Research for Lateral Penumbra and Dose Distribution When Air Gap Changing in Proton Therapy Case

In the treatment of high-energy protons Air gap (the distance between the patient and the exit Beam) Lateral Penumbra of the changes to the increase in the radiation fields can form unnecessary and Increase the maximum dose at the site of treatment and reduced the minimum dose homogeneity of dose distributions can decline. Air gap due to this change in dose distribution compared to investigate studied. Received proton therapy at our institution Lung, Liver patients were selected and the size of six other Air gap in Field A and Field B 2, 4, 6, 8, 10 cm Proton external beam planning system by setting up a treatment plan established. Air gap according to the Lateral Penumbra area and DVH (Dose Volume Histogram) to compare the maximum dose and minimum dose of PCTV areas were compared. In addition, the dose homogeneity within PCTV Homogeneity index to know the value and compared. Air gap (2, 4, 6, 8, 10 cm) at each change in field size were analyzed according to the Lateral Penumbra region Field A Change in the Air gap 2-10 cm by 1.36-1.75 cm, the average continuously increased about 28.7% and Field B Change in the Air gap 2-10 cm by 1.36-1.75 cm, the average continuously increased about 31.6%. The result of DVH analysis for relative dose of the maximum dose According to Air gap 2-10 cm is the mean average of 110.3% from 108.1% to a sustained increased by approximately 2.03% and The average relative dose of minimum dose is the mean average of 93.9% percent to 90.8 percent from the continuous decrease of about 3.31 percent. The result of Homogeneity index value to the according to Air gap 2-10 cm is the 2-fold increase from 1.09 to 2.6. In proton therapy case, we can see the increasing of lateral penumbra area when airgap getting increase. And increasing of Dmax and decreasing Dmin in the field are making increase homogeneity index, So we can realize there are not so good homogeneity in the PCTV. Therefore we should try to minimize air gap in proton therapy case.