Dosimetric Study to Estimate Deviations in Delivered Radiation Dose due to Occluded Air Spaces in Vaginal Vault Brachytherapy Applications.

BACKGROUND: To develop a dosimetric tool to estimate the dose delivered in the presence of air pockets with EBT3 film while simulating the conditions of vaginal vault brachytherapy (VVBT) with 3.0 diameter cylindrical applicator at a prescription dose distance of 5mm from the surface of it. MATERIALS AND METHOD: Six acrylic plates (10 cm x 10 cm, 0.5 cm thick) with four different types of slots were designed and produced locally. They can hold a cylindrical vaginal brachytherapy applicator in the centre, air equivalent material from the applicator's surface [(sizes 4.5 mm (A), 3.0 mm (B), and 2.0 mm (C)], EBT3 film at the prescribed dose distance, and holder rods. Plates were layered together with acrylic rods and assembled in a holding box in a water phantom. Three treatment plans done in TPS with prescription doses of 2 Gy, 3 Gy, and 4 Gy at 5.0 mm with a treatment length of 6 cm, and were executed in Co-60-based HDR brachytherapy unit (M/s SagiNova, Germany) with & without the placement of air equivalent material, and the dose received at slot locations A, B, & C were noted. RESULTS: The mean percentage deviation of measured dose without and with presence of air pocket at A, B and C was 13.9%, 11.0% and 6.4% respectively for all dose prescriptions. As the air pocket size expanded radially from 2.0 mm to 4.5 mm, the increase in dosage ranged from 6.4% to 13.9% which was due to the fact that the film was held at dosage prescription distance and the lack of attenuation of photons radially through air pocket. CONCLUSIONS: The present study can be carried out with a 3D printed phantom that simulates VVBT application having air pockets of different dimensions at different locations and also can be analyzed with Monte Carlo simulations.

Estimation of Surface Dose in the Presence of Unwanted Air Gaps under the Bolus in Postmastectomy Radiation Therapy: A Phantom Dosimetric Study.

BACKGROUND: The aim of this study is to design and fabricate a thorax phantom with irregularly shaped trapezoidal slots across the left side of the chest wall, allowing for the creation of unwanted air gaps under the bolus. METHOD: Surface dose (Dsurf) measurements were made with Gaf Chromic EBT3 films at air gaps (0.0, 5.0, 10.0 and 15.0 mm) under gel bolus of thickness (5.0 mm & 10.0 mm), for 3DCRT technique (2 and 3 field) with clinical 6 MV photon beam under uniform and non-uniform air gap condition. The obtained values were compared with TPS estimated ones. RESULTS: In the presence of 15.0 mm uniform air gap, the mean estimated and measured Dsurf values with two and three field techniques decreased by 14.0 % to 15.2% and 14.7% to 17.4% under 5.0 mm and 10.0 mm bolus applications respectively. In presence of non-uniform air gap condition, the effect on Dsurf was minimal (3 to 3.5%) compared with the uniform air gap condition. CONCLUSIONS: Based on the study's findings, it is recommended that when using bolus in clinical radiotherapy applications, special care be taken to avoid unwanted air gaps under the bolus in order to achieve a uniform surface dose across the treatment region, where a customized 3D printed bolus may be a better option.

Fabrication and development of a thorax phantom to evaluate 3D CRT treatment planning techniques for post-mastectomy chest wall radiotherapy.

The aim of this study was to design and fabricate a thorax phantom to quantify the radiation doses to the region of the chest wall (with 3 ionization chambers), the organ at risk (OAR) (lung), and the surface using radiochromic films (EBT3) for three different 3D CRT treatment planning techniques. Anthropomorphic phantoms are one of the best tools for verifying the quality of the radiotherapy treatment plans generated by treatment planning systems since they can provide equivalent human tissue densities. Thirty acrylic plates were cut into ellipses 21 cm in height and 31 cm in width, and slots were created to insert lung equivalent cork material and bone equivalent Teflon material. Three treatment planning techniques were designed: (A) tangential pair beams, (B) tangential pair beams with wedges and (C) tangential beams followed by an anterior oblique beam. The percentage difference between the measured point doses and the calculated doses (measured with three CC13 ionization chambers) ranged from - 3.2 to 1.6%, with a mean deviation of - 1.04 ± 1.3%. The measured mean percentage doses on the target surface with EBT3 film were 90.3% and 95.1% of the prescribed dose with 5-mm and 10-mm boluses, respectively. Finally, the average absolute dose difference between the measured and calculated surface doses was within 10 cGy in all three planning techniques. The developed thorax phantom is suitable for point dose measurements using ionization chambers and for surface dose measurements using EBT3 Gafchromic films in post-mastectomy chest wall radiotherapy.