Session Item

To develop an end-to-end test for empirical validation of 3D dose distributions in prostate and rectum using an anthropomorphic pelvis phantom.

Polymer dosimetry gel was filled into the 3D printed prostate organ shell of the phantom. A stack of nine thermoluminescent detectors (TLDs) mounted onto a specifically designed holder was placed into the rectum posterior to the prostate. The phantom was scanned in treatment position with an in-room CT (Somatom Emotion, Siemens Healthineers). Using Raystation8A (Raysearch Laboratories), a treatment plan simulating the clinical situation with seven beams aiming at a prescribed dose of 4.5 Gy to the whole prostate volume including a 4 mm margin was calculated. Phantom irradiation was performed on a 6 MV Linac (Artiste, Siemens). The dosimetry gel was evaluated using MRI and the TLDs were read out with a harshaw hot gas reader. For the dosimetry gel, the measured 3D dose distribution was compared to the calculated dose distribution with a 3D gamma analysis (Verisoft, PTW, passing criteria: 3% dose difference and 3 mm distance to agreement). The TLDs were used to evaluate punctual doses in the rectum and the measured doses were compared to calculated doses by the treatment planning software on a point-by-point basis.

The 3D gamma index comparing measured and calculated dose distributions in the prostate was 97.7%. For the TLDs, the average dose difference between calculated and measured doses was 0.08 Gy (range: 0.02 – 0.21 Gy, with 0.08 Gy referring to 1.78 % of the prescribed dose). Table 1 summarizes the calculated and measured doses in the rectum.

Empirical validation of dose distributions in prostate and rectum is feasible using 3D dosimetry gel and TLD dose measurements. This study describes a novel method for absolute 3D dosimetry in a complex anthropomorphic phantom.