Development of in-house heterogeneous thorax phantom and evaluation of pretreatment patient-specific transit dosimetry for intensity-modulated radiotherapy and volumetric modulated arc therapy plans.

BACKGROUND

Patient-specific dosimetry before patient treatment plays a crucial role in radiotherapy (RT) treatment. Absolute point dosimetry and relative dosimetry using homogeneous phantom are the regular methods which are employed for dose verification in RT department. However, this method does not imitate the realistic radiation interaction taking place inside the patient's body as it is heterogeneous in nature. Hence, to perform the relative dosimetry inside the heterogeneous medium which can very well replicate the actual patient scenario, in the present work, we studied the radiological properties of in-house developed heterogeneous thorax phantom (HTP) phantom for different photon energies. And in the second part, we performed the patient-specific relative transit dosimetry by using the design cost-effective HTP.

MATERIALS AND METHODS

HTP was constructed with porous sawdust of pinewood of density 0.24 g/cm, honeybee's wax of density 0.86 g/cm, and rib cage of density 1.84 g/cm to mimic the actual human thorax. To assess the radiological properties of designed HTP, the mean depth of central axis isodose curves was measured on computed tomography images of homogeneous slab phantom (HSP), actual patient, and on HTP. To evaluate the performance of treatment planning system (TPS), quality assurance (QA) plans of 30 patients were generated on HTP, and the two-dimensional dose fluence calculated by TPS was compared with that of the acquired dose fluence on a linear accelerator. Global γ index passing criteria (dose difference of 3% and distance-to-agreement of 3 mm) were used to evaluate the closeness between the calculated and measured fluence maps.

RESULTS

The depth of various isodose lines along the central axis was found to be similar in HTP and actual patients as compared to HSP for different photon energies using varied gantry angles. The γ values for relative exit dosimetry were found to be <1 for >97% of data set points and the correlation factor r was found to be positive ≤1 for all QA plans which indicates the good correlation between calculated and acquired dose fluence.

CONCLUSIONS

In-house developed HTP is a cost-effective phantom which resembles with that of the human thorax in terms of its radiological properties. Moreover, it can be a better QA medium for pretreatment plan verification of the actual patients.