The accuracy of treatment planning system dose modelling in the presence of brass mesh bolus

Effect of variation of silicone rubber RTV 52 and bluesil catalyst 60 R composition on bolus material for electron beam radiotherapy application

A bolus is a material equivalent to soft tissue and is directly placed on the skin surface during radiotherapy. It is commonly used to increase the dose on the skin surface in electron beam radiation. A typical material for a bolus is silicone rubber (SR). We made a bolus with dimensions of 17 × 17 × 1 cm³by varying silicone rubber (SR) RTV 52 and hardening material (bluesil catalyst 60 R) using a simple molded method. We characterized it using a CT scan to find the relative electron density (RED) and examined it using the electron beam of a linear accelerator (LINAC) at energies of 5 and 7 MeV to investigate the percentage of surface dose (PSD). The PSD value is relative to the dose at maximum doses (d_max). The RED value of the bolus was from 1.168 ± 0.021 to 1.176 ± 0.019, higher than the soft tissue (muscle) value of 1.043. The percentage of surface dose (PSD) test at 5 and 7 MeV LINAC energy showed that the highest PSD without using a bolus were 84.79±0.06% and 86.03±0.07%, respectively. With a bolus, the PSD values were 112.52±0.16% and 111.14±0.03%, respectively. The results indicate that bolus fabrication using SR RTV 52 and bluesil 60R is very effective for radiotherapy in the treatment of skin cancer due to an increase in surface dose.

Effects of lung tissue characterization in radiotherapy of breast cancer under deep inspiration breath hold when using Monte Carlo dosimetry

PURPOSE

To investigate the sensitivity of Monte Carlo (MC) calculated lung dose distributions to lung tissue characterization in external beam radiotherapy of breast cancer under Deep Inspiration Breath Hold (DIBH).

METHODS

EGSnrc based MC software was employed. Mean lung densities for one hundred patients were analysed. CT number frequency and clinical dose distributions were calculated for 15 patients with mean lung density below 0.14 g/cm³. Lung volume with a pre-defined CT numbers was also considered. Lung tissue was characterized by applying different CT calibrations in the low-density region and air-lung tissue thresholds. Dose impact was estimated by Dose Volume Histogram (DVH) parameters.

RESULTS

Mean lung densities below 0.14 g/cm³ were found in 10% of the patients. CT numbers below -960 HU dominated the CT frequency distributions with a high rate of CT numbers at -990 HU. Mass density conversion approach influenced the DVH shape. V_4Gy and V_8Gy varied by 7% and 5% for the selected patients and by 9% and 3.5% for the pre-defined lung volume. V_16Gy and V_20Gy, were within 2.5%. Regions above 20 Gy were affected. Variations in air- lung tissue differentiation resulted in DVH parameters within 1%. Threshold at -990 HU was confirmed by the CT number frequency distributions.

CONCLUSIONS

Lung dose distributions were more sensitive to variations in the CT calibration curve below lung (inhale) density than to air-lung tissue differentiation. Low dose regions were mostly affected. The dosimetry effects were found to be potentially important to 10% of the patients treated under DIBH.