Date Approved


Graduate Degree Type


Degree Name

Medical Dosimetry (M.S.)

Degree Program

Health Professions

First Advisor

Kristen Vu

Academic Year




A mechanical limitation of modern linacs affecting the quality of radiotherapy treatments of large tumor volumes is the travel length of multicollimator leaves. Considering this limitation, research was carried out to evaluate the dosimetric differences between a split-field fixed-jaw volumetric arc radiotherapy (SF-FJ VMAT) technique and a jaw tracking VMAT (JT VMAT) technique for the treatment of prostate cancer with involved lymph nodes.


Plans for 16 patients treated with the same dose regimen were created using both techniques. Initial JT-VMAT plans were planned with two arcs to encompass the entire tumor volume without jaw restrictions and jaw tracking feature activated. SF-FJ VMAT initial plans utilized three arcs with each arc focused on different areas of the tumor volume and X-jaws restricted to ≤15cm. Identical boost plans were used for each patient. Dosimetric and statistical comparison of plans were conducted using dose value histogram data and paired t-tests.


Monitor units were found to be on average much higher (1028.04 ± 149.45 MUs) in the SF-FJ VMAT plans while average PTV45 V105 (1.34 ± 1.52 cGy) and average PTV45 D2% (8278.23 ± 54.54 cGy) were found to be significantly higher in the JT VMAT plans after statistical analysis. Sparing of small bowel and rectum was very similar. Combined femurs had statistically significant higher average max dose (4659.27 ± 374.02 cGy) when using JT VMAT technique. Bladder sparing was mixed with average V80 (5.22 ± 4.21 cGy) and mean dose (4507.34 ± 826.21 cGy) being greater in the SF-FJ VMAT plans while V50 (33.01 ± 18.55 cGy) was greater in the JT VMAT plans.


Dosimetric advantages were found to be mostly in favor of the SF-FJ VMAT plans, however the resulting higher monitor units can lead to significantly longer treatment times. A cost-benefit analysis regarding potential advantages of this technique will have to be undertaken on a case by- case basis with the decision ultimately being physician-dependent.