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/cm3, honeybee's wax of density 0.86 g/cm3, and rib cage of density 1.84 g/cm3 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.

Validation of absolute point dosimetry by the analytical anisotropic algorithm and Acuros XB algorithm employing intensity-modulated radiotherapy technique on an in-house develop cost-effective heterogeneous thorax phantom.

INTRODUCTION: Dose validation inside the human body needs a medium which can simulate the actual heterogeneities of a specific body site. The aim of the present work is to study the properties of a cost-effective heterogeneous thorax phantom (HTP) developed in-house by the author and its application for the evaluation of patient-specific absolute point dosimetry by employing analytic anisotropic algorithm (AAA) and Acuros XB (AXB) algorithm. MATERIALS AND METHODS: HTP was made from the dust of porous pinewood, rib cage, and honeybee's wax. Density and central axis isodose depth distribution was measured on computed tomography images of actual patient and on HTP. Absolute point dose verification of 35 patients was done using AAA and AXB algorithm. The difference in the calculated dose by AAA and AXB was compared using the Wilcoxon signed-rank test. RESULTS: Density distribution and central axis depth dose inside the HTP compare well with that of an actual patient. The mean percentage variation between the planned and the measured doses inside the HTP was found to be 4.85 (standard deviation [SD] = 3.38) and 1.3 (SD = 2.93), respectively, using AAA and AXB algorithm. The difference in the measured dose and the planned dose was found to be significant for AAA with the significance level of 0.01 (p-value < 0.00001), whereas it was found to be insignificant (p-value < 0.00001) for AXB. CONCLUSION: The results of this study showed that the HTP is resembled with the human thorax in terms of its heterogeneities and radiological properties and can be used for pretreatment plan verification.

To evaluate volume changes on computerized tomography scan and magnetic resonance imaging-based delineation during radiotherapy treatment planning in prostate cancer.

AIM: The aim of the present study was to evaluate the impact of magnetic resonance imaging (MRI) on radiotherapy target volume changes in prostate cancer. MATERIALS AND METHODS: Ten patients with localized prostate cancer receiving radical radiotherapy were included in the study. Computerized tomography (CT) simulation was done with adequate immobilization, and pelvic MRI was also done at the same time. The two were then registered on eclipse planning system and fused. Target delineation (gross tumor volume [GTV] and clinical target volume [CTV]) was done on both the image sets separately and their volumes were compared. RESULTS: In the current study, it has been found that the CT image-based contouring overestimated the GTV and CTV with 35.4% and 21.7%, respectively, as compared to that by MRI images. The difference observed was statistically significant in the case of GTV, whereas it was not statistically significant for CTV. CONCLUSIONS: It can be concluded that MRI is found to be a better modality for GTV delineation, as it gives superior soft-tissue contrast.

Dosimetric analysis of Co-60 source based high dose rate (HDR) brachytherapy: A case series of ten patients with carcinoma of the uterine cervix.

AIM: To analyse the dosimetric parameters of Co-60 based high dose rate (HDR) brachytherapy plans for patients of carcinoma uterine cervix. BACKGROUND: Co-60 high dose rate (HDR) brachytherapy unit has been introduced in past few years and is gaining importance owing to its long half life, economical benefits and comparable clinical outcome compared to Ir-192 HDR brachytherapy. MATERIALS AND METHODS: A study was conducted on ten patients with locally advanced carcinoma of the uterine cervix (Ca Cx). Computed tomography (CT) images were taken after three channel applicator insertions. The planning for 7 Gray per fraction (7 Gy/#) was done for Co-60 HDR brachytherapy unit following the American Brachytherapy Society (ABS) guidelines. All the patients were treated with 3# with one week interval between fractions. RESULTS: The mean dose to high risk clinical target volumes (HRCTV) for D90 (dose to 90% volume) was found to be 102.05% (Standard Deviation (SD): 3.07). The mean D2cc (dose to 2 cubic centimeter volume) of the bladder, rectum and sigmoid were found to be 15.9 Gy (SD: 0.58), 11.5 Gy (SD: 0.91) and 4.1 Gy (SD: 1.52), respectively. CONCLUSION: The target coverage and doses to organs at risk (OARs) were achieved as per the ABS guidelines. Hence, it can be concluded that the Co-60 HDR brachytherapy unit is a good choice especially for the centers with a small number of brachytherapy procedures as no frequent source replacement is required like in an Ir-192 HDR unit.