Purpose: To evaluate whether the standardized helmet technique is adequate to reliably cover the clinical target volume (whole brain including cranial meninges) during treatment planning and treatment delivery.
Methods and materials: In 21 patients undergoing irradiation of the brain in acute lymphoblastic leukemia or primary cerebral lymphoma, the coverage of the clinical target volume was checked with a repeat computed tomography (CT) in the treatment position (head fixation with face mask). The accuracy of field alignment was quantitatively assessed with sequential verification films. For each patient, linear and rotational discrepancies were measured between the simulation and first check film, and between five consecutive verification films.
Results: Coverage 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 normally distributed with mean values between -1.2 mm and 1.5 mm and standard deviations of 2.9 mm to 3.7 mm for linear discrepancies, and 0.3 degrees +/- 3.2 degrees for rotational discrepancies. For all patients, deviations were similar for the transition from simulation to the treatment 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 currently used helmet technique is inadequate to cover the clinical target volume. Repeat CT examinations are a useful method to delineate the clinical target volume on an individual patient basis. In addition, statistical fluctuations of field displacements up to 1.0 cm have to be considered when prescribing safety margins for reliable coverage of the clinical target volume during treatment planning and delivery.