The scaling method applied to beta particle line sources with a finite diameter

Therapeutic application of CCK2R-targeting PP-F11: influence of particle range, activity and peptide amount

BACKGROUND

Targeted radionuclide therapy with high-energy beta-emitters is generally considered suboptimal to cure small tumours (<300 mg). Tumour targeting of the CCK2 receptor-binding minigastrin analogue PP-F11 was determined in a tumour-bearing mouse model at increasing peptide amounts. The optimal therapy was analysed for PP-F11 labelled with (90)Y, (177)Lu or (213)Bi, accounting for the radionuclide specific activities (SAs), the tumour absorbed doses and tumour (radio) biology.

METHODS

Tumour uptake of (111)In-PP-F11 was determined in nude mice bearing CCK2 receptor-transfected A431 xenografts at 1 and 4 h post-injection for escalating peptide masses of 0.03 to 15 nmol/mouse. The absorbed tumour dose was estimated, assuming comparable biodistributions of the (90)Y, (177)Lu or (213)Bi radiolabelled peptides. The linear-quadratic (LQ) model was used to calculate the tumour control probabilities (TCP) as a function of tumour mass and growth.

RESULTS

Practically achievable maximum SAs for PP-F11 labelled with (90)Y and (177)Lu were 400 MBq (90)Y/nmol and 120 MBq(177)Lu/nmol. Both the large elution volume from the 220 MBq (225)Ac generator used and reaction kinetics diminished the maximum achieved (213)Bi SA in practice: 40 MBq (213)Bi/nmol. Tumour uptakes decreased rapidly with increasing peptide amounts, following a logarithmic curve with ED50 = 0.5 nmol. At 0.03 nmol peptide, the (300 mg) tumour dose was 9 Gy after 12 MBq (90)Y-PP-F11, and for (111)In and (177)Lu, this was 1 Gy. A curative dose of 60 Gy could be achieved with a single administration of 111 MBq (90)Y labelled to 0.28 nmol PP-F11 or with 4 × 17 MBq (213)Bi (0.41 nmol) when its α-radiation relative biological effectiveness (RBE) was assumed to be 3.4. Repeated dosing is preferable to avoid complete tumour receptor saturation. Tumours larger than 200 mg are curable with (90)Y-PP-F11; the other radionuclides perform better in smaller tumours. Furthermore, (177)Lu is not optimal for curing fast-growing tumours.

CONCLUSIONS

Receptor saturation, specific radiopharmaceutical activities and absorbed doses in the tumour together favour therapy with the CCK2 receptor-binding peptide PP-F11 labelled with (90)Y, despite its longer β-particle range in tissue, certainly for tumours larger than 300 mg. The predicted TCPs are of theoretical nature and need to be compared with the outcome of targeted radionuclide experiments.

Monte Carlo dose calculations using MCNP4C and EGSnrc/BEAMnrc codes to study the energy dependence of the radiochromic film response to beta-emitting sources

The energy dependence of the radiochromic film (RCF) response to beta-emitting sources was studied by dose theoretical calculations, employing the MCNP4C and EGSnrc/BEAMnrc Monte Carlo codes. Irradiations with virtual monochromatic electron sources, electron and photon clinical beams, a (32)P intravascular brachytherapy (IVB) source and other beta-emitting radioisotopes ((188)Re, (90)Y, (90)Sr/(90)Y,(32)P) were simulated. The MD-55-2 and HS radiochromic films (RCFs) were considered, in a planar or cylindrical irradiation geometry, with water or polystyrene as the surrounding medium. For virtual monochromatic sources, a monotonic decrease with energy of the dose absorbed to the film, with respect to that absorbed to the surrounding medium, was evidenced. Considering the IVB (32)P source and the MD-55-2 in a cylindrical geometry, the calibration with a 6 MeV electron beam would yield dose underestimations from 14 to 23%, increasing the source-to-film radial distance from 1 to 6 mm. For the planar beta-emitting sources in water, calibrations with photon or electron clinical beams would yield dose underestimations between 5 and 12%. Calibrating the RCF with (90)Sr/(90)Y, the MD-55-2 would yield dose underestimations between 3 and 5% for (32)P and discrepancies within +/-2% for (188)Re and (90)Y, whereas for the HS the dose underestimation would reach 4% with (188)Re and 6% with (32)P.

Patient and personnel dosimetry in endovascular radiotherapy with 90Sr/90Y sources

Endovascular brachytherapy (EVBT) is an established treatment to reduce the probability of restenosis after a percutaneous coronary intervention. The purpose of this study was to assess (1) the manufacturer's stated dosimetric data for (90)Sr/(90)Y source trains to be used in EVBT and (2) the procedure-related radiation burden. The radiation fields in water around six (90)Sr/(90)Y source trains were studied using phantoms made of 'solid water' and MD55-2 radiochromic films. The water equivalence of the phantom material was tested by applying quantitative computed tomography. Thermoluminescence dosemeters were used to assess personal radiation burden and crosscheck the dose distribution along the source trains. Technical failure was observed in one source train and this train was excluded from analysis. The measured dose rate in water at 2 mm radial distance was on average 8% higher than the manufacture's stated value (range of measured to stated values 1.05--1.15). The dose rate decreased exponentially with radial distance between 2 and 6 mm. The dose rate in contact with the source viewing window of the delivery devices ranged between 0.5 and 7.5 mGy h(-1). Low-energy photons were the main contributors to personal dose.

Basic treatment planning parameters for a 90Sr / 90Y source train used in endovascular brachytherapy

Working groups of the AAPM, DGMP, and ESTRO have published recommendations for endovascular brachytherapy, introducing concepts of relevant parameters for dose specification and treatment planning. However, the procedures for this treatment remain often mainly based on trial protocols and manufacturer instructions. Treatment planning requires the essential knowledge of the radial and longitudinal dose distribution, as well as information about geometrical uncertainties. The present study includes a whole data set for daily clinical practice using a commercially available device for endovascular brachytherapy (Novoste Betacath). The dose distribution around the 90Sr seed train was calculated with Monte-Carlo algorithms and verified by film dosimetry. The radial dose profile was determined starting from the surface of the delivery catheter Calculated dose profiles were in good agreement to measured values. The geometrical uncertainties were estimated with a retrospective analysis of 51 patient treatments. This shows the importance of using a safety margin of at least 10 mm between Intervention Length and Reference Isodose Length. Based on the longitudinal dose profile and the necessary safety margins, the maximum treatable intervention length is 25 mm and 45 mm for a 40 mm and 60 mm source train, respectively.

Clinical quality assurance for endovascular brachytherapy devices

BACKGROUND AND PURPOSE

Endovascular brachytherapy is still an important therapy modality with a high number of treated patients per year. Quality assurance of devices used has been addressed already in several publications (AAPM, DGMP, ESTRO, NCS). However, there are no clear recommendations given on test procedures and related equipment. Our experience with four different devices containing beta- ((32)P, (90)Sr/Y) and gamma-sources ((192)Ir), which were used in clinical routine during the last 3 years is described.

PATIENTS AND METHODS

The incoming check includes leakage radiation, missing catheter interlock, positioning test, timer check, interrupt button check, power-off test and verification of the manual retraction facility. Dose profiles are measured using GafChromic film. Source strength verification is performed using well type chambers or air-kerma measurements. In addition, the proposed reference absorbed dose rate at 2 mm distance from the source centre is measured with a dedicated film dosimetry technique where two additional films are exposed to two known doses in a (60)Co field for calibration.

RESULTS

Dosimetrical parameters (dose profiles, source strength) are found to be within +/-10% of the manufacturers specifications. The reference dose rate measured with film is on average +3.1% for 13 (90)Sr seed trains, +8.1% for three (32)P wire sources and -3.7% for one (192)Ir seed ribbon compared to the source certificate. The activity of 30 individual (32)P wire sources measured by using a calibrated well type chamber showed a deviation of mean -0.3%, the activity of 16 (192)Ir seed ribbons determined with air kerma measurements a deviation of mean 2.8%.

CONCLUSIONS

The QA programme introduced in our department provides methods to verify all relevant parameters proposed by international recommendations. Film dosimetry can be used as independent verification of the reference dose rate within a 10% limit.