Introduction: Stereotactic body radiation therapy (SBRT) is a method for treating of lung cancer currently used at Christchurch Hospital. The combined effects of lung tumour motion and the limitations of treatment planning systems calculating dose in lung regions creates uncertainty in the dose delivered to the tumour and surrounding normal tissues. In turn, this uncertainty has the possibility to negatively affect the clinical outcomes of the patient. This study overrides the treatment volumes to different densities between water and lung, and looked at the effect on plan quality and accuracy.

Method & Materials: Modified cork cylindrical inserts, each with a spherical tumour of either 15mm, 22mm, or 30mm diameter, and the QUASAR phantom were used to simulate the motion of a lung tumour. A four-dimensional computed tomography (4DCT) scan was taken for each insert on a Siemens Somatom RT Pro CT scanner. A treatment plan was generated on Monaco 5.1 treatment planning software, based on clinical protocols. Two standard plans (free breathing (FB), average intensity projection (AIP)) were compared to eight density overridden plans that focused on different target volumes (internal target volume (ITV), planning target volume (PTV), and a hybrid plan (HPTV)). The target volumes were set to a variety of densities between lung-equivalence and water-equivalence. The Dose Volume Histogram (DVH) statistics were used to compare the target coverage, conformity, and heterogeneity of the plans. The plans were delivered on an Elekta Synergy linac, and verified using ionisation chamber and radiographic film measurements.

Results: Minimal differences were seen in the 30mm tumour in terms of target coverage, plan conformity, and improved dosimetric accuracy. For the smaller tumours, a PTV override showed improved target coverage as well as plan conformity compared to the baseline plans. Minimal differences were seen with the ITV overrides. In terms of dosimetric accuracy, the ITV plans showed the highest gamma pass rate agreement between treatment planning system (TPS) and measured dose (p<0.05). However the low density PTV and HPTV plans also showed a significantly improved gamma pass rates (p<0.05)

Discussion & Conclusion: This study suggests that low density PTV overrides improves the plan quality and accuracy for small tumours only. Although an ITV override generates the most significant increase in accuracy, the low density PTV plans have the additional benefit of plan quality improvement. Although this study and others agree that density overrides improve the treatment of SBRT, the optimal density override and the conditions under which it should be applied depend on the department.