Session Item

The GammaPod™ (GP, Xcision Med. Syst.) is a recently-introduced system for stereotactic radiotherapy of the breast. It consists of 25 high-activity Co-60 sources rotating during the patient treatment with two possible collimators (15 and 25 mm of nominal diameter, ∅). Precise dose painting is obtained  by means of table translations. GP TPS requires the determination of ∅25 dose rate and ∅15 OF in PMMA. Moreover, a water phantom is used in our center for patient-specific treatment verifications. To date GP dosimetry has not been included in dosimetry codes of practice as IAEA-AAPM TRS 483. Recently Yi and Becker proposed a practical method for determining dose in PMMA through the TRS 483 formalism  and the Exradin A1SL (Standard Imaging) ion chamber (Physica Medica 88, 138–141, 2021). The authors themselves admitted some limits in their approach and the need of further investigations. Our work had a two-fold purpose: to compare the GP A1SL dosimetry vs. detectors other than ion chambers, and provide  correction factors referred to in Eq. 25 of TRS 483, for different chambers in both PMMA and water.

GP system was installed in our center in June 2021. Since no static irradiation setup can be set, the machine reference field was obtained with all the rotating sources simultaneously out and ∅25-collimated. The in-PMMA A1SL dose results were validated vs. both OSL and GaFchromic™ EBT3 films. A dose rate to activity coefficient, i.e. the ∅25 dose rate divided by the total activity amount at the same time, was determined as practical reference. Moreover, the following ion chambers were cross-calibrated against A1SL in both PMMA and water phantoms: PTW 31021 and 31016, IBA Razor, Razor Nano, and CC01. ∅15 OF results were compared with the Monte Carlo (MC)-calculated value as provided by Xcision: for this purpose also the PTW 60019 microDiamond was used in addition to all the above detectors.

Based on A1SL measurements, the dose rate to activity coefficient resulted to be 1.94 cGy /(min TBq). The deviation from A1SL of OSL and EBT3 films resulted to be 6.2 % and 2.6% on the average, respectively. For all the investigated detectors the resulting OFs agreed with the MC value less than 1% except OSL (-1.2% on the average). Correction factors from cross-calibration are shown in Table 1.

A1SL dose determination agrees with both OSL and GaFchromic findings within their own uncertainties. The obtained dose rate to activity coefficient can be useful for intercomparisons among GammaPod centers. The MC OF value, as calculated by Xcision, well agrees with the performed measurements. Correction factors for 5 ion chambers other than A1SL are provided. A completion of the detector intercomparison by means of TLD dosimetry is on going. In addition, more ion chambers of the same model will allow taking into account manufacturing reproducibility.