Dosimetric dependences of bone heterogeneity and beam angle on the unflattened and flattened photon beams: A Monte Carlo comparison

• Dosimetric impact of bone heterogeneity in the presence of unflattened and flattened photon beams was compared.
• Surface and depth dose of unflattened and flattened photon beams varying with the beam angle were compared.
• Depth dose deviation due to the presence of bone was sensitive to the beam obliquity.
• Surface dose deviation between the unflattened and flattened beams became smaller with an increase of beam angle.
• Surface dose and range of depth dose ratios (unflattened to flattened beam) decreased with an increase of beam angle.

The variations of depth and surface dose on the bone heterogeneity and beam angle were compared between unflattened and flattened photon beams using Monte Carlo simulations. Phase-space files of the 6 MV photon beams with field size of 10×10 cm2 were generated with and without the flattening filter based on a Varian TrueBeam linac. Depth and surface doses were calculated in a bone and water phantoms using Monte Carlo simulations (the EGSnrc-based code). Dose calculations were repeated with angles of the unflattened and flattened beams turned from 0° to 15°, 30°, 45°, 60°, 75° and 90° in the bone and water phantoms. Monte Carlo results of depth doses showed that compared to the flattened beam the unflattened photon beam had a higher dose in the build-up region but lower dose beyond the depth of maximum dose. Dose ratios of the unflattened to flattened beams were calculated in the range of 1.6–2.6 with beam angle varying from 0° to 90° in water. Similar results were found in the bone phantom. In addition, higher surface doses of about 2.5 times were found with beam angles equal to 0° and 15° in the bone and water phantoms. However, surface dose deviation between the unflattened and flattened beams became smaller with increasing beam angle. Dose enhancements due to the bone backscatter were also found at the water–bone and bone–water interfaces for both the unflattened and flattened beams in the bone phantom. With Monte Carlo beams cross-calibrated to the monitor unit in simulations, variations of depth and surface dose on the bone heterogeneity and beam angle were investigated and compared using Monte Carlo simulations. For the unflattened and flattened photon beams, the surface dose and range of depth dose ratios (unflattened to flattened beam) decreased with increasing beam angle. The dosimetric comparison in this study is useful in understanding the characteristics of unflattened photon beam on the depth and surface dose with bone heterogeneity.