Leaf settings and corresponding EPID images for the four test fields , each with 2 segments. The collimator jaw edges are shown (thick grey lines) for the X1, X2, Y1, and Y2 sides (indicated for segment 1, tests A and C). MLC banks are located on the Y1 and Y2 sides; leaves are shown as thin lines. Test fields A and B produce on- and off-axis cold spots, where segments overlap in regions. Test fields C and D result in abutting segments; the abutment region is parallel to the leaf motion. In both C and D, the abutment side is blocked by the jaw (X2) in segment 1, and by the MLC (X1) in segment 2. The minimum leaf gap under the jaw is located centrally in test C, and shifted in the Y1 direction in test D.
Isocentre dose values for pre-treatment verification of 20 IMRT prostate plans. Dose values (a) and ratios (b) are given for the plan, EPID and ionization chamber. Both sets of measured dose values agree, with an average ratio of 1.00 [(b), diamonds], and both fall below the planned dose, with an average ratio of 0.99 [(b), squares and crosses].
Histogram of and , derived from evaluations, of 100 IMRT prostate fields (20 plans). Each planned field was compared with a 2D EPID dose image, reconstructed in the phantom at the plane perpendicular to the beam, intersecting the isocenter. The acceptance criteria for two parameters are also indicated. Two fields (from 1 plan) had and 4 fields (from 2 plans) had
(a) evaluation for one IMRT field comparing EPID dose distributions with the plan and a film measurement, with criteria . The scale represents gamma values. A discrepancy “cold spot” in the EPID versus plan image was not found when comparing EPID versus film. (b) Profiles for three dose distributions. In the region with the largest discrepancy, the EPID and film dose values agree , and are 16% lower than the planned dose values . (c) A evaluation of the total plan, comparing the film and planned dose distributions (isocentric coronal plane, all fields, original gantry angles). The dose discrepancy in one field resulted in an 8% local dose discrepancy in the total plan [(c) arrow].
Transversal slice of the CT scan and planned dose distribution of the same field, as shown in Fig. 4. The intended low dose region along the length of the beam ray shows the location in the patient that would be affected by the discrepancy.
A evaluation for test fields A and B, EPID versus plan, with criteria . A is the combination of 2 rectangular segments with a leaf over-travel of , designed to produce a “cold spot” at the isocenter. A similar design was applied to B, with the intended cold spot regions located either side of the center, off-axis. In both cases, dose discrepancies were 9% (of ) and 12% in A and B, respectively, however the distance-to-agreement was within . Therefore the discrepancy was considered minor. The under-dosage at the junction of opposing leaves, however, lead to discrepancies up to 16% along the thin region where the segments meet.
Segment shapes and evaluations for test fields C and D, and a clinical pre-treatment IMRT field, using criteria of . The segments are outlined with the 50% field-edge detection line to indicate the respective segment shapes. The 2 segments in the IMRT field were delivered as separate beams with (upper) and (lower). The calculated dose in the abutting region decreased in all 3 cases after the tongue-and-groove width parameter in the TPS was increased from 0.06 to . In addition, there is a region of “agreement” in the middle of the red discrepancy line (along the abutment region), of test C and the IMRT field, and shifted to the Y1 direction (right) in test D. This is due to the leaf gap scatter from under the collimator, increasing the measured dose and cancelling out the under-dosage.
EPID image and dose line profile for test D. The dose distributions agree within at either side of the abutment region. The EPID (black line) gave an (9%) lower dose at the junction of the segments than the plan with (grey solid line). By effectively widening the “groove” width in the dose calculation model to (grey dotted line), greater attenuation leads to a reduction in the calculated dose, better matching the measured dose distribution.
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