A detailed radiobiological and dosimetric analysis of biochemical outcomes in a case‐control study of permanent prostate brachytherapy patients

Abstract

The purpose of this study is to determine dosimetric and radiobiological predictors of biochemical control after recalculation of prostate implant dosimetry using updated AAPM Task Group 43 (TG-43) parameters and the radiobiological parameters recommended by TG-137. All biochemical failures among patients implanted with 125I Or 103Pd sources between 1994 and March 2006 were matched 2:1 with nonfailure controls. The individual matching was by risk group, radionuclide, prescribed dose, and time of implant (one match before and one after the failed patient) resulting in a median follow-up of 10.9 years. Complete dose volume histogram (DVH) data were recalculated for all 55 cases and 110 controls after updating the original source strength by the retrospectively determined ratios of TG-43. Differential DVH data were acquired in 179 increments of prostate volume versus percentage prescribed dose. At each incremental dose level i, the biologically equivalent dose BEDi, equivalent uniform dose EUDi, and tumor control probability TCPi were calculated from the implant dose plus any external beam delivered to the patient. Total BED, EUD, and TCP were then derived from the incremental values for comparison with single point dosimetric quality parameters and DVH-based averages. There was no significant difference between failures and controls in terms of total BED (143 vs 142 Gy), EUD (95 vs 94 Gy), or TCP (0.87 vs 0.89). Conditional logistic regression analysis factored out the matching variables and stratified the cohort into each case and its controls, but no radiobiological parameter was predictive of biochemical failure. However, there was a significant difference between radiobiological parameters of 125I and 103Pd due to less complete coverage of the target volume by the former isotope. The implant BED and TCP were highly correlated with the D90 and natural prescription doses and a series of mean DVH-based doses such as the harmonic mean and expressions of the generalized EUD. In this case-control study of prostate brachytherapy biochemical failures and nonfailures, there were no radiobiological parameters derived from detailed DVH-based analysis that predicted for biochemical control. This may indicate that in our approach, implant dosimetry is at or near the limits of clinically effective dose escalation.

Direct intratumoral infusion of liposome encapsulated rhenium radionuclides for cancer therapy: Effects of nonuniform intratumoral dose distribution

“…EUD, defined as the uniform absorbed dose of which produces the same biological outcome as the absorbed dose of which is delivered heterogeneously throughout the tumor, 28 was referenced to external beam radiotherapy ͑EBRT͒ delivering a daily fraction of 2.0 Gy within a few minutes for five fractions per week to mimic a conventional EBRT treatment regimen. Assuming the response of the tissue to be the clonogen weighted mean of the voxels, the EUD is determined from the linear-quadratic model and was calculated for each tumor using 28,29 …”

Influence of Prostatic Edema on 131Cs Permanent Prostate Seed Implants: A Dosimetric and Radiobiological Study

“…Other investigators (12,26,32) have studied the equivalent uniform dose of initial postimplant plans or in combination with pre-and initial postimplant plans for 125 I and 103 Pd implants. But no one has evaluated any biological parameters for 131 Cs implants at different postimplant times.…”

“…For BED calculations, a = 0.15 Gy À1 ; a/b = 3.1 Gy; m = 62.38/ day[i.e. m = ln(2)/t rep , where t rep = 16 min]; and T p = 42 days (25,26). In this work, the value of T 1/2_edema was 9.72 days (15) and was used to calculate l x and T eff .…”

Inverse planning optimization for hybrid prostate permanent‐seed implant brachytherapy plans using two source strengths

“…In fact, some recent studies have shown no significant difference between biochemical failure and control as a function of biological equivalent dose (BED), equivalent uniform dose (EUD), or tumor control probability (TCP) when comparing plans using ${\hspace{0.17em}}^{125}\mathrm{I}$ and ${\hspace{0.17em}}^{103}\text{Pd}$. ( 6 ) However, even setting aside the uncertainty in CT‐reconstructed target volumes, there is a lack of consensus about the actual value of the biological parameters that govern the definition of the BED, EUD, and TCP, with values of α/β differing by more than a factor of 2. ( 7 ) The uncertainty in the models that predict the biological effect of the specific characteristics naturally propagates to the dosimetry of brachytherapy plans that incorporate multiple radionuclides.…”