Dose estimation to critical organs from vertex field treatment of brain tumors

Radiation management of intracranial tumors may require a noncoplanar vertex field that often irradiates the entire length of the body. In view of radiation related risks to the normal tissues dose estimation to the extracranial organs such as the thyroid gland, spinal cord, heart, and genitalia is performed for a vertex field.

A vertex field used clinically was reproduced on an anthropomorphic Rando phantom to measure radiation dose to various organs in the primary beam. Three photon beams (4, 6, and 10 MV), and two high energy electron beams (16 and 20 MeV) were used. Dosimetry was performed with an ion chamber sandwiched between phantom slices at the appropriate positions. All doses were normalized to the target dose at a depth of 5 cm. The effect of the head position was studied by rotating the gantry angle up to +/-20 degrees to mimic the extension and flexion of the head. Theoretical calculation was performed using an exponential best fit to the depth dose table to estimate the dose to various points and compare with the measured dose.

The measured normalized dose to the cervical cord, thyroid, heart, and female and male gonads are 60, 36, 16, 2.5, and 1.6%, respectively, for a 6 MV photon beam. The dose from 4 MV and 10 MV are slightly lower and higher, respectively. Doses from electron beams are about a factor of 4-10 lower than those of the photon beams. The measured gonadal dose from the primary beam is <5% of the target dose for all energies used in the study. The actual value, however, is dependent on the body structure, length, and the posture of the patient. A +5 degree head flexion had little effect on the dose to the various parts of the body. The head rotations greater than +/-10 degrees produced relatively lower doses by a factor of 10(-2) to the organs at distances greater than 40 cm from the prescription point. The radiation doses to the different critical organs estimated from the fitted curves are lower than the measured doses up to 35%.

When a vertex field is used for the treatment of the brain tumors, the entire axial length of the body is irradiated which adds to the integral dose. Unlike the scattered and leakage radiation, the primary dose to extracranial critical organs is greater for higher energies. For a 10 MV beam the ovary and testis at a distance of 80 cm and 90 cm may receive a dose of 4.2 and 3%, respectively, of the target dose. The gonadal dose could be quite significant if the entire treatment is delivered using a vertex field. For pediatric and smaller patients, dose to the critical organs at known distances could be estimated from the empirical equation obtained from the measured data. While the risk-benefit ratio is often evaluated and acceptable for treating malignant tumors, the long-term complications need thorough assessment in younger and curable patients. In view of radiation carcinogenesis and genetic burden, dose reduction to critical organs should be considered using a 3D planning system to arrange beams in other nonaxial planes and by considering electron beams for the vertex field.