Measurements of exit dose profiles in 60Co beams with a conventional portal film system

Effective Penumbra and Scalloping Effect: A Dosimetric Study in Multifield Radiotherapy with Multileaf Collimator for Prostate Cancer Treatment

Aims and background

The aim of the study was to test whether the multileaf collimator can be used to replace divergent alloy blocks for field shaping and to evaluate the dosimetric impact of the scalloping effect and the effective penumbra in multifield isocentric configurations routinely used at our department to treat early stage prostate tumor in supine patients. Deviations between measurements and treatment planning system calculations are also discussed in order to estimate the accuracy of effective penumbra calculations carried out by the software in the presence of blocks or a multileaf collimator.

Methods

The multileaf collimator installed on a dual energy (6 and 18 MV) linear accelerator Varian CLINAC 2100 C/D is an add-on component positioned below the standard jaws, with 40 computer-controlled opposed pairs of tungsten leaves. Transmission, effective penumbra and scalloping measurements were performed with films placed at different depths in a (30 × 30 × 20 cm3) acrylic phantom. A laser scanning photo-densitometer was used to obtain the optical density and the relative dose profile. Effective penumbra and scalloping effect measured data were tested on a software phantom; the phantom, automatically performed with the treatment planning system, was a regular parallelepiped measuring 30 × 30 × 20 cm3 and the acrylic electronic density value.

Results

For one multileaf collimator-shaped field, the width of effective penumbra was about 2 mm wider than penumbra for cerrobend blocks, at a 45° angle between leaf motion direction and the field edge. Collimator rotation, automatically performed by the treatment planning system, by minimizing the sum of over- and underblocked areas, reduces the differences between the multileaf collimator and blocks. The differences between measured and treatment planning system calculated data were within the treatment planning system dose calculation accuracy limits, as recommended in ICRU Report No. 42.

Conclusions

Penumbra and, for the multileaf collimator, scalloping effect values seem to depend on the gantry angle, i.e., on the fields path of entry. The values for even fields are higher than the those for odd fields, because the dose gradient at the target edges is steeper for a single field than for two opposite fields, and the interplay of doses from the individual beams increases the distance between the isodose levels. Therefore, in order to reduce scalloping and effective penumbra values, it would be better to rotate the collimators, above all in even-number field techniques. In particular, the six-field technique, used mostly in prostate treatment, shows the same effective penumbra values with the multileaf collimator, with proper collimator rotation, and in blocked fields.

In vivo estimation of midline dose maps by transit dosimetry in head and neck radiotherapy

The aim of the present study is to compare the calculated midline dose map with the in vivo measured midline dose map, using portal detectors in conjunction with a pair of diodes. Measurements were performed in 10 patients treated for head/neck cancer and irradiated with lateral opposed 6 MV X-ray beams. The relative exit dose map, derived from transmission dose data of a portal film combined with the absolute entrance/exit dose measured by the diodes, can be used to derive the corresponding midline dose map by applying appropriate algorithms. Midplane dose values were estimated in eight relevant anatomic positions and compared with the corresponding calculated values with our three-dimensional (3D) treatment planning system using two-dimensional (2D) (Batho) and 3D (ETAR) inhomogeneity correction algorithms. In vivo estimated midplane doses agree within +/-3.5% relative to treatment planning calculations in 89 of 116 measurements points, with only 4 of 116 points outside +/-5%. A variation between measured and calculated dose can be found according to anatomical location. For air inhomogeneity, mean deviations were +2.2% (1 standard deviation (SD) approximately 1.7%) for both Batho and ETAR algorithms; for bone structures, mean deviations were approximately -0.6% (1 SD approximately 2.7%) for both algorithms. The worst agreement was found in the anterior neck where the mean deviation between measured and calculated midline dose was +3.1% (1 SD=1.4%) and +3.4% (1 SD= 2%) using Batho and ETAR, respectively. Sufficiently accurate 2D midplane dose maps may be simply obtained in vivo in the irradiation of head/neck cancer by using a portal detector in combination with a pair of diodes, in order to verify the dose actually delivered during treatment.

A simplified approach for exit dose in vivo measurements in radiotherapy and its clinical application

This is a study using LiF:Mg;Ti thermoluminescent dosimeter (TLD) rods in phantoms to investigate the effect of lack of backscatter on exit dose. Comparing the measured dose with anticipated dose calculated using tissue maximum ratio (TMR) or percentage depth dose (PDD) gives rise to a correction factor. This correction factor may be applied to in-vivo dosimetry results to derive true dose to a point within the patient. Measurements in a specially designed humanoid breast phantom as well as patients undergoing radiotherapy treatment were also been done. TLDs with reproducibility of within +/- 3% (1 SD) are irradiated in a series of measurements for 6 and 10 MV photon beams from a medical linear accelerator. The measured exit doses for the different phantom thickness for 6 MV beams are found to be lowered by 10.9 to 14.0% compared to the dose derived from theoretical estimation (normalized dose at dmax). The same measurements for 10 MV beams are lowered by 9.0 to 13.5%. The variations of measured exit dose for different field sizes are found to be within 2.5%. The exit doses with added backscatter material from 2 mm up to 15 cm, shows gradual increase and the saturated values agreed within 1.5% with the expected results for both beams. The measured exit doses in humanoid breast phantom as well as in the clinical trial on patients undergoing radiotherapy also agreed with the predicted results based on phantom measurements. The authors' viewpoint is that this technique provides sufficient information to design exit surface bolus to restore build down effect in cases where part of the exit surface is being considered as a target volume. It indicates that the technique could be translated for in vivo dose measurements, which may be a conspicuous step of quality assurance in clinical practice.

Portal imaging

Portal imaging is the acquisition of images with a radiotherapy beam. Imaging theory suggests that the quality of portal images could be much higher if the efficiency of the imaging media in detecting radiation could be improved. Introduction of new media (films and electronic portal imaging devices) has confirmed this by markedly increasing the quality of portal images. Images from these devices can then be used to verify a patient's treatment. Geometric verification requires the portal image to be registered with a reference image. Dosimetric verification requires the portal imager to be calibrated for dose. This review gives a brief overview of the current areas of interest in portal imaging: imaging theory; imaging media, film and electronic portal imaging devices; image registration; and dosimetry using these devices.

A simple and robust method for in vivo midline dose map estimations using diodes and portal detectors

INTRODUCTION

This work investigates the possibility of using a pair of diodes on the beam axis in conjunction with a portal imaging detector to estimate in vivo midline dose distributions, without any additional patient information, related to the external body contour.

MATERIALS AND METHODS

In the proposed method, the patient is considered equivalent to a parallelepiped phantom with a thickness z equal to the patient's physical thickness on the field axis with a variable electronic density rho, depending on the water-equivalent thickness. Based on this assumption, if the air gap between portal detector and patient is kept small (within 10-15 cm), the relative exit dose map may be assumed to be equal to the corresponding map measured at the portal detector level by geometrical back projection to the corresponding exit points. The relative exit dose map is then normalized at the on-axis value measured by the exit diode. The entrance dose map is derived by correcting the absolute dose value measured with the diode at the entrance surface by the off-axis ratios. For each pair of entrance and exit doses, the midline dose may be estimated by applying algorithms reported in literature. The method was tested in 6 MV beams using portal film as detector and the Huyskens and Rizzotti algorithms for midline dose estimation. Tests on homogeneous cubic phantoms, homogeneous phantoms with varying thickness symmetrically (simulating head and neck regions) and asymmetrically (simulating abdomen/pelvis region), and a half-sphere phantom with simulating the breast, were performed. Midline doses estimated with the proposed method have been compared with corresponding ones measured by ionisation chamber.

RESULTS AND DISCUSSION

Results confirm that the proposed method can be used to estimate midplane dose maps within 2-3% for most clinically suitable situations. For homogeneous symmetrical phantoms the agreement between estimated and measured midline doses decreases with the phantom-portal film distance, the field sizes and the thickness. For homogeneous asymmetrical phantoms the percentage deviations are generally within 3%. Discrepancies larger than 3% (up to 5-6%) are found only for "stressed" irradiation geometries which are not linked with any clinical condition.

CONCLUSIONS

The obtained results not only show the accuracy of the proposed method but, due to its simplicity, suggest a rapid clinical implementation of this method in relevant clinical situations such as head-neck, breast and abdomen/pelvis irradiation. Previous investigations which confirmed the possibility of using portal detectors for transit dosimetry in inhomogeneous regions suggest the further exploration of the accuracy and the limits of the proposed method in such cases.