Feasibility of reducing differences in estimated doses in nuclear medicine between a patient-specific and a reference phantom

Abstract

The feasibility of reducing the differences between patient-specific internal doses and doses estimated using reference phantoms was evaluated. Relatively simple adjustments to a polygon-surface ICRP adult male reference phantom were applied to fit selected individual dimensions using the software Rhinoceros®4.0. We tested this approach on two patient-specific phantoms: the biggest and the smallest phantoms from the Helmholtz Zentrum München library. These phantoms have unrelated anatomy and large differences in body-mass-index. Three models approximating each patient's anatomy were considered: the voxel and the polygon-surface ICRP adult male reference phantoms and the adjusted polygon-surface reference phantom. The Specific Absorbed Fractions (SAFs) for internal photon and electron sources were calculated with the Monte Carlo code EGSnrc. Employing the time-integrated activity coefficients of a radiopharmaceutical (S)-4-(3-F-fluoropropyl)-l-glutamic acid and the calculated SAFs, organ absorbed-dose coefficients were computed following the formalism promulgated by the Committee on Medical Internal Radiation Dose. We compared the absorbed-dose coefficients between each patient-specific phantom and other models considered with emphasis on the cross-fire component. The corresponding differences for most organs were notably lower for the adjusted reference models compared to the case when reference models were employed. Overall, the proposed approach provided reliable dose estimates for both tested patient-specific models despite the pronounced differences in their anatomy. To capture the full range of inter-individual anatomic variability more patient-specific phantoms are required. The results of this test study suggest a feasibility of estimating patient-specific doses within a relative uncertainty of 25% or less using adjusted reference models, when only simple phantom scaling is applied.

Impact of interpatient variability on organ dose estimates according to MIRD schema: Uncertainty and variance‐based sensitivity analysis

“…The absorbed dose coefficient for each source region r S was, of course, highly dependent on the TIAC(r S ). For the relatively low energies of the b + -spectrum of 18 F the dose absorbed in r S is mainly due to the self-absorption in this region. The applied mass-scaling of the self-absorption SAFs eliminated the interphantom variability in these values and thus the uncertainty in the output dose coefficients for r S was mainly caused by the variability in the respective TIACs.…”

“… Note that P‐SCP‐AM and P‐BCP‐AM are denoted as Pat2M and Pat1M, respectively, in the referenced study …”

“…We used the SAFs of monoenergetic photon and electron sources calculated for the considered phantoms (see Table and Fig. ) with the Monte Carlo code EGSnrc in our previous work . Since the cross‐sections for electrons and positrons of the same energies are very similar in the considered energy range of the β + ‐spectrum of 18 F (from zero to the end‐point energy E 0 = 633.5 keV), the SAFs of electron sources with the additional two 511 keV annihilation photons were used instead of the respective SAFs of positrons for the internal dosimetry in this work.…”

“…1) with the Monte Carlo code EGSnrc 29 in our previous work. 18 Since the cross-sections for electrons and positrons of the same energies are very similar in the considered energy range of the b + -spectrum of 18 F (from zero to the endpoint energy E 0 = 633.5 keV), 30 the SAFs of electron sources with the additional two 511 keV annihilation photons were used instead of the respective SAFs of positrons for the internal dosimetry in this work. The SAFs of monoenergetic electrons of various energies ranging from zero to E 0 were subsequently integrated over the detailed b + -spectrum of 18 F from ICRP 16 to obtain the S-values according to Eq.…”

“…Hence, no Note that P-SCP-AM and P-BCP-AM are denoted as Pat2M and Pat1M, respectively, in the referenced study. 18 variability of SAF(UB wall UB cont ) among the anatomical models was considered (where UB wall and UB cont refer to urinary bladder wall and urinary bladder content, respectively). The mass-scaling of the SAFs to the reference organ masses of RCP-AM was applied.…”