Incidence
Advances in technology have resulted in improved RT dose delivery which, in turn, has improved biochemical disease-free survival.6 There are various approaches to increasing the dose the patient receives, including high-dose photons using IMRT,6 charged particles (protons),7 and combinations of external beam and BT.8,9 Despite increased dose conformity, rectal injuries have not been eliminated, owing to the proximity of the rectum to the prostate.
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Giordano et al10 analyzed the SEER database for claims of gastrointestinal (GI) diagnoses 6–60 months after RT (n = 24,130), and compared this to a group of men with prostate cancer treated without RT (n = 33,835). They found that late lower GI toxicity after RT may be more common than previously reported. Rates of GI diagnoses were 19% higher in the RT group, with bleeding being the most common diagnosis (40% vs 18–20% in patients treated without RT). Lower endoscopies were performed in 32% of RT patients versus 12% of patients treated with radical prostatectomy. These patients were treated between 1992 and 1999; thus, today’s improved techniques of irradiation (IMRT and BT) might reduce the risk of GI injury, but not eliminate it. Conversely, these values may more accurately estimate the risk of GI injury throughout the radiation oncology community, as opposed to what is reported from high-volume centers. Table 1 summarizes data on the incidence of late rectal injury after various types of radiotherapy for prostate cancer.
TABLE 1 HERE
(To view a larger version of Table 1, click here.)
IMRT
Dose escalation has become the standard of care for patients with prostate cancer after randomized trials showed improvement in disease control with increasing RT dose.7,11–13 The primary clinical problem radiation oncologists face is the ability to achieve high doses of irradiation at the tumor site without causing severe complication in normal tissues that are in the path of the treatment beam. Over the past two decades, there have been significant advances in technology that now allow three-dimensional (3D) tailoring of RT dose delivery as well as proton therapy. IMRT is a technique that uses multiple beam angles and robotically controlled beam-shaping devices located within the linear accelerator to conform the dose to the prostate as closely as possible.
The utilization of IMRT in the treatment of prostate cancer has led to decreases in rectal injury. An example of this comes from a study by Zelefsky et al14 in which they examined 1,571 patients treated with 3DCRT or IMRT. The doses ranged from 66 to 81 Gy and all patients who received IMRT were prescribed 81 Gy. With a median follow-up of 10 years, they found that the incidence of Grade ≥2 GI toxicity was significantly lower at 5% in the IMRT group versus 13% in the 3DCRT group (P < 0.0001), despite the fact that patients in the IMRT group received higher doses of RT to the prostate. Alicikus et al15 examined 10-year outcomes of 170 patients who received 81 Gy RT using a 5-field IMRT technique. They found that late Grade 2 GI toxicity occurred in 4 patients (2%), and late Grade 3 GI toxicity occurred in 2 patients (1%). Among patients who developed late Grade 2 GI toxicity, 2 developed rectal bleeding at a median of 24.5 months after therapy completion. No Grade 4 rectal toxicities were noted. The 10-year incidence of late Grade 2 GI toxicity was 3.7%, much lower than in historical controls using 3DCRT.11,13,16
Whereas IMRT has improved the ability to safely dose escalate in patients with prostate cancer, IMRT per se cannot take sole credit for reduction in toxicities. Given that larger doses are prescribed in prostate IMRT plans, the margins around the clinical target volume are significantly reduced as compared to 3DCRT plans and accurate image guidance is required to safely deliver these treatments.
