A Practical Approach of Small Field Dosimetry Measurement for Patient-Specific IMRT/SRS/SBRT
X Wu*, R Popple, Y Yuan, J FIveash, S Shen, J Duan, R Cardan, I Brezovich, Univ Alabama Birmingham, Birmingham, ALSU-E-P-14 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
To design and implement a small-field IMRT/SRS/SBRT dosimetry measurement with a regular-size ion chamber and films.
An acrylic phantom was constructed and commissioned to sandwich a Kodak EDR2 radiographic film. After a patient QA plan was delivered, the phantom was shifted superiorly by 10 cm and a reference plan was delivered on the same film. The reference plan has four-field-box beam geometry with 4 x 4 cm2 field size. Since the dose distribution of the reference plan was uniform and large, the absolute dose of the reference plan can be accurately measured separately with a regular-size ion chamber (diameter: 0.6 cm). After normalization, a two-dimensional absolute dose distribution can be obtained and compared to that calculated by Eclipse Treatment Planning System. An in-house software written in MATLAB was used to analyze films. Two-dimensional gamma indexes were calculated to evaluate patient QA plans.
Three patient-specific IMRT/SRS/SBRT QA plans were used to verify the feasibility of the method. The prescription dose of the reference plan was 3.5 Gy. The scatter dose from one plan to the other plan (which is 10 cm or more away) is very small (<1 cGy) and can be ignored. Compared with Eclipse calculation, the measured dose errors of the reference plan were -0.1%, -0.3%, and -0.8%, respectively. For the three patient-specific QA plans, the point dose measurement errors were 2.1%, 0.8% and 0.2%, respectively, and the γ>1 failure rates were 0.2%, 0.2%, and 0.1%, respectively.
Since the scatter dose from another plan 10 cm (or more) away was very small, this method allows one to measure the dose of any small target with a regular-size ion chamber and films without volume limitation.