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Verification of Stereotactic Radiation Therapy Plan with Small Fields by Using Portal Dosimetry and A26 Ionization Chamber

D Zhu , Z Huang*, Y Feng , East Carolina Univ, Rockville, MD


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To verify Stereotactic Radiation Therapyâ‚?SRTâ‚Žplan with small fields by using portal dosimetry device and A 26 ionization chamber.

Methods: MRI and CT images were taken and fused together. GTV was contoured based on MRI images. 2 mm margin was added to create PTV. PTV volume was 1.07 cm3. The prescription dose was 30 Gy/5fxs. VMAT technique was used to develop a treatment plan with 3 partial arcs in Elcipse (version 11). MAX-HD SRS phantom with Exradin A26 chamber were scanned in CT-simulation room and imported into Eclipse. A verification plan was created. The collection volume of A26 was contoured as a structure in Eclipse. 3 partial arcs were aligned to the collection volume of A26. In treatment room, MAX-HD SRS phantom was placed on the 6 degrees of freedom head adjuster couch top. CBCT was performed to align phantom to the treatment position. Treatment plan was delivered by using VARIAN Trilogy machine. The measured dose was compared to the mean dose of collecting volume of SRT verification plan. Dose distribution was verified by using portal dosimetry device.

Results: In portal dosimetry software, using the criteria of improved gamma evaluation with 2.0% dose tolerance and 2.0mm DTA, area gamma passing percentage was 99.6%, 99.0%, and 98.7% for 1st, 2nd, and 3rd partial arcs, respectively. The difference between measured dose and planned dose was -3.77%, 2.12%, and -2.08% for 1st, 2nd, and 3rd partial arcs, respectively. The difference between total measured dose and total planned dose was -1.63%.

Conclusion: Portal dosimetry device and MAX-HD SRS phantom with A26 are useful to verify SRT plan with small fields which PTV’s volume is about 1.0 cm3.


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