Film measurements of kilovoltage x-ray percentage depth doses for different energies and field sizes

Abstract

Objectives: Kilovoltage x-ray therapy, which includes superficial x-rays and orthovoltage x-rays, is particularly suited for treatments near the skin surface. Radiation dose deposition at depth is typically characterized by percentage depth dose (PDD) curves. However, PDD data are difficult to obtain in kilovoltage x-ray therapy, hence clinics often rely on British Journal of Radiology Supplement 25 (BJR-25) archival data. This study measured PDD for different energies and field sizes on the Xstrahl 300 kilovoltage x-ray therapy unit for comparison with BJR-25 data.

Methods: We irradiated EBT3 film in water using different energy and field size combinations, then, compared the acquired PDD curves with BJR-25 data. We used 75, 125, and 250 kVp beams, for 10 × 10 cm2 open field, 4 × 4 cm2 closed-ended square applicator, and open-ended circular applicators of 1.5, 2.0, 2.8, and 4.7 cm diameter.

Results: PDD for different energies and field sizes on the Xstrahl 300 kilovoltage x-ray therapy unit agreed well with BJR-25 data. The average difference between measured and BJR-25 data was -0.3 ± 1.8%; 85.9% of the individual local differences were within ±3%, 94.8% within ±4%, and 99.5% within ±5%.

Conclusions: The PDD curves presented here, together with BJR-25 data, can serve as useful comparison datasets for the commissioning of kilovoltage x-ray machines.

Advances in knowledge: Our film-based measurement technique provides high spatial resolution PDD datapoints for a modern kilovoltage x-ray unit, adding to the limited published data in the kilovoltage energy range.

Keywords: kilovoltage x-ray; orthovoltage; percentage depth dose; superficial.

Figure 1.

Setup for percentage depth dose measurements with film in water on the Xstrahl 300 kilovoltage x-ray therapy unit.

Figure 2.

Energy-specific film calibration curves to convert net optical density to dose.

Figure 3.

Percentage depth dose curves for 75 kVp (2.2 mm Al HVL), 125 kVp (3.5 mm Al HVL), and 250 kVp (1.3 mm Cu HVL) beams based on film measurements and compared with BJR-25 data, grouped by field sizes: (A) 10 × 10 cm² open (no applicator), (B) 4 × 4 cm² square applicator, (C) 1.5 cm diameter circular applicator, (D) 2.0 cm diameter circular applicator, (E) 2.8 cm diameter circular applicator, and (F) 4.7 cm diameter circular applicator.

Figure 4.

Depth of dose penetration at various dose levels, 90% of the maximum dose (dark green), 80% of the maximum dose (light green), and 50% of the maximum dose (yellow), for 75 kVp (2.2 mm Al HVL), 125 kVp (3.5 mm Al HVL), and 250 kVp (1.3 mm Cu HVL) beams, using circular applicators of diameters 1.5 cm, 2.0 cm, 2.8 cm, and 4.7 cm.

Figure 5.

Comparison of percentage depth doses for kilovoltage photon energies (75, 125, and 250 kVp) with those of megavoltage electron energies (6, 9, 12, 16, and 20 MeV) using a 4 × 4 cm² square applicator.