Akpochafor M O

Akpochafor M O

Lagos University Teaching Hospital, Nigeria

Title: Precision comparison of different monitor unit algorithms using an in-house designed phantom



A phantom for use in radiotherapy treatment planning of human pelvic anatomical region has been designed with six hollows for inserting materials mimicking different biological tissues and the ionization chamber. The yellow plaster of Paris was used to mimic the bone, Styrofoam for the lung and water for soft tissue. The phantom was scanned with Toshiba-Asteion CT-scanner and the images were transferred to the CMS-XiO Treatment Planning System with 3 different algorithms. Measurements of monitor units were conducted using 6 MeV photon beams from the ELEKTA-Precise clinical linear accelerator with iso-centric set up. The test of the phantom was done using Fast Superposition (FSS), the Superposition (S) and the Convolution (C) algorithms. Results with FSS algorithm showed better accuracy than S and C. The standard deviation of measurements with bone heterogeneity for all plans varied between +2% and -3%. FSS has faster computation speed than other algorithms; however C has a good balance of speed versus precision in homogeneous medium. Choice of algorithm for use should not be based on the speed of computation alone but also on the accuracy, especially for applications with modern radiotherapy techniques such as intensity modulated radiation therapy (IMRT).