Program Information

Vertically-Oriented Farmer-Type Chamber for Small-Field Applications

M Ahmad

M Ahmad*, A Chu , H Lincoln , Z Chen , J Deng , R Nath , Yale New Haven Hospital, New Haven, CT

Presentations

SU-E-T-431 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
To introduce a non-conventional measurement setup using Farmer-type chambers to accommodate several situations of small-field dose measurements without compromising accuracy. The validation of this technique was demonstrated for photon small-field output measurements, and electron small-field percentage depth-dose (PDD) measurements.

Methods:
Initial chamber alignment was performed using the conventional (horizontally-oriented) chamber setup. A PDD was acquired for a 4x4 cm² field size using this arrangement. This PDD was used as a positional reference for the vertically-oriented chamber (VOC) configuration. Next, a PDD was acquired for a 4x4 cm² field size with the VOC. The PDD’s were superimposed to find the effective shift of the VOC. Using the shifted VOC setup, photon small-field output factors were measured and compared to stereotactic diode output factor measurements. Additionally, electron small-field PDD's were acquired using the VOC setup and results were compared to electron Monte Carlo (eMC) predictions in the Eclipse treatment planning system (TPS).

Results:
(1) For photon small-field output factors field-sizes 2x2 cm² and larger, the difference between the VOC setup and SFD measurements were less than 0.8%. For field sizes less than 2x2 cm² discrepancies ranged from 4.0 to 10.6%.

(2) PDD’s measured by VOC setup show better than 1.6% agreement as compared to eMC for all electron energies measured down to the 80% depth on the 2x2 cm² PDD curve. Disagreement between the VOC setup measurement and eMC calculations for depths down to the 50% depth on the PDD curve is 3.6% or less.

Conclusion:
Using the VOC setup, it is possible to use a conventional farmer chamber for small field-size measurements down to 2x2 cm² field size without sacrificing the accuracy of measurements.


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