L. Tsvang

An interobserver study of prostatic fossa clinical target volume delineation in clinical practice: are regions of recurrence adequately targeted?

Objectives:To study interphysician variability of delineation of the prostatic fossa clinical target volume (pfCTV) to be irradiated in patients with residual or recurrent microscopic prostate cancer following radical prostatectomy and to estimate the risk for a geographical miss. Methods:Thirty-eight pfCTV were delineated on postradical prostatectomy computerized tomography scans of 8 patients by 5 observers. To estimate the risk of a geographical miss, a high risk volume (HRV) was defined and the percentage of “missed” HRV was calculated for each pfCTV. Results:Interphysician variability was considerable with a mean pfCTV of 39.09 cm3 (range, 11.8–72.5 cm3). At least 25% of the HRV at the bladder neck/anastomosis and the retro-vesical space was excluded in 11 pfCTVs. The mean “missed” HRV was 27.5% (range, 2.3%–78.7%). A pfCTV of less than 30 cm3 was associated with a geographical miss in 66% of cases versus 17.2% for pfCTV of 30 cm3 or more (P = 0.006). Observer identity was significantly associated with excluded HRV (P = 0.03). Conclusions:pfCTV delineation is subject to considerable interobserver variability associated with a significant risk of inadequate targeting of the anastomosis/bladder neck region and the retrovesical space. The failure to recognize regions at high risk for harboring microscopic disease may be due to a lack of familiarity with tissue redistribution following radical surgery, and a lack of literature-based guidelines for pfCTV delineation. A strategy to improve pfCTV delineation is proposed.

Spine radiosurgery: lessons learned from the first 100 treatment sessions.

OBJECTIVE Local therapy to spine tumors has been shown to be effective in selected cases. Spinal radiosurgery (SRS) is an evolving radiotherapy regimen allowing for noninvasive, highly efficacious local treatment. The learning curve can compromise the results of any newly employed technology and should be studied to minimize its effects. In this paper the first 100 SRSs performed at several medical centers are presented and analyzed for the effects of the learning curve on outcome. METHODS A retrospective analysis was undertaken to evaluate data from patients treated with SRS at Sheba Medical Center and Assuta Medical Centers in the period from September 2011 to February 2016. Medical history, clinical and neurological findings, pathological diagnoses, SRS variables, complications, and follow-up data were collected and analyzed. Local control rates were calculated, and local treatment failure cases were qualitatively studied. RESULTS One hundred treatment sessions were performed for 118 lesions at 179 spinal levels in 80 patients. The complication rate was low and did not correlate with a learning curve. Mean follow-up time was 302 days, and the overall local control rate was 95%. The local control rate was dose dependent and increased from 87% (among 35 patients receiving a dose of 16 Gy) to 97% (among 65 patients receiving a dose of 18 Gy). The 6 treatment failure cases are discussed in detail. CONCLUSIONS Spinal radiosurgery is a safe and effective treatment. Comprehensive education of the treating team and continuous communication are essential to limit the effects of the learning curve on outcome.

Urethra-sparing stereotactic body radiotherapy for prostate cancer: how much can the rectal wall dose be reduced with or without an endorectal balloon?

BackgroundThis is a dosimetric comparative study intended to establish appropriate low-to-intermediate dose-constraints for the rectal wall (Rwall) in the context of a randomized phase-II trial on urethra-sparing stereotactic body radiotherapy (SBRT) for prostate cancer. The effect of plan optimization on low-to-intermediate Rwall dose and the potential benefit of an endorectal balloon (ERB) are investigated.MethodsTen prostate cancer patients, simulated with and without an ERB, were planned to receive 36.25Gy (7.25Gyx5) to the planning treatment volume (PTV) and 32.5Gy to the urethral planning risk volume (uPRV). Reference plans with and without the ERB, optimized with respect to PTV and uPRV coverage objectives and the organs at risk dose constraints, were further optimized using a standardized stepwise approach to push down dose constraints to the Rwall in the low to intermediate range in five sequential steps to obtain paired plans with and without ERB (Vm1 to Vm5). Homogeneity index for the PTV and the uPRV, and the Dice similarity coefficient (DSC) for the PTV were analyzed. Dosimetric parameters for Rwall including the median dose and the dose received by 10 to 60% of the Rwall, bladder wall (Bwall) and femoral heads (FHeads) were compared. The monitor units (MU) per plan were recorded.ResultsVm4 reduced by half D30%, D40%, D50%, and Dmed for Rwall and decreased by a third D60% while HIPTV, HIuPRV and DSC remained stable with and without ERB compared to Vmref. HIPTV worsened at Vm5 both with and without ERB. No statistical differences were observed between paired plans on Rwall, Bwall except a higher D2% for Fheads with and without an ERB.ConclusionsFurther optimization to the Rwall in the context of urethra sparing prostate SBRT is feasible without compromising the dose homogeneity to the target. Independent of the use or not of an ERB, low-to-intermediate doses to the Rwall can be significantly reduced using a four-step sequential optimization approach.

A Retrospective Feasibility Study of Salvage Pelvic Nodal Radiation in 6 Patients With Biochemical Failure Following Prostate Fossa Radiation: An Alternative to Androgen Deprivation Therapy (ADT).

Purpose:To evaluate salvage pelvic nodal radiation as an alternative to androgen deprivation therapy (ADT) in patients with biochemical failure and lymph node recurrence following salvage prostate fossa radiation. Methods:Six patients with biochemical failure and lymph node recurrence following prostate fossa radiation were treated with salvage pelvic nodal radiation therapy. A gross target volume was contoured using Choline PET/CT, CT, or MRI imaging. The clinical target volume included pelvic nodes. Avoidance structures were created using isodose lines from previous prostate fossa radiation plans. Radiation was delivered using IMRT or VMAT techniques. Failure was defined as a confirmed rise of prostate-specific antigen (PSA) over 0.2 ng/mL. Results:Four patients had presalvage PSA values <1 and 2 patients had PSAs >1. Dose to the clinical target volume was 54 to 60 Gy. The gross target volume dose was 60 to 73.6 Gy. One of the 2 patients with a high PSA received 6 months of concomitant ADT. Mean follow-up after RT for all patients was 24.9 months (range, 18.1 to 33.0 mo). All 5 patients with no ADT had significant PSA responses. PSA reduction was 80% (62% to 100%) of pre-RT PSA. At last follow-up, 2 patients with initial PSA<1 ng/mL remain free of biochemical progression at 33 and 20 months. Four patients have had PSA rise and meet criteria for failure. This included both patients with initial PSA values > 1. Duration of response before failure was 18.1 to 30.7 months. ADT for failure has been started in 1 patient. There was no grade ≥2 GI or GU toxicity. Conclusions:Salvage lymph node irradiation for patients with early biochemical recurrence and radiologic evidence of pelvic nodal metastases is well tolerated and associated with a durable biochemical response and may be an alternative to or may delay the need for ADT in some patients.

SU-E-T-599: Pelvic Volumetric Modulated Arc Therapy with Hypofractionated Concomitant Integrated Prostate Boost: An Isoeffective Dose Comparison

PURPOSE Pelvic irradiation is controversial in prostate cancer and has been associated with significant late toxicity, but may be beneficial in patients with high risk disease. To evaluate dose reduction to organs at risk (OAR), we compared volumetric intensity modulated arc therapy with hypofractionated concomitant integrated prostate boost (VMAT) to sequential 3-D conformal 4 field pelvis plan and prostate IMRT boost (Sequential IMRT). METHODS The planning CTs of 20 consecutive high risk prostate cancer patients were utilized for this study. Dose planning using triple A algorithm (Varian Eclipse). CTV included prostate and proximal seminal vesicles. These were expanded by 6 mm for the PTV1 . Pelvic LN were delineated and expanded by 3 mm for the PTV2. Volume based equivalent dose at 2Gy/fx (EQD2) doses were calculated using alpha/beta ratios of 1.5 Gy for the prostate and 3 Gy for normal tissues. VMAT prescription was 73.6 Gy (EQD2-80 Gy) to PTV1 and 54.4 Gy to PTV2 (EQD2-50 Gy) in 32 fractions of 2.3 Gy and 1.7 Gy, respectively. Sequential prescription was 46 Gy to the pelvis and 34 Gy for IMRT boost to a total of 80 Gy in 40 fractions of 2 Gy each. RESULTS A significant reduction in mean dose was observed for rectum, bladder, bowel, femur head, and penile bulb for VMAT vs. Sequential IMRT (p<0.001) (table 1). Rectal V75 and V70 was 6.2% and 11.2% for VMAT and 13.6% and 19% for Sequential IMRT (p<0.001). Bowel V50 was 0.35 cm3 vs. 24.1 cm3 (p<0.001), respectively. The 98% isodose conformity index for the PTV was 1.44 for VMAT vs. 1.69 for Sequential IMRT (p<0.001). CONCLUSIONS VMAT with concomitant boost significantly reduced dose to OAR compared to Sequential IMRT, potentially leading to less late toxicity, while providing excellent target coverage and conformity. Furthermore, reduction of treatment planning time and both individual fraction and overall treatment delivery time inherent in this approach, offer significant advantages for both providers and patients.

SU‐E‐T‐290: Dosimetry Comparison of Gating V.S. ITV Approach for Moving Targets in SBRT, Dynamic Treatments

Purpose: To measure and determined the accuracy of delivered dynamic dose IMRT or RapidArc (RA) in cases of moving targets using gating vs. internal target volumes (ITV) approach for SBRT of lung or any other moving target. Methods: A Varian breathing RPM phantom was attached to a toy wagon for simulating horizontal movements. A 2 slice Perspex phantom was placed on the wagon with option to use mini ion chamber and films for dose measurements. We investigated different setup combinations with different target moving distances 0.5, 0.7, 1.2 and 2.2cm, target sizes and shapes. All setup combinations underwent 4DCT. ITV and gated targets were delineated with ECLIPSE TPS contouring tools for all 4DCT phases. Dynamic SBRT plans ware created (RA, IMRT) with different dose rates (300, 600 MU/min) to be delivered in different number of breathing cycles, and different MLC leaves direction, parallel and perpendicular to target moving direction. Collimator position in 0, 45 and 90 deg.) Point dose and 2D dose distribution were measured and compared to the calculated dose using DVH and Gamma test. Results: The combination of dynamic SBRT and ITV approach may lead in some setup combinations to dosimetric errors of >5% in dose and >5mm in isodose distance. In general Gated SBRT is more accurate from the dosimetry point of view. Conclusions: ITV approach in SBRT dynamic treatments should be used with great caution and every dynamic SBRT protocol should take into account the dosimetry inaccuracy as a result of combination of: target moving distances; target size and shape; dose rate; number of breathing cycles; MLC leaves direction.

SU‐FF‐T‐350: Post Radical‐Prostatectomy Radiotherapy: A New Technique for Minimization of the Bladder Dose

Purpose: Patients undergoing either rescue or adjuvant post radical‐prostatectomy radiation therapy are exposed to significant potential toxicities of both surgery and radiation. The inclusion of the bladder neck and anastamoses in the CTV may result in significant dose to the entire bladder due to prolapse of the bladder into the prostatic fossa following surgery. Thus we compared five 3‐D conformal planning strategies to minimize bladder dose.Methods and Materials:CT scans of 20 patients treated with post prostatectomy radiation with three‐dimensional conformal (3D‐CRT) were utilized for this study. Patients were immobilized in the supine position with a knee rest. The following structures were contoured: CTV expanded by 1 cm for PTV‐I and 0.5cm to rectum direction for PTV‐II, bladder, rectum, penile bulb, femurs and testis. Prescription dose was 54 Gy to PTV‐I and 16 Gy to PTV‐II. All patients were planned by a single physicist. Five treatment techniques were compared: a bladder sparing 6 field non‐coplanar plan developed in our department, 4 and 6 field co‐planar plans, Dynamic arc therapy and IMRT. The mean dose for each structure normalized to the prescription dose were calculated for each technique. Results: A correlation between a critical organsdose and CTV, bladder and rectum volumes was detected in used treatment planes. The non‐coplanar 6 field provided the lowest dose to the bladder compared to the other techniques. Overall this technique offers excellent target coverage a favorable dose profile to the rectum and femur compared with other treatment techniques. Conclusions: A 6 field non‐coplanar technique offers the best therapeutic ratio for 3‐D prostatic fossa radiation providing the best bladder sparing radiation compared to other conformal treatment plans.