Radiotherapy of the breast: changes in the volume of the lung covered in the treatment fields and resulting changes in dose distribution

In breast cancer adjuvant therapy, respiratory movements continuously modify the irradiated volumes and the anatomical shape of this body region. Fifteen patients were submitted to 3 Computed Tomography (CT) sequences for treatment planning: the first one without any indications to the patient (the standard sequence) and the second and the third one with spontaneous stopped inspiration and expiration, respectively; the patient was always in the same position. The treatment was planned on standard CT images and then applied to the other sequences, maintaining all parameters unvaried, including isocenter position and treatment time. The lung volumes within the fields (and those included in the 95%, 100%, 105% isodoses referred to the prescribed dose) were evaluated with dose/volume histograms. The average irradiated lung was 69 cm3 (DS 28) in standard sequences, 136 cm3 (DS 67) in inspiration and 41 cm3 (DS 25) in expiration. The pulmonary volume within the above isodoses exhibited similar changes. In other words, the lung volume actually irradiated during the whole treatment is smaller than the one which can be calculated on standard CT sequences and it corresponds to expiration volume. The remaining part falls into a wide "twilight zone" relative to dose. Therefore, the true risk of lung toxicity can be similarly lower than the calculable one on standard CT images. Thus, the complication risk (based on dose/volume histograms and normal tissue control probability parameters) could be assessed in new prospective studies, introducing a corrective factor for the irradiated lung volume, because the latter is smaller than that shown by standard CT.