4. An analysis of the doses calculated and measured in-vivo during total body irradiation

Abstract

Purpose The aim of this work was to estimate the error in dose calculations, to check the agreement between the measured and calculated doses and to analyse dose discrepancies in the group of thirty patients undergoing total body irradiation. Method A group of thirty consecutive patients, children aged from 5 to 12 years, was taken for dose comparison. A combination of lateral and anterior – posterior fields with electron boost was used in eight fractions and on four consecutive days. Cobalt unit was used as a source of radiation. Lung shields were used at two anterior-posterior fields. Compensators were attached at the neck, head and legs. Doses were preliminarily calculated and then measured in-vivo by thermoluminescent (TLD) and semiconductor (SEM) detectors attached to the body in ten representative transverse cross-sections. Calculations and measurements were carried out for the beam at the body entry and exit. The error in dose calculations was estimated for each dosimetrical point. A total dose of 12,6 Gy was prescribed in the body midline on the central beam axis (CAX). A dose of 8.2 Gy was delivered from six lateral fields and a dose of 4,4 Gy from two AP/PA fields. In the lungs dose was below 10 Gy. Lateral fields were set at a source to skin distance (SSD) of 305 cm with the dose rate to midline of 5,9 cGy/min and AP/PA fields were set at 210 cm with 11,5 cGy/min, respectively. A fractional dose specified on CAX and midline was 2.5 Gy during all anterior-posterior fields (from each side), 1.5 Gy (0.75 Gy from each side) during four lateral fields, and 1.1 Gy (0.55 Gy from each side) during two lateral fields. Results The error in the preliminary dose calculations was determined by a total differential method, taking into account of the accuracy of determining the following parameters: SSD, body thickness and contour, the thickness of compensators, lung shields, filter shape, lung dimensions and body density, where applicable. Calculations were done separately for each cross-section and for the beam entry and exit. The values of this error were rounded up to 3% for CAX, from 5% to 10% for head, neck, shoulders, abdomen, elbows, wrist, knee and feet and increased to 15% for lung at AP/PA fields. Per cent deviations for hole group of patients for lateral and AP/PA fields (on entry and exit) were below 10% (TLD and SEM detectors) with the exceptions for the neck at lateral and for the lungs at AP/PA fields where the errors exceeded 10%. Conclusions For the group of patients the per cent deviations exceeded 10% for the neck exit in lateral fields and for the lung exit in anterior – posterior fields. Standard deviations exceeded 10% at the neck and lung exits in lateral fields and at the lung exit in anterior – posterior fields.