Evaluation of target dose based on water-equivalent thickness in external beam radiotherapy

Abstract

In vivo dosimetry was carried out for 152 patients receiving external beam radiotherapy and the treatment sites were divided into two main groups: Thorax, Abdomen, and Pelvic (120 fields) and Head and Neck (52 fields). Combined entrance and exit dose measurements were performed using LiF: Mg, Cu, P thermoluminescent dosimeters (TLDs). Water-equivalent (effective) thicknesses and target dose were evaluated using dose transmission data. The ratio of measured to expected value for each quantity was considered as an indicator for the accuracy of the parameter. The average ratio of the entrance dose was evaluated as 1.01 ± 0.07. In the diameter measurement, the mean ratio of effective depth divided by the contour depth is 1.00 ± 0.13 that shows a wide distribution which reflects the influence of contour inaccuracies as well as tissue inhomogeneities. At the target level, the mean ratio of measured to the prescribed dose is 1.00 ± 0.07. According to our findings, the difference between effective depth and patient depth has a direct relation to target dose discrepancies. There are some inevitable sources which may cause the difference. Evaluation and application of effective diameter in treatment calculations would lead to a more reliable target dose, especially for fields which involve Thorax, Abdomen, and Pelvic.

Keywords: External beam radiotherapy; in vivo do simetry; target dose; thermoluminescent dosimetry.

Figure 1

Samples of glow curve for two different annealing procedures. (a) TLDs were annealed in the oven for 10 min. (b) Post-read annealing cycle in the reader was followed (240°C for 10 s). Deformed glow curve was noticed during the successive measurement following the first annealing procedure

Figure 2

Schematic representation for entrance dose calibration of TLDs. TLD badges were positioned at the surface of standard water phantom (30 × 30 × 10 cm³) in reference geometry and covered with 0.5-cm build-up material. The aver age TLDs’ measured signal was correlated to the IC measured dose in the depth d_m (D_en). The geometry for exit dose calibration was the same except that TLD badges were stuck at the beam exit point

Figure 3

Dose–response (calibration) functions of TLDs. The function was evaluated for the entrance and exit surfaces separately

Figure 4

The difference between measured and delivered dose (recorded by IC) values at the exit surface. TLDs’ readings for different dose values at the exit surface were analyzed using both entrance calibration function (patterned bars) and also exit calibration function (gray bars) and the respective sets of ECCs. Application of entrance calibration function and ECCs for TL data of the exit surface could lead to a systematic underestimation of the exit dose values

Figure 5

SSD correction factor at the entrance surface (a) and the exit surface (b). All data have been evaluated for a 10 × 10 cm2 field size (at the collimator), then normalized to the response at the reference geometry

Figure 6

Field-size correction factor at the entrance surface (a) and the exit surface (b). All data have been evaluated for an SSD = 80 cm and different field sizes, then normalized to the response at the reference geometry

Figure 7

Results for entrance dose measurements. The histogram shows the distribution for the ratio of measured to expected entrance doses. Light-gray bars represent T-A-P treatments (n = 106) and dark-gray bars represent H-N treatments (n = 52). Patterned bars correspond to T-A-P data, which were collected during malfunction of SSD indicator (n = 14)

Figure 8

Results for water-equivalent diameter measurements. The histogram shows the distribution for the ratio of water-equivalent to contour diameters. Light-gray bars represent T-A-P treatments (n = 106) and patterned bars represent H-N treatments (n = 52). Dark-gray bars correspond to T-A-P data, which were collected during malfunction of SSD indicator (n = 14)

Figure 9

The distribution of discrepancies between patient and contour diameters. Patients’ diameters were checked with caliper and compared to contour data. All patients were treated for T-A-P

Figure 10

Results for midline dose measurements. The histogram shows the distribution for the ratio of measured to expected (prescribed) midline doses. Light-gray bars represent T-A-P treatments (n = 106) and patterned bars represent H-N treatments (n = 52). Dark-gray bars correspond to T-A-P data, which were collected during malfunction of SSD indicator (n = 14)