Author: Cheewai Cheng, Indra J. Das, Jeffrey Yui Hei Wong, Poonam Yadav 👨🔬
Affiliation: Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine 🌍
Purpose: Radiation treatment machine replacement brings chaos and urgency of timely completion of machine commissioning for every procedure used including TBI that are time consuming. Based on past experiences and MU calculation, an analytical approach is provided that can be used on a new machine with minimum efforts without compromising the accuracy.
Methods: Our center has been using TBI for the last 25 years and has treated over 1000 pediatrics and adult patients. In the last 10 years, 492 TBI patients were treated on two different Elekta machines with 440.2 cm or 466.3 cm SAD with identical treatment techniques (AP/PA decubitus, 10 MV beam with or without lung blocks). In-vivo dose measurements were verified using either TLD/MOSFET on central axis and off-axis points. The Elekta machines have been replaced with Varian TrueBeams in the past 10 months, thus impetus for this analysis. The past MUs were analyzed with respect to central axis depth (d) as MU = Fx (SSDref/SSDnew)2. Where Fx is either straight line or a 2nd order polynomial in terms of patients’ depth (d), thickness at umbilicus.
Results: MU calculation based on analytical approach derived from 492 patients with central axis depths ranging from 5-26 cm shows either a straight line or 2nd order polynomial. For a reference SAD of 460.2 cm, Fx=1618+ 47.09(d) and Fx=1767.9+21.307(d)+1.057(d2) provide r2 values of 0.994 and 1.00 respectively. This approach is verified using MOSFET system and found to be accurate on the new machines.
Conclusion: Simple analytical functions can be used to fit MU data derived from past TBI patients and used for second MU check on new machines with excellent agreements. The implementation is extremely accurate and straight forward. The method can be used in any department when a machine is replaced as long as treatment technique remains the same.