Seed Marker-Based IGRT for Prostate Cancer: Excellent Preliminary Toxicity Outcomes

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

In 2004, we completed extensive seed marker-based image guided radiation therapy (IGRT) measurements to correct for the setup/targeting error inherent in external beam radiotherapy of the prostate (Scarbrough et al., IJROBP 2006;65:378–87). Our calculations indicated that seed marker-based IGRT could substantially shrink the necessary PTV margins (in the ∼3 mm range) versus what we or others had used previously. Validation of the calculations via post-repositioning analyses followed (Ting et al., Hematol Oncol Clin North Am, 2006;20:63–86). This is a preliminary report of our toxicity outcomes using IMRT and IGRT for prostate cancer.

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Materials/Methods 

Only pts followed a minimum of 6 months post-treatment were included in this retrospective analysis; a continuous cohort of 231 pts meet this criteria and have a median followup of 1.4 years. The seed marker insertion and setup method is previously described (Scarbrough et al., IJROBP 2006;65:378–87). All pts received a minimum prostate dose of 81 Gy/45 fx. For pts receiving pelvic nodal plus seminal vesicle (21/231 pts) or seminal vesicle (151/231) elective irradiation, the combined CTV (elective structures plus prostate) was expanded 1.0 cm circumferentially/0.5 cm posteriorly to create PTV1. PTV1 received 45 Gy/25 fx. The prostate was expanded 4 mm circumferentially to create PTV2 which received 27 Gy/15 fx. The prostate was expanded 4 mm circumferentially/0 mm posteriorly to create PTV3 which received 9 Gy/5 fx. In low-risk pts, the prostate was expanded circumferentially by 4 mm to create PTV1 which received 72 Gy/40 fx, and by 4 mm/0 mm posteriorly to create PTV2 which received 9 Gy/5 fx. This yielded an “average rectal margin” of ∼3–4 mm for all plans. PTVs were normalized so ≥95% of the volume received 100% of the Rx dose (keeping the prostate minimum dose always 81 Gy); this yielded a mean dose maximum of 85.7 Gy for all pts. Most pts' (75%) prostates were contoured using thin-slice (2.5 mm) MRI with marker-to-marker/MRI-to-CT fusion. All pts were treated using sliding-window IMRT (600 MU/min dose rate), 6MV energy, 7–11 discrete beams, and kV X-ray seed marker-based IGRT; 132/231 pts received anti-androgen therapy of 4–8 months duration. Toxicities were assessed in followup according to CTCAE v.3.0 for rectal morbidity and International Prostate Symptom Score (IPSS) for GU morbidity. Pts (220/231) also completed sexual satisfaction questionnaires in followup.

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Results 

At a median followup of 1.4 years, 226/231 (97.8%) pts have grade 0 rectal toxicity, 3/231 have grade 1 (1.3%), and 2/231 have grade 2 (0.9%); there are no grade 3 or greater toxicities of any sort in any pts. Pre-treatment IPSS is 7/35 and post-treatment IPSS is 8/35. Of 220 pts completing sexual satisfaction questionnaires post-treatment, 105/220 were potent pre-treatment; 76/105 (72.3%) maintain sexual activity post-treatment. As the data matures, we will report oncologic outcomes at a later date.

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Conclusions 

The precision of high-dose, tight margin IMRT–made possible via the accuracy offered by seed marker-based IGRT–results in excellent and very low treatment morbidity. If these outcomes hold over time and can be reproduced by others, IMRT+IGRT for prostate cancer will yield some of the lowest toxicity rates of any of the definitive local treatments for this disease.