITH METASTASIS SIMULTANEOUS INTEGRATED BOOST AT LIGA CONTRA O CANCER

The use of intensity modulated techniques for whole brain irradiation with simultaneous integrated boost in metastasis and hippocampus sparring is shown in the RTOG 0933. The aim of this work is to validate this technique using an in house built anthropomorphic phantom with heterogeneous materials. The validation consisted in evaluate the uncertainty of the measured dose and the target localization. It was done every step of radiotherapy planning using an in-house bult anthropomorphic phantom, from the tomographic images acquisition to the dose delivery in the machine. It was placed five pieces of graphchromic films EBT3 and a micro ion chamber Exradin A16 (0.007 cc) in our phantom. Physicians drew five metastases like structures in the images at the regions where the films where placed and a sixth structure at the location of the ion chamber. The irradiation plan was done using the Varian Eclipse planning system and AAA calculation algorithm. The planned dose was 300 cGy reported to the whole brain and 400 cGy to the metastases. We followed restrictions recommended by RTOG 0933 in the optimization process, including hippocampus sparing. The graphchromic film was cross-calibrated with the chamber inside our phantom. The A16 ion chamber was cross-calibrated with a farmer chamber and used the bragg-gray formalism to calculate the dose read in the chamber inside a acrylic phantom. The uncertainty measured was out the order of ± 5%. Using the same images it was delineated two gold spheres encrusted within the phantom to simulate two metastases and through the evaluation of their position with respect to the center of the irradiated field we evaluated the delivery error. It was used the MV repositioning of Varian Medical Systems as our localization system. The difference among the measured dose and the calculated dose by the TPS was of 0.1% in the 400 cGy region and 3.7% in the region of 300 cGy measured with ion chamber. The difference of dose with EBT3 films against TPS was 1.5 % in average, and reached maximum of 5 %. The location error for one metastases was, in average, 2 mm and 3 mm maximum. We conclude that for our system it is needed a PTV with margins of at least 3 mm to account the position uncertainty. The tolerance error between the measured dose and the calculated dose for simulators with density heterogeneity is 7 % (TRS 430), The largest error found was 5 %. Then we conclude this technique is validated in our department.

Publication History

Article published online:
23 October 2019