Linac-based isocentric electron-photon treatment of radically operated breast carcinoma with enhanced dose uniformity in the field gap area

Background and purpose: Isocentric treatment technique is a standard method in photon radiotherapy with the primary advantage of requiring only a single patient set-up procedure for multiple fields. However, in electron treatments the size of the standard applicators does not generally allow to use an isocentric treatment technique. In this work we have modified and dosimetrically tested electron applicators for isocentric treatments in combination with photons. An isocentric treatment technique with photons and electrons for postmastectomy radiation therapy (PMRT) has been developed with special emphasis on improving the dose uniformity in the field gap area. Materials and methods: Standard electron applicators of two Varian Clinac 2100CD linear accelerators were shortened by 10 cm allowing isocentric treatments of 90 cm < SSD < 100 cm in electron fields. Shortened applicators were commissioned and configured for the electron calculation algorithm of the treatment planning system. The field arrangement of PMRT was modified by combining three photon field segments with different gaps and overlaps with the electron field to improve dose uniformity. The developed technique and two other methods for PMRT were compared with each other in the group of 20 patients. Results: Depth dose characteristics of the shortened applicators remained unchanged from those of the standard applicators. Penumbrae were broadened by 0-3 mm depending on electron energy and depth as the air gap was increased from 5 cm (standard applicator at SSD = 100 cm) to 10 cm (shortened applicator at SSD = 95 cm). The dose calculation performance of the modified applicators at 95 cm < SSD < 100 cm was considered similar as with standard applicators at SSD = 100 cm using the Gaussian pencil beam electron dose calculation algorithm of the treatment planning system (Varian Eclipse). The modified isocentric treatment technique for PMRT was superior than the traditional two-dimensional technique. However, with the tangential photon fields without electrons the even better dose uniformity within PTV could be achieved but with increased irradiation of healthy tissues (lung, heart, and contralateral breast). The modified isocentric technique was also found faster than the traditional technique with SSD = 100 cm fields. Conclusions: It is possible to apply an isocentric treatment technique in PMRT with electrons and photons. The homogeneity of the dose distribution can be improved by adding more photon field segments. With the isocentric technique it is possible to achieve even some time sparing in treatment delivery compared with the traditional SSD = 100 cm technique. (C) 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 93 (2009) 80-86