Price, Michael

14597 Background: Permanent seed implant (PSI) brachytherapy is a common treatment modality for low-risk prostate cancer. Post implant dosimetry (PID) is utilized to asses the quality of the implant. Significant prostate swelling occurs as a result of the implant procedure, and this swelling subsides over time. PID completed on Day 0 after the implant procedure captures the prostate swelling from the procedure. Conversely, PID completed one month later does not have this swelling. PID can therefore show great variation, depending on the timing of the analysis. It is hypothesized that PID completed on Day 0 demonstrates lower dosimetric parameters than PID completed one month later. Methods: Thirteen low risk prostate cancer patients, (Stage ≤ T2, PSA < 13.9, Gleason Score ≤7) were implanted with 125Iodine seeds, with a prescription dose of 145Gy to the prostate plus a 5mm margin. Computed Tomography (CT) PID was completed for each patient on day 0 and on average 33 days following the implant. The prostate was contoured on each axial CT image and the data was analyzed using commercially available PSI planning software. The dose which encompassed 90% of the prostate volume (D90) was calculated for day 0 and day 33 PID. Results: On average, the prostate volume contoured was larger on day 0 PID (Avg. 44.9 cc; range 19–97 cc) compared to day 33 PID (Avg. 38.9 cc; range 18–59 cc) (P = 0.068). The D90 values however, were significantly higher on day 33 PID (Avg.163.7 Gy; range 125–212 Gy) than on day 0 PID (Avg.149 Gy; range 112–166 Gy) (P = 0.003). This D90 relationship was even demonstrated paradoxically in two patients whose contoured prostate volume was larger on the day 33 PID as compared to the day 0 PID. Conclusions: Timing does matter in the analysis of post implant dosimetry for PSI brachytherapy. The D90 values were significantly greater on day 33 PID compared to Day 0 PID while the contoured prostate volumes were not. Future studies which use PID planning to evaluate implant quality should specify the timing of the PID, as this would facilitate cross study comparison. No significant financial relationships to disclose.

15639 Background: Permanent prostate brachytherapy (PPB) assesses implant quality but may suffer significant variability due to: 1) Prostate edema may occur during the procedure which usually subsides within several months, and 2) interobserver variability of CT- derived prostate volumes. An analysis is described, which uses the seeds as prostate fiducials to produce an implant reference volume (IRV). The ratio of the IRV volumes at day 0 and day 33 provides a unique way to quantify prostate edema; thus reducing this factor as a source of variability. Methods: Thirteen low risk prostate cancer patients, (Stage ≤ T2, PSA<13.9, Gleason Score ≤7) were implanted with 125Iodine seeds (77% stranded; prescription dose of 145 Gy). PPB was completed for each patient on day 0 and 33 ±3 days following the implant. The IRV was defined as the volume obtained by contouring the outer most peripheral seeds, circumferentially, on each axial image, resulting in a 3D volume. Contouring was performed by three separate investigators (KJM, DCG, EKM), who were blinded with respect to patient identification. The IRV's were compared both by investigator and by time of PPB (day 0 vs. day 33). The radiation dose encompassing 90% (D90) of the IRV and Prostate (PST) was calculated and compared . Results: The IRVs contoured were reproducible between the investigators with a maximum mean difference of .56 (SD .33) cc. There was no statistical difference between any of the investigators contoured volume (minimum Spearman correlation was 0.978). The average IRV was significantly larger on day 0 (Avg. 52.7;SD 19.7 cc) compared to day 33 (Avg. 44.6; SD 19.1 cc) (p<0.001). The day 0 IRV was an average of 1.18 times larger than the Day 33 PRV. There was no difference between either the day 0 D90PST (Avg.149.1; SD18.7 cGy) vs the day 0 D90IRV (Avg.149.8, SD 27.5 cGy) (p=.442) nor between the day 33 D90PST (Avg.163.7; SD22.7 cGy) vs the day 33 D90IRV (Avg.166.6, SD 30.3 cGy) (p=.257). Both the D90IRV and D90PST were significantly different at day 0 vs 33 (p=.0003, p=<.0001 respectively). Conclusions: The IRV analysis demonstrates no significant interobserver variability, provides accurate quantification of prostate swelling after PPB, and is an accurate objective surrogate analysis for PSI quality. No significant financial relationships to disclose.

In the present work, we investigated the accuracy of the electron pencil‐beam redefinition algorithm (PBRA) in calculating central‐axis percent depth dose in water for rectangular fields. The PBRA energy correction factor C(E) was determined so that PBRA‐calculated percent depth dose best matched the percent depth dose measured in water. The hypothesis tested was that a method can be implemented into the PBRA that will enable the algorithm to calculate central‐axis percent depth dose in water at a 100‐cm source‐to‐surface distance (SSD) with an accuracy of 2% or 1‐mm distance to agreement for rectangular field sizes cm. Preliminary investigations showed that C(E), determined using a single percent depth dose for a large field (that is, having side‐scatter equilibrium), was insufficient for the PBRA to accurately calculate percent depth dose for all square fields cm. Therefore, two alternative methods for determining C(E) were investigated. In Method 1, C(E), modeled as a polynomial in energy, was determined by fitting the PBRA calculations to individual rectangular‐field percent depth doses. In Method 2, C(E) for square fields, described by a polynomial in both energy and side of square W [that is, ], was determined by fitting the PBRA calculations to measured percent depth dose for a small number of square fields. Using the function C(E, W), C(E) for other square fields was determined, and C(E) for rectangular field sizes was determined using the geometric mean of C(E) for the two measured square fields of the dimension of the rectangle (square root method). Using both methods, PBRA calculations were evaluated by comparison with measured square‐field and derived rectangular‐field percent depth doses at 100‐cm SSD for the Siemens Primus radiotherapy accelerator equipped with a ‐cm applicator at 10 MeV and 15 MeV. To improve the fit of C(E) and C(E, W) to the electron component of percent depth dose, it was necessary to modify the PBRA's photon depth dose model to include dose buildup. Results showed that, using both methods, the PBRA was able to predict percent depth dose within criteria for all square and rectangular fields. Results showed that second‐ or third‐order polynomials in energy (Methods 1 and 2) and in field size (Method 2) were typically required. Although the time for dose calculation using Method 1 is approximately twice that using Method 2, we recommend that Method 1 be used for clinical implementation of the PBRA because it is more accurate (most measured depth doses predicted within approximately 1%) and simpler to implement.PACS number: 87.53.Fs