A BrachyPhantom for verification of dose calculation of HDR brachytherapy planning system.

PURPOSE: To develop a calibration phantom for (192)Ir high dose rate (HDR) brachytherapy units that renders possible the direct measurement of absorbed dose to water and verification of treatment planning system. METHODS: A phantom, herein designated BrachyPhantom, consists of a Solid Water™ 8-cm high cylinder with a diameter of 14 cm cavity in its axis that allows the positioning of an A1SL ionization chamber with its reference measuring point at the midheight of the cylinder's axis. Inside the BrachyPhantom, at a 3-cm radial distance from the chamber's reference measuring point, there is a circular channel connected to a cylindrical-guide cavity that allows the insertion of a 6-French flexible plastic catheter from the BrachyPhantom surface. The PENELOPE Monte Carlo code was used to calculate a factor, P(sw)(lw), to correct the reading of the ionization chamber to a full scatter condition in liquid water. The verification of dose calculation of a HDR brachytherapy treatment planning system was performed by inserting a catheter with a dummy source in the phantom channel and scanning it with a CT. The CT scan was then transferred to the HDR computer program in which a multiple treatment plan was programmed to deliver a total dose of 150 cGy to the ionization chamber. The instrument reading was then converted to absorbed dose to water using the N(gas) formalism and the P(sw)(lw) factor. Likewise, the absorbed dose to water was calculated using the source strength, Sk, values provided by 15 institutions visited in this work. RESULTS: A value of 1.020 (0.09%, k = 2) was found for P(sw)(lw). The expanded uncertainty in the absorbed dose assessed with the BrachyPhantom was found to be 2.12% (k = 1). To an associated Sk of 27.8 cGy m(2) h(-1), the total irradiation time to deliver 150 cGy to the ionization chamber point of reference was 161.0 s. The deviation between the absorbed doses to water assessed with the BrachyPhantom and those calculated by the treatment plans and using the Sk values did not exceed ± 3% and ± 1.6%, respectively. CONCLUSIONS: The BrachyPhantom may be conveniently used for quality assurance and/or verification of HDR planning system with a priori threshold level to spot problems of 2% and ± 3%, respectively, and in the long run save time for the medical physicist.

On the need for quality assurance in superficial kilovoltage radiotherapy.

External auditing of beam output and energy qualities of four therapeutic X-ray machines were performed in three radiation oncology centres in northeastern Brazil. The output and half-value layers (HVLs) were determined using a parallel-plate ionisation chamber and high-purity aluminium foils, respectively. The obtained values of absorbed dose to water and energy qualities were compared with those obtained by the respective institutions. The impact on the prescribed dose was analysed by determining the half-value depth (D(1/2)). The beam outputs presented percent differences ranging from -13 to +25%. The ratio between the HVL in use by the institution and the measurements obtained in this study ranged from 0.75 to 2.33. Such deviations in HVL result in percent differences in dose at D(1/2) ranging from -52 to +8%. It was concluded that dosimetric quality audit programmes in radiation therapy should be expanded to include dermatological radiation therapy and such audits should include HVL verification.

Absorbed dose determination with plane-parallel chambers.

According to IAEA TRS 398 recommendations the determination of absorbed dose with plane-parallel ionisation chambers calibrated in terms of N(K,Q0) can be done using N(D,W,Q0) = (M(Q0)(free air)/M(Q0)(surface)) N(K,Q0)B[(mu(en)/rho)(W,air)](free air) P(Q0). This equation takes into account only the scattering from the stem of the soft ionisation chamber, not the total scattering published in the scientific literature. That makes it difficult to perform dosimetry with field sizes different from those used in the standardisation laboratory to calibrate the chamber. This paper describes a method to calculate D(W,Q0) by using either N(K,Q0) or N(D,W,Q0) for different radiation field sizes.

A graphite transmission ionization chamber.

A pancake-type transmission chamber made of high-purity graphite and open to the atmosphere has been designed and constructed at the Secondary Standard Dosimetry Laboratory (SSDL-Rio de Janeiro). Tests performed on the chamber following the International Electrotechnical Commission recommendations indicate that its performance characteristics are comparable to those expected from a secondary standard ionization chamber.