Med Phys
-
In prostate brachytherapy, post implant dosimetry quality parameters may be strongly affected by edema brought on by the trauma of the implant procedure since the amount of edema and the time course of its resolution are highly variable from patient to patient. Edema was simulated from preplans on three prostates which had ultrasound prostate volumes of 18.7, 40.7 and 60.2 cm3 expanded to planning volumes of 32.9, 60.0 and 87.8 cm3, respectively. The preplans were designed so that identical seed distributions for a given prostate gave virtually identical target dose coverage of 99.7+/-0.3% of the planning volume when using either 125I or 103Pd. ⋯ A greater fraction of the defined prostate volume received doses in the range of likely therapeutic significance, from 75% to 125% of the prescribed minimal peripheral dose (mPD), from 125I implants than from 103Pd implants. These differences in dosimetric quality arise from two differences in the physical properties of the isotopes: more rapid attenuation of 103Pd photons with distance creates cool spots in an edematous prostate, and the shorter half-life of 103Pd causes a greater fraction of the isotope decay to consist of the prostate in an edematous state. An increase in 103Pd seed strength by about 10% beyond that required to achieve equal coverage with an identical seed distribution using 125I should minimize the differences brought on by edema.
-
This study investigates the feasibility of locating the urethra at the geometric center of peripherally loaded 125I prostate implant when a urinary catheter is not utilized for the postimplant CT scan. Twenty postimplant CT scans utilizing a urinary catheter were randomly selected. ⋯ Dose-volume histograms of the urethra and surrogate urethra were compiled and compared. The values obtained for the urethra D10, D25, and D50 were in good agreement and demonstrate that the urethral dose can be determined reliably by locating a surrogate urethra at the geometric center of the prostate in a peripherally loaded implant when the urethra cannot be visualized.
-
Comparative Study
Comparison of seed loading approaches in prostate brachytherapy.
Since uniform seed loading in prostate brachytherapy can produce an intolerably high dose along the urethra, some form of peripheral loading is commonly employed. We define three variants of peripheral loading and compare them in a small, medium, and large prostate in terms of coverage of the planning target volume (PTV), homogeneity, and ability to spare critical structures of excessive dose. Modified uniform loading has at least 2/3 of the seeds occupying sites on a 1 cm cubic grid keyed to the prostate base and the posterior border of the prostate. ⋯ These regions approach within 10 mm of the rectum or urethra, so these two approaches require greater accuracy in intraoperative execution of the plan. Although each of the three planning approaches can achieve the treatment goals of adequate coverage and critical structure sparing, modified uniform loading has a more homogeneous dose distribution. This approach may be more forgiving of systematic errors in seed placement.
-
Permanent transperineal interstitial 125I and 103Pd prostate implants are generally planned to deliver a specific dose to a clinically defined target volume; however, the post-implant evaluation usually reveals that the implant delivered a lower or higher dose than planned. This difference is generally attributed to such factors as source placement errors, overestimation of the prostate volume on CT, and post-implant edema. In the present work we investigate the impact of edema alone. ⋯ A comparison of the DVH of the static model to the 30 edema corrected DVHs revealed that the plan overestimated the total dose by an amount that increased with the magnitude of the edema and the edema half-life. The maximum overestimation was 15% for 125I and 32% for 103Pd. For more typical edema parameters (a 50% increase in volume and a 10 day half-life) the static plan for 125I overestimated the total dose by about 5%, whereas that for 103Pd overestimated it by about 12%.
-
Pulsed brachytherapy is an endeavor to mimic low dose rate (LDR) treatments using a single higher activity source (a medium dose rate) that is periodically introduced into the patient (i.e., pulsed) using a remote afterloader. It has been reported that by a careful choice of pulse length and frequency and using the ERD bioeffect dose model, therapeutic advantage (TA) values slightly less than unity can be achieved where TA has been defined as the ratio of tumor ERD for PB to tumor ERD for LDR treatments for constant late-reacting normal tissue ERD. These calculations are based upon a uniform average dose rate in each pulse and equal repair rate constants for both tumor and normal tissue. ⋯ Furthermore, for PB treatments the dose rate at a point of interest during each pulse is not uniform, since the treatment involves a single stepping source. A generalized ERD equation based on the linear quadratic model has been developed to account for the variation in the dose rate and, subsequently, to maximize the TA. Our calculations indicate that PB performed with 40 pulses in 120 hours with an irradiation time of 30 minutes per pulse with a delay time of two and a half hours is the best replacement for a LDR treatment that delivers 60 Gy in 120 hours.