Dose verification for liver target volumes undergoing respiratory motion
Respiratory motion has a significant impact on dose delivered to abdominal targets during radiotherapy treatment. Accurate treatment of liver tumours adjacent to the diaphragm is complicated by large respiratory movement, as well as differing tissue densities at the lung-liver interface. This study aims to evaluate the accuracy of dose delivered to superior liver tumours using passive respiratory monitoring, in the absence of gating technology, for a range of treatment techniques. An in-house respiratory phantom was designed and constructed to simulate the lung and liver anatomy. The phantom consisted of adjacent slabs of lung and liver equivalent materials and a cam drive system to emulate respiratory motion. A CC04 ionisation chamber and Gafchromic EBT3 film were used to perform point dose and dose plane measurements respectively. Plans were calculated using an Elekta Monaco treatment planning system (TPS) on exhale phase study sets for conformal, volume modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) techniques, with breathing rates of 8, 14 and 23 bpm. Analysis confirmed the conformal delivery protocol currently used for this site within the department is suitable. The experiments also determined that VMAT is a viable alternative technique for treatment of superior liver lesions undergoing respiratory motion and was superior to IMRT. Furthermore, the measurements highlighted the need for respiratory management in these cases. Displacements due to respiration exceeding planned margins could result in reduced coverage of the clinical target volume and much higher doses to the lung than anticipated.