Presentation of a matrix-based method to calculate dose distribution in brachytherapy with photon-emitting sources.

AIM OF STUDY: The report of the task group number 43(TG-43) of AAPM has been known as the most common method to obtain the dose distribution around brachytherapy sources. The error caused by independent obtaining and rounding and combinational error caused by algebraic operations of each TG-43 dosimetry parameters increase the total error in the calculation of the dose distribution around the brachytherapy sources. The aim of the present study is to present and evaluate a matrix-based approach for simplifying and reducing calculation errors. MATERIALS AND METHODS: In this study, the simulation method with MCNPX code was used to obtain the dose distribution. Four sources were simulated, and the dose matrix around these sources was obtained. Finally, the dose distribution obtained from the matrix-based method was compared with the dose distribution obtained from the method of TG-43 report. RESULTS: There is little difference between the values obtained from the two methods in some points. Absolute mean differences between the values obtained by these two methods were 1.4% for the 60Co, 3.52% for the 137Cs, 2.67% for the 192Ir, and 2.42% for the 103Pd sources. The advantage of the matrix-based method is its simple computing process and less computation time. CONCLUSION: Considering that the comparison of brachytherapy sources is not raised in calculations of treatment planning systems and also considering the more uncertainty in the calculation of the dose distribution in TG-43 method, it is recommended that dose distribution obtained from matrix-based method be used as input for treatment planning systems.

A comparison between skin dose of breast cancer patients at the breast region, measured by thermoluminescent dosimeter in the presence and absence of bolus.

AIM: The aim of this study was to measure entrance skin dose (ESD) on the breast of patients who had undergone radiotherapy following surgery, in the presence and absence of bolus. MATERIALS AND METHODS: In this study, the ESD on the breast of 22 female patients was measured using thermoluminescent dosimeter-100 chips. For each patient, the ESD was measured 3 times (once without bolus and twice using bolus). The bolus types used in this study include super flab and wax. RESULTS: The average ESDs on the breast of patients (from both medial and lateral tangential fields) in the presence of the super flab bolus and absence of bolus were 225.8 and 148.17 cGy, respectively, that when using the bolus, around 52% increasing in ESD was observed. The results showed a significant relationship between the ESD on the breast of patients and bolus types (P = 0.002); in addition, correlation coefficient between the two boluses (super flab and wax) was 0.615 (r = 0.615). CONCLUSION: When using the bolus in postmastectomy irradiation, it is noted that in dose delivery to the chest wall, surgical scar or skin of the treated region should be considered. The use of the bolus as a substance that increases of the skin dose can sometimes cause an excessive increase in skin dose that may cause severe skin reactions and underdosing of underlying tissues. Furthermore, using wax bolus in regions that do not require a lot of shaping of bolus is affordable.