Dose homogeneity as a function of source activity in optimized I-125 prostate implant treatment plans

Practical considerations in the selection of seed strength for prostate implants

There are advantages in using lower numbers of higher activity seeds for prostate seed implants. This work investigated the use of higher strength seeds for our manually optimized prostate implants. Following a planning study using a range of seeds strengths between 0.4 U and 0.7 U, a series of patients were implanted using seeds of strength ~ 0.7 U. Twenty consecutive patients were selected for this study; ten patients were implanted with 0.4 U seeds and the next ten patients implanted with 0.7 U seeds. Postimplant dosimetry for the target volume, urethra, and rectal wall was compared between the two groups. Our data showed a small and insignificant decrease in the total theatre time when implanting seeds of higher strength. The mean number of seeds required per implant decreased by over 30% for the 0.7 U implants, and the mean number of needles decreased by eight needles. The mean D90 (%) was marginally lower for the 0.7 U group, and spread over a wider range of values. Doses to the rectal wall were slightly higher for the 0.7 U group. At six years postimplant, the symptom scores for urinary and rectal toxicity and erectile function were similar to those reported before brachytherapy, with little differences between the 0.4 U and 0.7 U groups. Our experiences and practical advice in the selection of seed strength for prostate implants are reported in this paper.

Prostate brachytherapy post-implant dosimetry: A comparison between higher and lower source density

BACKGROUND AND PURPOSE

To compare post-implant dosimetry between high density source implants (HDI) and low density source implants (LDI).

MATERIALS AND METHOD

Dosimetric analysis of the whole prostate (V200, V150, V100, D90, D80, contiguous V200 and V150, external index), prostate quadrants (V200, V150, V100, D90), rectum (V150, V120, V100, V80, V60) and deviated surrogate urethra (V200, V150, V120, V100, V80) was performed on 39 consecutive prostate brachytherapy LDI and 39 volume matched HDI over the same time period. The distinction between LDI and HDI was based on differing prescribed dose using 125-Iodine sources, with MPD of 115 and 144 Gy, respectively, using a fixed source strength of 0.424 U (0.334 mCi). Cases were contoured by two independent blinded observers. Repeated measures analysis of variance was used to look at the effects of treatment arm, observer and their interaction.

RESULTS

Whole prostate (WP) volume did not differ significantly between the treatment arms, mean of 25.4 cc for LDI and 26.6 cc for HDI. There was no significant difference in any of the measured post-implant dosimetric parameters for the WP or quadrants, surrogate urethra or rectum.

CONCLUSIONS

No difference in post-implant dosimetric parameters was observed between Iodine 125 LDI and HDI. Neither dose homogeneity nor conformality is compromised with a lower source density. Higher strength sources have the potential for considerable cost saving and reduced morbidity.

Randomized trial of high- and low-source strength 125I prostate seed implants

PURPOSE

A range of (125)I isotope activities is used in permanent prostate implants. In this study, we compared the implant quality and cost in patients randomized to high-source or low-source strength permanent implants.

METHODS AND MATERIALS

Forty patients were randomized to receive high (0.76 microGy/m(2)/h) or low (0.4 microGy/m(2)/h) seed strength implants. The two treatment arms had a comparable mix of primary and boost patients and underwent implantation by the same team. The postimplant dosimetric evaluation was performed using CT (seed position) and T(2)-weighted MRI (prostate) scans registered using mutual information techniques. The implant quality parameters were assessed by dose indexes (ratio of achieved dose to planned dose) to quantify the relative error tolerance.

RESULTS

The high-source strength implants had better dose coverage as defined by the dose index; a larger percentage of volume receiving 100% of the prescribed dose as determined by CT (V(100)) (96.3% +/- 3.5% vs. 90.4% +/- 5.3%; p <0.002); lower seed cost (2400 US dollar vs. 3840 US dollar average/case); and took less operating room time on average (67 +/- 16 min vs. 85 +/- 20 min; p <0.004). Finally, the differences in the rectal and urethral doses were not statistically significant between the two treatment arms.

CONCLUSION

All 40 patients received an excellent implant as indicated by the CT V(100). Unless long-term toxicity differs, high-source strength seed implants improve the probability of excellent implant quality and decrease the average cost of permanent prostate implants.

Clinical and physical determinants for toxicity of 125-I seed prostate brachytherapy

BACKGROUND AND PURPOSE

To assess acute as well as long-term toxicity after permanent prostate seed implantation. To find predictive clinical or dosimetric factors for side effects in order to work out strategies for improvement.

PATIENTS AND METHODS

A group of 174 patients with localised prostate cancer was treated with permanent seed implantation between 1999 and 2001, either alone (140 patients) or in combination with external radiotherapy (34 patients). For the majority (114/174, i.e. 66%) a CT was performed four weeks after implantation and analysed in the planning system VariSeed. In the postimplant analysis, dosimetric descriptors (doses, volumes) were determined for the prostate and rectum and compared with the intraoperative values. In addition, a questionnaire was sent to all patients to assess and quantify acute and chronic toxicity (urinary, rectal, sexual) and the impact on subjective acceptance and quality of life (return rate of questionnaires 83%). The derived score changes were correlated with clinical and dosimetric factors.

RESULTS

In the mono-brachytherapy group 14% (16/140) required a bladder catheter, of them 8% (9/140) with a manifest urinary obstruction. Long-term rectal toxicity (<5%) and impairment of potency (<30%) are moderate and obviously below other treatment options. Urinary toxicity is dominant with an overall long-term dysuria up to 30% (at a mean observation interval of ten months), and a significant trend to decline with follow-up time. Conversely, the erectile function tends to deteriorate with follow-up time. Nevertheless, quality of life is not significantly reduced and acceptance is high. Our analysis suggests that the main factor for urinary toxicity and impaired erectile function is the dose load to larger portions of the prostate (D(50)>240 Gy), which appears to be attributed to unnecessarily high numbers of seeds (for a fixed activity per seed) and needles. The rectal toxicity is correlated with the high dose regions in the rectum (>/=145 Gy). Urinary toxicity is lower for combined-brachytherapy, while rectal toxicity and impairment of potency are slightly higher.

CONCLUSIONS

Toxicity spectrum and quality of life after permanent seed implantation for early prostate cancer are acceptable for nearly all patients (98%). To further improve tolerance we should attempt to achieve a better dose homogeneity, i.e. by reducing D(50). Therefore, special attention should be given to D(50) during the real-time planning process. The necessity of more homogeneous dose distributions might imply a reduction of the activity per seed, e.g. from 0.7 mCi down to 0.6 mCi.

The robustness of dose distributions to displacement and migration of 125I permanent seed implants over a wide range of seed number, activity, and designs

PURPOSE

To investigate the robustness of permanent prostate implant dosimetry for various (125)I seed activities and various seed models. The dosimetric impact of seed misplacement and seed migration (seed loss) is also taken into account using various standard dose indices.

METHODS AND MATERIALS

A dose-based inverse planning algorithm is used for automated dosimetric plan creation (45-60 s per plan) and provides an unbiased way to compare the robustness of various optimal dosimetric plans. Seed misplacement and seed migration are simulated by way of Monte Carlo, based on the measured displacement distributions from clinical postimplant cases. Plans were generated for seed activities between 0.2 and 1.4 mCi (0.25 to 1.78 U) and for 11 different seed models.

RESULTS

The numbers of seeds and needles are shown to decrease rapidly for a seed activity between 0.3 mCi and 0.6 mCi (0.38 and 0.76 U). The loss in V100, from 100%, because of seed misplacement is below 10% for an apparent activity ranging from 0.2 to 0.9 mCi (0.25 to 1.14 U). A minimum degradation in V100 is observed around 0.6-0.7 mCi (0.76-0.89 U). D90 increases from 150 to 170 Gy between 0.3 and 0.7 mCi (0.38 and 0.89 U) and decreases afterward to fall below 140 Gy at higher activity. V200 and D10 to the target volume both show an increase in hot spots up to 0.7 mCi, and then decrease linearly at higher activities for all seed models. V200 and D10 to the urethra remain about constant for all seed activities up to 0.8 mCi (1.02 U), at which point they start to decrease. All seed models follow this general trend.

CONCLUSIONS

Plans were shown to be robust to misplacement and migration of seeds over a wide range of seed activity and for various seed models. With a properly tuned inverse planning algorithm able to ensure the dose coverage and protection for the organs at risk in the presence of placement errors (displacement and migration), the choice of a preferred seed activity, in a range up to about 0.7 mCi (0.89 U), is open. The upper part of this range offers the opportunity to significantly reduce the number of seeds and needles, thus reducing surgical trauma to the patient, saving time in an operating room planning setting, and reducing the cost of a permanent prostate implant procedure.

Automatic post-implant needle reconstruction algorithm to characterize and improve implant robustness analyses

Post-implant analysis in permanent implant brachytherapy is an important process that provides a feedback on treatment quality. Random seed movements, edema, and needle related factors contribute to deteriorate dose coverage. For a complete study of these movements, it is important to reconstruct the post-implant seeds clusters but, up to now, this task was only possible via a long and difficult manual process. To facilitate post-implant analysis a simulated annealing algorithm was developed to perform automatic reconstructions. This process is fast (30-60 s on a 1.3 GHz pentium) and has a high level of success, even with up to 5% of seed loss. Tests on 21 clinical cases show that the algorithm yields exactly the same results as manual reconstructions. A realistic simulation tool was used to generate 58 synthetic post-implant data, in which cases the exact configuration was known. Even if some errors were found, pertinent information was extracted. For medium seed density [corresponding to seeds of 0.6 mCi (0.762 U)], 97% of seeds are matched with their correct needle and 89% are matched with their correct planned position. This method provides pertinent information that can be used to understand inhomogenous dose coverage in specific prostate quadrants; to make realistic post-implant simulations or to identify seeds belonging to a needle loaded with different seed types or activity.