A Robust and Simple IMRT Approach Reducing Dose to the Heart in Left-sided Breast Cancer

Purpose/Objective: Although 3D-treatment-planning reduced cardiac dose in postoperative breast radiotherapy, overall cardiac toxicity increased due to the more widespread use of anthracyclines in the adjuvant treatment. Modelling of toxicity based on data from patient series treated for breast cancer and for Hodgkin's disease suggests that reduction of the maximal dose to the left heart might be particularly beneficial. We compared treatment plans created with an aperture based IMRT-approach (an approach that offers some advantages over fully inverse treatment planning) to three-dimensionally planned tangent fields with regard to physical and biological optimisation of the dose distribution, its robustness to positioning errors and its treatment time requirements. Materials/Methods: CT- and structure data of 14 patients that had been treated postoperatively with external beam irradiation for early breast cancer at our department formed the basis of our plan comparison study. For each patient data set, a conventional 3D treatment plan and an IMRT plan were created on ELEKTA PrecisePlan®. Dose Volume Histograms (DVH) were evaluated with regard to target coverage, dose homogeneity, and dose to organs at risk (heart, lung, contralateral breast). Normal tissue complication probability (NTCP) for cardiac mortality as the endpoint was calculated for both approaches based on a relative seriality model. For a representative patient, the influence of positioning errors on the DVH was simulated by moving the isocenter in various directions. Results: IMRT reduced the maximum dose to the left ventricle from a mean of 49 Gy to a mean of 34 Gy. Heart volume that was exposed to >30 Gy was reduced from a mean of 45 ccm3 to a mean of 5.8 ccm3. Mean dose to the heart was increased by an average of 15 %. The change in DVH characteristics translated into a projected reduction of the probability for therapy associated long term cardiac death from 6 % (conventional 3D) to 2.5 % (IMRT) with the relative seriality model. On average, mean dose to the contralateral (right) breast was increased from 1.15 Gy to 5.4 Gy. Coverage of the ventral portion of the breast was robust to positioning errors, coverage of the posterior portion was not as a consequence of steep dose gradients towards the lung/heart. Monitor units, segment numbers and treatment time (<15 min) were within acceptable limits Conclusions: Aperture based IMRT for left sided breast cancer significantly reduces maximum dose to the left ventricle. This reduction may translate into reduced cardiac mortality based on an appropriate NTCP model. The clinical significance of increased exposure to the right breast is as of now unclear but may be less relevant than cardiac toxicity. As for any conformal technique, patient positioning has to be improved in the direction of the steep gradients (e.g. at the thoracic wall).

SSK25-08

A Robust and Simple IMRT Approach Reducing Dose to the Heart in Left-sided Breast Cancer