IS THE STANDARDIZED HELMET TECHNIQUE ADEQUATE FOR IRRADIATION OF THE BRAIN AND THE CRANIAL MENINGES

Purpose: To evaluate whether the standardized helmet technique is adeq uate to reliably cover the clinical target volume (whole brain includi ng cranial meninges) during treatment planning and treatment delivery. Methods and Materials: In 21 patients undergoing irradiation of the b rain in acute lymphoblastic leukemia or primary cerebral lymphoma, the coverage of the clinical target volume was checked with a repeat comp uted tomography (CT) in the treatment position (head fixation with fac e mask). The accuracy of field alignment was quantitatively assessed w ith sequential verification films. For each patient, linear and rotati onal discrepancies were measured between the simulation and first chec k film, and between five consecutive verification films. Results: Cove rage of clinical target volume. In 11 cases (52%), the CT examinations showed that parts of the subfrontal region and midcranial fossa were not included by the field assigned under simulation, Accuracy of field alignment,For the total group of patients, all deviations were normal ly distributed with mean values between -1.2 mm and 1.5 mm and standar d deviations of 2.9 mm to 3.7 mm for linear discrepancies, and 0.3 deg rees +/-3.2 degrees for rotational discrepancies, For all patients, de viations were similar for the transition from simulation to the treatm ent machine and for subsequent treatment delivery, with 50% and 95% of absolute differences being less than 2.0 mm and 6.5 mm, respectively. Maximum linear deviations were less than 9.5 mm. Conclusions: The cur rently used helmet technique is inadequate to cover the clinical targe t volume. Repeat CT examinations are a useful method to delineate the clinical target volume on an individual patient basis. In addition, st atistical fluctuations of field displacements up to 1.0 cm have to be considered when prescribing safety margins for reliable coverage of th e clinical target volume during treatment planning and delivery.