Radiation dose and cancer risk to out-of-field and partially in-field organs from radiotherapy for symptomatic vertebral hemangiomas.

Abstract

Vertebral hemangiomas (VHs) are the most common benign tumors of the spine that may cause bone resorption. Megavoltage irradiation is usually the treatment of choice for the management of symptomatic VHs. The current study was conducted to estimate the risk for carcinogenesis from radiotherapy of this benign disease on the basis of the calculated radiation doses to healthy organs. The Monte Carlo N-particle transport code was employed to simulate the irradiation with 6 MV x-rays of a VH presented in the cervical, upper thoracic, lower thoracic, and lumbar spine. The average radiation dose (Dav) received by each critical organ located outside the primarily irradiated area was calculated. Three-dimensional treatment plans were also generated for the VHs occurring at the four different sites of the spinal cord based on patients' computed tomography data. The organ equivalent dose (OED) to each radiosensitive structure, which was partly encompassed by the applied treatment fields, was calculated with the aid of differential dose-volume histograms. The Dav and the OED values were combined with a linear-no-threshold model and a nonlinear mechanistic model, respectively, to estimate the organ-, age-, and gender-specific lifetime attributable risks (LARs) for cancer development. The estimated risks were compared with the respective nominal lifetime intrinsic risks (LIRs) for the unexposed population. For a standard target dose of 34 Gy, the OED varied from 0.39-5.15 Gy by the organ of interest and the irradiation site. The Dav range for the out-of-field organs was 4.9 × 10(-4) to 0.56 Gy. The LAR for the appearance of malignancies in the partially in-field organs after radiotherapy of male and female patients was (0.08%-1.8%) and (0.09%-1.9%), respectively. These risk values were 1.5-15.5 times lower when compared to the respective LIRs. The lifetime probability for out-of-field cancer induction in irradiated males and females was (2.5 × 10(-4) to 7.7 × 10(-2))% and (1.4 × 10(-4) to 2.6 × 10(-1))%, respectively. The above risks were one to four orders of magnitude lower than the LIRs. The probability for the development of out-of-field malignancies due to radiotherapy for VHs is trivial with respect to the nominal risk for unexposed population. The respective cancer risks to partially in-field organs are smaller than the nominal probabilities but they should not be considered as inconsiderable. These risks may be taken into account during the follow-up of patients treated for a symptomatic VH.