Technical Note: On the feasibility of performing dosimetry in target and organ at risk using polymer dosimetry gel and thermoluminescence detectors in an anthropomorphic, deformable, and multimodal pelvis phantom

Experimental validation of a novel method of dose accumulation for the rectum

BACKGROUND

Dose-surface maps (DSMs) are an increasingly popular tool to evaluate spatial dose-outcome relationships for the rectum. Recently, DSM addition has been proposed as an alternative method of dose accumulation from deformable registration-based techniques. In this study, we performed the first experimental investigation of the accuracy at which DSM accumulation can capture the total dose delivered to a rectum's surface in the presence of inter-fraction motion.

MATERIAL AND METHODS

A custom PVC rectum phantom capable of representing typical rectum inter-fraction motion and filling variations was constructed for this project. The phantom allowed for the placement of EBT3 film sheets on the representative rectum surface to measure rectum surface dose. A multi-fraction prostate VMAT treatment was designed and delivered to the phantom in a water tank for a variety of inter-fraction motion scenarios. DSMs for each fraction were calculated in two ways using CBCT images acquired during delivery and summed to produce accumulated DSMs. Accumulated DSMs were then compared to film measurements using gamma analysis (3%/2 mm criteria). Similarity of isodose clusters between films and DSMs was also investigated.

RESULTS

Baseline agreement between film measurements and accumulated DSMs for a stationary rectum was 95.6%. Agreement between film and accumulated DSMs in the presence of different types of inter.-fraction motion was ≥92%, and isodose cluster mean distance to agreement was within 1.5 mm for most scenarios. Overall, DSM accumulation performed the best when using DSMs that accounted for changes in rectum path orientation.

CONCLUSION

Dose accumulation performed with DSMs was found to accurately replicate total delivered dose to a rectum phantom in the presence of inter-fraction motion.

Thermoluminescent Tb(III) and Dy(III) complexes with redox-active ligands: experimental and theoretical study

Thermoluminescence and persistent luminescent materials with unique delayed emission have attracted much attention and exhibit great promise in optical information storage. In this manuscript, to reveal the thermoluminescence mechanism, a combined experimental and theoretical study of ternary Ln(NO₃ )₂ Acac(Phen)₂ complexes, where Ln is Tb(III), Dy(III), Eu(III), Acac is acetylacetonate anion, and Phen is 1,10-phenanthroline, was carried out. The terbium and dysprosium complexes had thermoluminescence properties, while the europium complex did not. A thermoluminescence mechanism is proposed: the powerful double π-conjugate phenanthroline system appearance upon photoexcitation, the peculiarities of frontier orbitals, the abnormally small highest occupied molecular orbital-lowest unoccupied molecular orbital gap, and the geometrical changes in the terbium and dysprosium complexes led us to suggest that phenanthroline molecules serve as 'chemical' electron traps. Therefore, we succeeded in 'freezing' and storing the excited state of complexes I and II indefinitely. The obtained thermoluminescent materials with 'chemical' traps of electrons are capable of storing the energy from incident photons and exhibit a great opportunity in optical information storage and anticounterfeiting applications.

End-to-end test for fractionated online adaptive MR-guided radiotherapy using a deformable anthropomorphic pelvis phantom

Objective.In MR-guided radiotherapy (MRgRT) for prostate cancer treatments inter-fractional anatomy changes such as bladder and rectum fillings may be corrected by an online adaption of the treatment plan. To clinically implement such complex treatment procedures, however, specific end-to-end tests are required that are able to validate the overall accuracy of all treatment steps from pre-treatment imaging to dose delivery.Approach.In this study, an end-to-end test of a fractionated and online adapted MRgRT prostate irradiation was performed using the so-called ADAM-PETer phantom. The phantom was adapted to perform 3D polymer gel (PG) dosimetry in the prostate and rectum. Furthermore, thermoluminescence detectors (TLDs) were placed at the center and on the surface of the prostate for additional dose measurements as well as for an external dose renormalization of the PG. For the end-to-end test, a total of five online adapted irradiations were applied in sequence with different bladder and rectum fillings, respectively.Main results.A good agreement of measured and planned dose was found represented by highγ-index passing rates (3%/3mmcriterion) of the PG evaluation of98.9%in the prostate and93.7%in the rectum. TLDs used for PG renormalization at the center of the prostate showed a deviation of-2.3%.Significance.The presented end-to-end test, which allows for 3D dose verification in the prostate and rectum, demonstrates the feasibility and accuracy of fractionated and online-adapted prostate irradiations in presence of inter-fractional anatomy changes. Such tests are of high clinical importance for the commissioning of new image-guided treatment procedures such as online adaptive MRgRT.