Incidental Dose to Pelvic Nodal Regions in Prostate-Only Radiotherapy

Abstract

Objectives: Pelvic lymph nodal regions receive an incidental dose from conformal treatment of the prostate. This study was conducted to investigate the doses received by the different pelvic nodal regions with varying techniques used for prostate radiotherapy. Methods and Materials: Twenty patients of high-risk node-negative prostate cancer treated with intensitymodulated radiotherapy to the prostate alone were studied. Replanning was done for intensity-modulated radiotherapy, 3-dimensional conformal treatment, and 2-dimensional conventional radiotherapy with additional delineation of the pelvic nodal regions, namely, common iliac (upper and lower), presacral, internal iliac, obturator, and external iliac. Dose-volume parameters such as Dmean, D100%, D66%, D33%, V40, and V50 to each of the nodal regions were estimated for all patients. Results: The obturator nodes received the highest dose among all nodal regions. The mean dose received by obturator nodal region was 44, 29, and 22 Gy from 2-dimensional conventional radiotherapy, 3-dimensional conformal treatment, and intensity-modulated radiotherapy, respectively. The mean dose was significantly higher when compared between 2-dimensional conventional radiotherapy and 3-dimensional conformal treatment (P < .001), 2-dimensional conventional radiotherapy and intensity-modulated radiotherapy (P < .001), and 3-dimensional conformal treatment and intensity-modulated radiotherapy (P < .001). The D33% of the obturator region was 64, 39, and 37 Gy from 2-dimensional conventional radiotherapy, 3-dimensional conformal treatment, and intensity-modulated radiotherapy, respectively. The dose received by all other pelvic nodal regions was low and not clinically relevant. Conclusion: The incidental dose received by obturator regions is significant especially with 2-dimensional conventional radiotherapy and 3-dimensional conformal treatment techniques as used in the trials studying elective pelvic nodal irradiation. However, with intensity-modulated radiotherapy, this dose is lower, making elective pelvic irradiation more relevant. Advances in Knowledge: This study highlights that incidental dose received by obturator regions is significant especially with 2-dimensional conventional radiotherapy and 3-dimensional conformal treatment techniques.

Dose outside of the prostate is associated with improved outcomes for high-risk prostate cancer patients treated with brachytherapy boost

“…Our results support previous observations that incidental dose outside the target region is associated with reduced BCR in high-risk prostate cancer patients (18,24,45), providing further evidence for adapting the treatment volume for men with high-risk disease. Further, the regions observed in both IBDM methods do cover part of the obturator region and external iliac nodes, supporting previous observations (18) and PSMA-PET studies (26) providing further evidence of nodal disease and for inclusion of nearby lymph nodes in the treatment volume for men with high-risk disease.…”

“…Conversely, whilst radiotherapy techniques try to limit dose outside of the Planning Target Volume (PTV) to spare normal tissues, studies have shown that incidental dose outside of the PTV could be inadvertently treating subclinical disease (23). Witte et al (18) and Chen et al (24) used Image Based Data Mining technique (IBDM), where the local association between dose and outcome is explored on a per-voxel basis, and found that that incidental dose to the obturator was associated with improved outcomes (25), a region that had previously been associated with clinically relevant incidental dose (26). More recently, Witte et al suggested that the relationship between incidental dose and BCR is influenced by fractionation when they identified a region in the in obturator internus muscles where incidental dose was beneficial for patients treated with standard fractionation radiotherapy, but not in patients treated with a hypo-fractionated schedule (27).…”

Dosimetric and Hematologic Implications of Prostate-Only Versus Whole Pelvic Radiotherapy: Results of the Multicentric Phase 3 PROPER Study

“… 20 The largest benefit was observed in the obturator area close to the prostate where the mean dose was lowered from 44 Gy to 29 Gy and 22 Gy when switching from 2D EBRT to 3D conformal radiotherapy and IMRT, respectively. 20 …”

“…This is in line with the results reported in the study by Murthy et al, who also clearly showed that modern radiotherapy techniques reduce the incidental dose to the pelvic areas in the vicinity of the prostate due to steeper dose gradients. 20 The largest benefit was observed in the obturator area close to the prostate where the mean dose was lowered from 44 Gy to 29 Gy and 22 Gy when switching from 2D EBRT to 3D conformal radiotherapy and IMRT, respectively. 20 Recently, the benefit of applying WPRT in PCa patients with a high risk of pelvic LN involvement, defined as a risk of >20% based on the Roach formula, was shown in a randomized trial.…”