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

Background: In radiation therapy, the peripheral dose is important when anatomical structures with very low dose tolerances are involved. In this study, the two available calculation algorithms of the Varian Eclipse 8.6 treatment planning system(TPS), the anisotropic analytic algorithm (AAA) and pencil-beam convolution (PBC) was used to compare measured and calculated peripheral dose distribution 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 ionization chamber in the slab phantom. Measurements were performed using 15°, 30°, 45° and 60° PW and EDW for three different field sizes at dmax 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 distribution more accurately. Results: Both algorithms from the TPS adequately model the peripheral dose distribution up to 45 degrees. For large field sizes with 600 EDW, the largest deviation between calculated and measured dose distribution is less than 3.5% using the AAA, but can increase up to 9.7% of the distribution using PBC. Conclusion: The AAA models wedged peripheral dose distributions more accurately than the PBC does for all studied conditions; 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 600 EDW could lead to inaccuracies of clinical significance.