P2.05-047 Feasibility Study: Assessment of RT Dose Using Cardiac MRI Contouring Methodology on Retrospective Lung Planning CT Scans

Lung cancer related mortality remains high after radiotherapy (RT) despite advances in treatment. The RTOG 0617 study (1) reported increased mortality within the higher radiation dose treatment arm, though toxicity was similar between the two study arms. One possible explanation was, increased radiation dose to the heart. Typically radiation induced heart disease (RIHD) is considered a late effect of RT in lymphoma and breast cancer. But RT dose prescribed in lung cancer is greater, and may result in acute effects in a population of patients with underlying cardio-pulmonary disease. Hence detailed dosimetric predictors are required for different cardiac morbidity endpoints. The CART study (2) was a prospective study investigating RIHD with serial cardiac MRI scans and the team involved developed a technique to analyse radiation dose of cardiac substructures without the MRI scan. CT planning scan was reformatted to develop a standardised method to outline in detail, cardiac substructures such as the left ventricular (LV) myocardium segments supplied by the Coronary Arteries (As). Other cardiac substructures such as cardiac chambers, conduction system and valves were contoured. This technique was applied to the planning scans of lung cancer patients and the dose to structures was calculated. Initially 5 patients who died within 3 months after radical RT were assessed. The RT treatment was 55Gy in 20 fractions over 4 weeks using 3D conformal RT technique. 3 patients had underlying cardiac or pulmonary comorbidities and pulmonary function as acceptable for all patients to proceed with radical RT. Dose to OARs were acceptable and all patients completed treatment. Radiation dose to the heart –mean heart dose range 158cGy-1910cGy, maximum heart dose range 1521-5669cGy and V20 dose range 0%-36% and V50 dose ranged from 0%-19%. Dose to left ventricular myocardium – LAD territory was maximum dose range (MaxDR) 71-2086 (cGy) and mean dose range (MeanDR) 112-802 (cGy); LCX territory was MaxDR 71-2549 (cGy) and MeanDR was 81-562 (cGy), and RCA territory was MaxDR 45-372 (cGy) and MeanDR was 44-178 (cGy) - demonstrated that anterior and lateral areas of LV myocardium received higher radiation dose. Dose to conduction system was high – SA node maximum dose ranged from 1140-5372 (cGy) and AV node maximum dose range was 103-1660 (cGy). It is feasible to use the CT planning scan to analyse retrospective patients for RT dose of coronary A myocardial territories, conduction system, and other substructures. An analysis of a larger sample of patients is planned.