Prostate Bed and Organ-at-Risk Deformation: Prospective Volumetric and Dosimetric Data from a Phase II Trial of Stereotactic Body Radiotherapy after Radical Prostatectomy

Abstract

Stereotactic body radiotherapy (SBRT) for localized prostate cancer has emerged as an attractive treatment currently being studied in phase III trials. SBRT in post-prostatectomy setting, however, has been minimally investigated. A major concern for this setting is the highly deformable nature of adjacent organs-at-risk (OARs) and prostate bed clinical target volume (CTV). We report a volumetric and dosimetric analysis of kilovoltage cone-beam CT (CBCT) data from 18 patients enrolled on a phase II clinical trial of post-prostatectomy SBRT. We hypothesized that with instructions on bladder filling and rectal preparation, we could achieve acceptable CTV coverage and avoid overdosing OARs. Eighteen patients receiving 5 fractions of 6-6.8 Gy to prostate bed were included. Patients received instructions on bladder filling and rectal preparation, specifically to urinate 60 minutes and drink 16-24 oz of water prior to treatment, as well as take enemas the night and morning before treatment. CBCT prior to each fraction was taken for anatomic verification and image guidance, and a second CBCT halfway through treatment for intrafractional motion analysis. CBCTs were imported into a deformable registration algorithm. Prostate bed CTV and OARs were contoured for each image. Percent changes in inter and intrafractional volume and mean dose change were calculated for all structures; V95 was also calculated for each CTV. Interfractional volumetric and dosimetric changes are summarized in Table 1. CTV volume was relatively constant (median change +5.7% [IQR -1.7% to + 9.8%]) as was CTV V95 (median change -0.74% [IQR -9.2% to -0.1%]). Rectal volume showed greatest change between treatment sessions and treatment plan, with median change of +21.0% [IQR +5.1% to 34.6%]. Intrafractional volumetric and dosimetric changes were overall nonsignificant. Among 11 patients with at least 3 months follow-up, there was one instance of acute grade 2 gastrointestinal toxicity by CTCAE 4.03, and no instances of grade 2 genitourinary toxicity. So far, one patient had biochemical failure which may be due to an out of field relapse. All others have had PSA response. Dosimetric and volumetric data from this prospective trial indicate CTV coverage is well maintained without overdosing OARs despite the highly deformable anatomy involved with post-prostatectomy SBRT. Acute toxicity is favorable, though longer follow-up and more patients are needed to evaluate late toxicity and efficacy.Abstract 1253; Table 1Interfractional percent change in volume and dose of critical treatment structuresMeanMedianQ1Q3CTV volume2.95%5.69%-1.73%9.84%CTV mean dose-1.63%-0.40%-2.43%-0.17%Bladder volume0.85%2.17%-28.19%10.55%Bladder mean dose0.30%-1.33%-11.77%6.55%Rectum volume25.38%20.97%5.10%34.64%Rectum mean dose-6.98%-8.53%-15.34%-2.86%Rectal wall volume23.76%23.14%3.46%40.13%Rectal wall mean dose-11.72%-12.79%-19.58%-6.76%CTV V95-5.05%-0.74%-9.15%-0.07% Open table in a new tab