DOSE ESTIMATION TO CRITICAL ORGANS FROM VERTEX FIELD TREATMENT OF BRAIN-TUMORS

Purpose: Radiation management of intracranial tumors may require a non coplanar vertex field that often irradiates the entire length of the b ody, In view of radiation related risks to the normal tissues dose est imation to the extracranial organs such as the thyroid gland, spinal c ord, heart, and genitalia is performed for a vertex held. Methods and Materials: A vertex field used clinically was reproduced on an anthrop omorphic Rando phantom to measure radiation dose to various organs in the primary beam. Three photon beams (4, 6, and 10 MV), and two high e nergy electron beams (16 and 20 MeV) were used. Dosimetry was performe d with an ion chamber sandwiched between phantom slices at the appropr iate positions. All doses were normalized to the target dose at a dept h 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 o f the head. Theoretical calculation was performed using an exponential best fit to the depth dose table to estimate the dose to various poin ts and compare with the measured dose. Results: The measured normalize d dose to the cervical cord, thyroid, heart, and female and male gonad s 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, respectiv ely. Doses from electron beams are about a factor of 4-10 lower than t hose of the photon beams. The measured gonadal dose from the primary b eam 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 degrees head flexion had little ef fect 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 pres cription point. The radiation doses to the different critical organs e stimated from the fitted curves are lower than the measured doses up t o 35%. Conclusions: When a vertex field is used for the treatment of t he brain tumors, the entire axial length of the body is irradiated whi ch adds to the integral dose. Unlike the scattered and leakage radiati on, the primary dose to extracranial critical organs is greater for hi gher 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 entir e treatment is delivered using a vertex field. For pediatric and small er patients, dose to the critical organs at known distances could be e stimated from the empirical equation obtained from the measured data. While the risk-benefit ratio is often evaluated and acceptable for tre ating malignant tumors, the long-term complications need thorough asse ssment in younger and curable patients. In view of radiation carcinoge nesis and genetic burden, dose reduction to critical organs should be considered using a 3D planning system to arrange beams in other nonaxi al planes and by considering electron beams for the vertex field. (C) 1997 Elsevier Science Inc.