Integral dose conservation in radiotherapy

Abstract

Treatment planners frequently modify beam arrangements and use IMRT to improve target dose coverage while satisfying dose constraints on normal tissues. The authors herein analyze the limitations of these strategies and quantitatively assess the extent to which dose can be redistributed within the patient volume. Specifically, the authors hypothesize that (1) the normalized integral dose is constant across concentric shells of normal tissue surrounding the target (normalized to the average integral shell dose), (2) the normalized integral shell dose is constant across plans with different numbers and orientations of beams, and (3) the normalized integral shell dose is constant across plans when reducing the dose to a critical structure. Using the images of seven patients previously irradiated for cancer of brain or prostate cancer and one idealized scenario, competing three-dimensional conformal and IMRT plans were generated using different beam configurations. Within a given plan and for competing plans with a constant mean target dose, the normalized integral doses within concentric "shells" of surrounding normal tissue were quantitatively compared. Within each patient, the normalized integral dose to shells of normal tissue surrounding the target was relatively constant (1). Similarly, for each clinical scenario, the normalized integral dose for a given shell was also relatively constant regardless of the number and orientation of beams (2) or degree of sparing of a critical structure (3). 3D and IMRT planning tools can redistribute, rather than eliminate dose to the surrounding normal tissues (intuitively known by planners). More specifically, dose cannot be moved between shells surrounding the target but only within a shell. This implies that there are limitations in the extent to which a critical structure can be spared based on the location and geometry of the critical structure relative to the target.

Leukoencephalopathy After Stereotactic Radiosurgery for Brain Metastases

“…To define additional predictive factors we investigated the whole brain integral dose in patients without WBRT and found that more than 3 J was associated with a statistically significant increase in leukoencephalopathy. These findings are consistent with the observation that integral dose is relatively constant in concentric volumes despite increasing conformality around the target [58]. To our knowledge, this is the first report identifying a parameter to guide physicians in limiting leukoencephalopathy in patients treated with SRS (namely integral dose).…”

Patient geometry‐driven information retrieval for IMRT treatment plan quality control

“…Given consistent target coverage across patients, the relative spatial configuration of an OAR with respect to a target determines, to a large extent, the dose distribution of the OAR: OARs distant from the target are easy to spare, while proximal or overlapping OARs are difficult. 7 Several head-and-neck studies have used the overlap volume between an OAR and a target as a descriptor of the relative spatial configuration. [8][9][10][11] However, the overlap volume is not meaningful when the OAR and target do not overlap.…”

Radiation Therapy and Late Mortality From Second Sarcoma, Carcinoma, and Hematological Malignancies After a Solid Cancer in Childhood

“…The dose calculation algorithm took into account primary radiation from the treatment machine and scattered radiation from the patient and from beam collimation, leakage, radiation, and lung heterogeneity. The integral dose may be considered the volume-weighted sum of the doses, the mean dose multiplied by the volume, or the total energy deposited (in joules [J]) (31)(32)(33).…”