Interstitial brachytherapy as a curative treatment option for prostate cancer is well established in various international guidelines [7,8,9,10]. It is recommended not only for low-risk cases, where brachytherapy alone suffices, but also for intermediate- to high-risk cases in combination with external-beam radiation therapy (EBRT), marking it a state-of-the-art therapy with a favorable toxicity profile [11]. This fact is also reflected in numerous international guidelines for the treatment of prostate cancer and the recommendation has remained unchanged for several years [7,8,9,10].

Additionally, brachytherapy shows high efficacy in the case of recurrence without metastasis after primary treatment (definitive radiotherapy or radical prostatectomy with postoperative EBRT) [12]. When pretreated with radiotherapy, the challenge in such situations is mainly related to the localization of the recurrent tumors with respect to already applied doses to organs at risk. While salvage surgery or EBRT could be considered, these treatments often carry a significant risk of both early and late toxicity. Given the known dose–response relationship, the use of EBRT is very limited due to already exhausted normal tissue tolerance [13, 14]. In contrast, salvage brachytherapy offers an acceptable toxicity profile with high local control rates and is therefore the treatment of choice [15, 16]. One out of 5 patients in our analysis presented with recurrent PCA. He had received pretreatment with iodine seeds for low-risk prostate cancer. Despite having a primary PCA diagnosis, the other patients (4/5) had mostly also been pretreated with EBRT for rectal cancer, which has to be considered for treatment planning. As a result, primary surgical removal and EBRT were not favored options due to the high risk of serious genitourinary (GU) and gastrointestinal (GI) toxicity [17]. ABS guidelines generally recommend transrectal ultrasound as the imaging method [18, 19]. However, also CT-based planning systems are mentioned, analogous to the planning of several other entities in the area of brachytherapy such as breast cancer or cervical carcinoma [20, 21].

The application of brachytherapy in patients who have undergone rectal extirpation and subsequently develop metachronous prostate cancer has been reported in just a couple of case series to date [22, 23]. Similarly to our approach, Koutrovelis et al. used CT-assisted image guidance for low-dose-rate (LDR) brachytherapy with iodine seeds alone (in patients with mostly intermediate- to low-intermediate-risk prostate cancer). After a median follow-up of 18.6 months, excellent biochemical control was reported, despite one instance of biochemical failure. Importantly, patients did not experience any gastrointestinal morbidity. One patient had a stricture of the distal ureter which requiring stenting [22]. Notably, we found similar toxicity in the same follow-up period without documenting serious toxicity requiring intervention. Jabbari et al. also investigated rectum-extirpated patients with newly diagnosed prostate cancer but used transperineal ultrasonographic image guidance. Excellent local control during the follow-up period was reported. Notably, 2 patients reported late grade 2 genitourinary toxicity which might be explained by insufficient image quality during the procedures [23].

Given the advancements in cancer therapy, the improved prognosis of colorectal cancer, and a noticeable increase in the prostate cancer incidence following rectal cancer, it is likely that the number of patients with these specific clinical features will increase in the future [1]. In this context, the novelty we were able to contribute with our analyses is that we offer an option to patients who are otherwise inaccessible to curative treatment. A recent meta-analysis described 89 patients with metachronic colorectal and prostate cancer of whom 23 received brachytherapy; 9 patients in 2 studies were described as rectally extirpated and receiving interstitial BT [24]. This shows that the scenario described in our analysis is prevalent, but there is a significant lack of data for a standardized treatment approach.

In both of the abovementioned studies [22, 23], brachy-therapy alone was used. In our report, we are able to demonstrate for 2 patients that also a combined approach, normofractionated EBRT and BT, is feasible. Since one patient remained radiotherapy naïve after the earlier rectal cancer treatment, reirradiation was nonproblematic. For another patient, radiographic evidence suggested nodal involvement, which is why we decided to treat the pelvic nodal regions as well. Interestingly, this patient did not report increased GI or GU toxicity during the course of follow-up.

Another novelty we were able to demonstrate is that the suggested method is also feasible for patients with recurrent disease after prostatectomy. To our knowledge, there are no reports of such cases in the literature.

The presented analysis has several limitations. Certainly, the low number of evaluated procedures in this analysis must be considered. In addition, the follow-up period might seem rather short in order to give conclusive results regarding efficacy compared to other image-guided brachytherapy approaches. Furthermore, the presented procedure is highly dependent on the executing brachytherapy expert in terms of experience and technical equipment, thus restricting this treatment option to high-volume institutions with a specialization in interstitial brachytherapy. However, for patients with prostate cancer who have undergone surgery and chemoradiation for rectal cancer and are thus not candidates for ultrasound-guided brachytherapy or radical surgery due to contraindications or personal preference, CT-guided brachytherapy remains a possible curative option. Nevertheless, further data are required to draw conclusions about the efficacy of the proposed method.

In conclusion, our findings suggest that protocol-based CT-guided brachytherapy is a feasible and safe approach for prostate cancer patients who have previously undergone rectal extirpation with or without radiotherapy.