Comparison of Calculated Percentage Depth Doses at Extended Source-to-Surface Distance for 6 MV And 15 MV Photon Beam of a Linear Accelerator

Background: Research findings from percentage depth dose (PDD) are crucial in evaluating patient doses received in radiation therapy. Objective: To compare calculated percentage depth doses at an extended source-to-surface distance (SSD) for 6 MV and 15 MV photon beams of a linear accelerator. Methodology: Measured PDD values of the 100 cm source to surface distance (SSD) and calculated values at extended SSDs for 6 MV and 15 MV photon beams of an Elekta NHA SLi 1998 linear accelerator were analyzed. The PDD data was collected by placing ionization chamber inside water phantom for depths ranging from z = 0 - 30 cm in a water phantom and using a square field sizes of 10 x 10 cm2. Photon energies of 6 MV and 15 MV were used for the measurement, with both gantry and collimator angles fixed at zero degree. PDD was calculated at extended SSD of 110 cm, 120 cm, 130 cm, and 140 cm from the measured PDD values of 100 cm SSD for both 6 MV and 15 MV photon energies using Mayneord factor. Results:  The depth dose maximum (Dmax) for field size 10 x 10 cm2  for 6 MV and 15 MV photon energies were 1.62 cm and 2.65 cm, respectively and the PDD at 10 cm  (D10) were 67.9% and 75.9%, respectively. The mean deviation of the calculated PDD at extended SSDs was found to be between 0.2% and 1%. Conclusion: The calculated PDD values at extended SSDs are considered suitable for clinical use at all clinically relevant depths and field sizes.