Practical aspects of 153Gd as a radioactive source for use in brachytherapy

Liquid scintillation efficiencies, gamma-ray emission intensities, and half-life for Gd-153

Gadolinium-153 was standardized for activity by live-timed anticoincidence counting and an ampoule was submitted to the international reference system (SIR). Absolute emission intensities for the main γ rays were determined with calibrated high-purity germanium (HPGe) and lithium-drifted silicon (Si(Li)) detectors. A revised decay scheme is indicated, with no probability of direct electron capture to the 153Eu ground state. Triple-to-double coincidence ratio (TDCR) efficiency curves indicate that the revised decay scheme is consistent with experiment. Half-life measurements agree with a previous NIST determination and show no sensitivity to chemical environment.

Dosimetric impact of a robust optimization approach to mitigate effects from rotational uncertainty in prostate intensity-modulated brachytherapy

BACKGROUND

Intensity-modulated brachytherapy (IMBT) is an emerging technology for cancer treatment, in which radiation sources are shielded to shape the dose distribution. The rotatable shields provide an additional degree of freedom, but also introduce an additional, directional, type of uncertainty, compared to conventional high-dose-rate brachytherapy (HDR BT).

PURPOSE

We propose and evaluate a robust optimization approach to mitigate the effects of rotational uncertainty in the shields with respect to planning criteria.

METHODS

A previously suggested prototype for platinum-shielded prostate ¹⁶⁹ Yb-based dynamic IMBT is considered. We study a retrospective patient data set (anatomical contours and catheter placement) from two clinics, consisting of six patients that had previously undergone conventional ¹⁹² Ir HDR BT treatment. The Monte Carlo-based treatment planning software RapidBrachyMCTPS is used for dose calculations. In our computational experiments, we investigate systematic rotational shield errors of ±10° and ±20°, and the same systematic error is applied to all dwell positions in each scenario. This gives us three scenarios, one nominal and two with errors. The robust optimization approach finds a compromise between the average and worst-case scenario outcomes.

RESULTS

We compare dose plans obtained from standard models and their robust counterparts. With dwell times obtained from a linear penalty model (LPM), for 10° errors, the dose to urethra ( D 0.1 c c $D_{0.1cc}$ ) and rectum ( D 0.1 c c $D_{0.1cc}$ and D 1 c c $D_{1cc}$ ) increase with up to 5% and 7%, respectively, in the worst-case scenario, while with the robust counterpart, the corresponding increases were 3% and 3%. For all patients and all evaluated criteria, the worst-case scenario outcome with the robust approach had lower deviation compared to the standard model, without compromising target coverage. We also evaluated shield errors up to 20° and while the deviations increased to a large extent with the standard models, the robust models were capable of handling even such large errors.

CONCLUSIONS

We conclude that robust optimization can be used to mitigate the effects from rotational uncertainty and to ensure the treatment plan quality of IMBT.

Effectiveness of Rotating Shield Brachytherapy for Prostate Cancer Dose Escalation and Urethral Sparing

PURPOSE

To compare single-fraction 153Gd-based rotating shield brachytherapy (RSBT) for prostate cancer with conventional 192Ir-based high-dose-rate brachytherapy (HDR-BT) in a planning study that radiobiologically accounts for dose rate and relative biological effectiveness. RSBT was used for planning target volume (PTV) dose escalation without increasing urethral dose for monotherapy, or for urethral sparing without decreasing PTV dose as a boost to external beam radiation therapy.

METHODS AND MATERIALS

Twenty-six patients were studied. PTV doses were expressed as equivalent dose delivered in 2 Gy fractions (EQD2), accounting for relative biological effectiveness (1.00 for 192Ir and 1.15 for 153Gd), dose protraction (114-minute repair half-time), and tumor dose response (α/β of 3.41 Gy). HDR-BT dose was prescribed such that 90% of the PTV received 110% of the prescription dose of 19 Gy for dose escalation and 15 Gy for urethral sparing, corresponding to EQD290% values (minimum EQD2 to the hottest 90% of the PTV) of 93.9 GyEQD2 and 60.7 GyEQD2, respectively. Twenty 90.95 GBq 153Gd RSBT sources and one 370 GBq 192Ir HDR-BT source were modeled.

RESULTS

For dose escalation with fresh sources, RSBT increased PTV EQD290% by 42.5% ± 8.4% (average ± standard deviation) without increasing urethral D10%, with treatment times of 216.8 ± 28.9 minutes versus 15.1 ± 2.1 minutes. After 1 half-life (240.4 days for 153Gd and 73.8 days for 192Ir), EQD290% increased 20.5% ± 9.1%. For urethral sparing with fresh sources, RSBT decreased urethral D10% by 26.0% ± 3.4% without decreasing PTV EQD290%, with treatment times of 133.6 ± 16.5 minutes versus 12.0 ± 1.7 minutes. After 1 half-life, urethral D10% decreased 20.2% ± 4.8%.

CONCLUSIONS

RSBT can increase PTV EQD90% or decrease urethral D10% relative to HDR-BT at the cost of increased treatment time. Source aging reduces RSBT benefit, but RSBT remains theoretically superior to HDR-BT by >20% after 1 half-life has elapsed.