Treatment planning and dosimetry
Beam calibration and MU fundamentals
Photon beams are calibrated with the TG-51 protocol at 10 cm depth, 10 × 10 cm² field, 100 cm SSD, and the result is reported as cGy/MU at dmax. Percentage depth dose (PDD) depends on SSD, field size, and energy; tissue-maximum ratio (TMR) is independent of SSD, which is why isocentric calculations use TMR while fixed-SSD calculations use PDD. Head scatter (Sc) and phantom scatter (Sp) are handled separately, and a rectangular field is converted to its equivalent square, 4A/P.
Heterogeneity and algorithm choice
Path-length corrections ignore changes in lateral scatter. Pencil-beam algorithms tend to overestimate dose in low-density lung and in small fields where lateral electronic equilibrium is lost; convolution/superposition and Monte Carlo are preferred in those settings. At lower energies, photoelectric absorption can make bone dose higher than a water-equivalent calculation predicts.
Plan evaluation
Report DVH metrics with explicit definitions: PTV D95 and D2, OAR V20, D2cc or D0.03cc, mean lung dose; ICRU-83 uses D98/D50/D2 plus homogeneity and conformity indices. Common traps: quoting median instead of near-maximum dose for an OAR, judging coverage from a single slice, and forgetting that the reported dose belongs to a specific algorithm and grid.
Worked example
The PDD at 10 cm is 66.5% at 100 cm SSD. Convert to 120 cm SSD with the Mayneord factor: F = [(120 + 1.5)/(120 + 10)]² × [(100 + 10)/(100 + 1.5)]² = (0.935)²(1.084)² = 1.026, giving about 68.2%. Mayneord ignores the change in scatter, so it slightly overestimates; commissioning data should drive clinical plans.