The impact of dose calculatıon algorıthms for perıpheral dose dıstrıbutıons of enhanced dynamıc and physıcal wedges

Abstract

Background : In radiaon therapy, the peripheral dose is important when anatomical structures with very low dose tolerances are involved. In this study, the two available calculaon algorithms of the Varian Eclipse 8.6 treatment planning system(TPS), the anisotropic analyc algorithm (AAA) and pencil-beam convoluon (PBC) was used to compare measured and calculated peripheral dose distribuon of physical wedged (PW) and enhanced dynamic wedged fields (EDW). Materials and Methods : Peripheral dose measurements were carried out for 6 and 18 MV photons using a 0.6cc Farmer-type ionizaon chamber in the slab phantom. Measurements were performed using 15°, 30°, 45° and 60° PW and EDW for three different field sizes at d max and up to a maximum distance of 50 cm beyond the field edges. peripheral dose was further computed using two different algorithms of a TPS. The measured and calculated datas were then compared to find which algorithm calculates peripheral dose distribuon more accurately. Results: Both algorithms from the TPS adequately model the peripheral dose distribuon up to 45 degrees. For large field sizes with 60 0 EDW, the largest deviaon between calculated and measured dose distribuon is less than 3.5% using the AAA, but can increase up to 9.7% of the distribuon using PBC. Conclusion: The AAA models wedged peripheral dose distribuons more accurately than the PBC does for all studied condions; the difference between the algorithms are more significant for large wedge angles and large field sizes. It must be emphasized that the use of PBC for planning large-field treatments with 60 0 EDW could lead to inaccuracies of clinical significance.