<![CDATA[An alternative approach to estimate the Tissue Phantom Ratio (TPR) at depths of 20 cm and 10 cm (TPR₂₀,₁₀) under non-reference conditions is required to address situations where a 10 × 10 cm² field size is not achievable on a specific Linear Accelerator (LINAC) during a beam quality test. This study aims to estimate the TPR20,10 under non-reference conditions using a geometric sequence approach, and to compare it with the TPR₂₀,₁₀ under non-reference conditions estimated using the Sauer method, the Palmas method, a linear fit approach, as well as with the TPR₂₀,₁₀ under reference conditions calculated using the TRS-398 protocol. The first step in this study was measuring the percentage depth dose (PDD), D20cm, and D10cm with field size variations from 4 × 4 cm² to 10 × 10 cm² for both 6 MV and 10 MV X-ray beams. The PDD were used to estimate the TPR₂₀,₁₀ using a geometric sequence approach, the Sauer method, the Palmans method, and a linear fit approach, and to calculate the TPR₂₀,₁₀ using the TRS-398 protocol. The D20cm and D10cm were also used to calculate the TPR₂₀,₁₀ using the TRS-398 protocol. The TPR₂₀,₁₀ for 6 MV and 10 MV X-ray beams estimated using the geometric sequence approach were 0.683 ± 0.004 and 0.742 ± 0.005, respectively. The level of precision that could be reached by the geometric sequence approach is potentially equivalent to the TRS-398 protocol, the Sauer method, the Palmans method, and the linear fit approach. The TPR₂₀,₁₀ for 6 MV and 10 MV X-ray beams estimated using the geometric sequence method did not show a significant difference compared with the TPR₂₀,₁₀ calculated using the TRS-398 protocol. However, the TPR₂₀,₁₀ for 6 MV and 10 MV X-ray beams estimated using the geometric sequence approach showed a significant difference compared with those TPR₂₀,₁₀ estimated using the Sauer method and the Palmans method.]]>
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