The effect of electronic disequilibrium on the received dose by lung in small fields with photon beams: Measurements and Monte Carlo study

Background: Prediction of the absorbed dose in irradiated volume plays an important role in the outcome of radiotherapy. Application of small fields for radiotherapy of thorax makes the dose calculation process inaccurate due to the existence of electronic disequilibrium and intrinsic deficiencies in dose calculation algorithms. To study the lung absorbed dose in radiotherapy with small fields, the central axis absorbed dose in heterogeneous thorax phantom was measured by ionization chamber and calculated for small fields by Monte Carlo (MC) method. Materials and Methods: A solid slab phantom consisting of unit and low density materials was used for dose measurements. The 6 and 18 MV photon beams of Elekta SL25 linac were simulated using MCNP4C MC Code. The model was validated by comparing the calculated depth dose and beam profiles with measurements in a water phantom. The MC model was used to calculate the depth doses in unit density and low density materials resembling the soft tissue and lung, respectively. Two small field sizes including 5×5 and 2×2 cm2 were used in this study. Results: The measured depth dose values were in good agreement with MC results and the difference less than 2% was observed. A large dose reduction was seen in lung for field size of 2×2 cm2 due to the lateral electronic disequilibrium and it reached up to 16.2% and 33.3% for 6 and 18 MV beams, respectively. Dose build up and down at material interfaces was predicted by MC method. Conclusion: Our study showed that the dose reductions with small fields in lung and dose variations at interfaces was very considerable, and inaccurate prediction of absorbed dose in lung using small fields and photon beams may lead to critical consequences for patients.