• © 2010 by American Society of Clinical Oncology

Reply to T. Bölling and N. Willich

We thank Tobias Bölling and Normann Willich¹ for their interest in our article on long-term survivors of malignancies in childhood and adolescence. Their letter has prompted us to reply with the following comments and answers.

In general, we agree with the authors that dose estimations performed retrospectively, from 1992 to 1995, on patients treated before 1985 cannot be compared with detailed dose calculations using modern computed tomography–based three-dimensional radiotherapy treatment planning.

However, as a result of an error and a lack of clarity in our article, Bölling and Willich misunderstood our dose estimation method. First, in our article, distances between the central and the six other heart points should be 1.3 to 8.3 cm rather than, as we erroneously stated by, 13 to 83 cm. Second, these points were centered around the heart center, rather than, as stated by Bölling and Willich, around a central point of the radiation field. To be clearer, the position of these points is shown in Figure 1. Additionally, for the sake of brevity, we probably oversimplified the description of dosimetry in our article. In fact, we used all information in radiotherapy technical charts, including external contours of patients, and simulation films, which were available for a large majority of the patients, as well as trunk inclination and back extension of the head to fit the position of patients during radiotherapy. Heart point locations were determined by studying the relationship between the position of the heart and the dimensions of the rib cage from recent patients' radiotherapy planning computed tomography images. Depending on the patient-adjusted phantom size, the distance between the central heart point and the six other points varied from 1.3 to 8.3 cm.

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Fig 1.

Locations of the seven heart points in the Dos_EG mathematical phantom. (A) Coronal view and (B) sagittal view. The points represent ¹ the center or barycenter, ² left (near the atrium and left ventricle, mitral valve, and left coronary artery), ³ right (near the atrium and right ventricle, tricuspid valve, and right coronary artery), (4) anterior (near the right and left ventricle, aorta, pulmonary valve, and aortic valve), (5) posterior (near the right and left ventricle, aorta, and right coronary artery), (6) upper (near the superior vena cava, ascending aorta, pulmonary artery, right coronary artery, and left coronary artery), and (7) lower parts of the heart (near the right and left ventricle, descending aorta, and inferior vena cava).

To calculate the dose to anatomic sites, beams were positioned on the individual patient-adjusted phantom² according to details from the patient's record as well as information on equipment, treatment techniques, and guidelines used at the time of the treatment. Radiotherapy parameters included beam size, shape, and inclination; location; radiation energy; and delivered treatment dose. The dose calculation algorithm accounted for primary radiation from the treatment machine and scattered radiation from the patient and from beam collimation, leakage radiation, and lung heterogeneity. Computations were based on measurements performed on 28 different treatment machines. Doses could be computed for 50-kV to 31-MV photon beams and for 4- to 32-MeV electron beams. The dose could be evaluated up to 180 cm from the central axis of the radiotherapy beam. Experimental validation of the Dos_EG software package, using an Alderson-Rando phantom, showed an agreement between thermoluminescent dosimeter measurements and the calculated doses better than 2% for dose estimations at points located inside the beam and 10% for points located outside the beam.³ When investigating average, minimal, and maximal dose and the number of points that received more than a given threshold, only the average dose played a significant role in cardiac disease risk. Nevertheless, as stated by Bölling and Willich, prospective studies will be more reliable and comprehensive, using all modern available data on anatomy and dose distributions.