Rectal Dose Sparing Research Articles

Biological Effective Dose (BED) in Evaluation of Rectal Dose in Prostate Cancer Patients with Hydrogel Spacer Who Underwent an EBRT with Different Fractionation Schemes

To evaluate rectal dose sparing in prostate cancer patients with hydrogel spacer who underwent an external beam radiotherapy (EBRT) with different fractionation schemes. In a previous study, we have reported that rectal sparing was achieved in prostate cancer patients with hydrogel spacer who underwent a combination therapy of EBRT and LDR brachytherapy (Zhang H, et al. J Appl Clin Med Phys, 2022). In this study, we focused on evaluation of spacer-related rectal sparing in patients who underwent an EBRT with conventional fractionation (81 Gy in 45 fractions), hypofractionation (70 Gy in 28 fractions), or stereotactic body radiotherapy (SBRT) for 40 Gy or 42.5 Gy in 5 fractions. In 172 patients, 119 patients were applied with injection of polyethylene-glycol hydrogel and others without the injection as control. In VMAT plans, doses to rectal wall were examined and converted to BED according to AAPM TG-137 (α/β ratio = 4 Gy). On average, an interspace of 1cm between prostate and rectum was achieved by spacer insertion. Rectal wall was defined as 0.4cm thickness inside the outer contour of rectum, and the volume ranged from 4.5 cm3 to 21.3cm3 for patients with spacer and from 8.2cm3 to 35.1cm3 for those with no spacer. In conventional scheme, an average rectal BED was 120.0 Gy, 116.9 Gy, 114.0 Gy, 108.6 Gy, 87.6 Gy and 55.6 Gy to the volume of 0.1, 0.5, 1, 2, 5 and 10cm3, respectively, and BEDmean was 55.3 Gy in patients with no spacer. The BED decreased to 105.1 Gy, 96.3 Gy, 86.1 Gy, 75.4 Gy, 51.7 Gy and 26.9 Gy to the volume of 0.1, 0.5, 1, 2, 5 and 10cm3, respectively (p < 0.01), and BEDmean was 46.0 Gy in patients with spacer. In hypofractionation, rectal BED was in average of 115.8 Gy, 112.3 Gy, 109.8 Gy, 103 Gy, 81.6 and 52.8 Gy to the volume of 0.1, 0.5, 1, 2, 5 and 10cm3, respectively, and BEDmean was 48.6 Gy in patients with no spacer. Patients with spacer had significantly decreases of rectal BED, an average of 103 Gy, 91.7 Gy, 84.1 Gy, 72.8 Gy, 48.8 Gy and 26.0 Gy to the volume of 0.1, 0.5, 1, 2, 5 and 10cm3, respectively (p < 0.01), BEDmean was 42.4 Gy. In SBRT, average rectal BED in patients with spacer decreased to 93.6 Gy, 78.4 Gy, 69 Gy, 57.5 Gy, 38.5 Gy and 21.5 Gy, in comparison of 124.1 Gy, 117,4 Gy, 110.9 Gy, 97.3 Gy, 59.7 Gy and 32.3 Gy to the volume of 0.1, 0.5, 1, 2, 5 and 10cm3 in those without spacer, respectively (p <0.01). BEDmean decreased to 32.0 Gy in patient with spacer, compared with 43.2 Gy in those with no spacer. Insertion of hydrogel spacer significantly improved rectal dose sparing, about 11.1% to 50.8% in conventional scheme, 12.4% to 51.6% in hypofractionation scheme, and 24.6% to 40.9% in SBRT patients. Furthermore, relatively more dose sparing on rectum was seen on the volume from 0.1cm3 to 2cm3 in SBRT patients.

Automated Planning for Prostate Stereotactic Body Radiation Therapy on the 1.5 T MR-Linac

PurposeAdaptive stereotactic body radiation therapy (SBRT) for prostate cancer (PC) by the 1.5 T MR-linac currently requires online planning by an expert user. A fully automated and user-independent solution to adaptive planning (mCycle) of PC-SBRT was compared with user's plans for the 1.5 T MR-linac. Methods and MaterialsFifty adapted plans on daily magnetic resonance imaging scans for 10 patients with PC treated by 35 Gy (prescription dose [Dp]) in 5 fractions were reoptimized offline from scratch, both by an expert planner (manual) and by mCycle. Manual plans consisted of multicriterial optimization (MCO) of the fluence map plus manual tweaking in segmentation, whereas in mCycle plans, the objectives were sequentially optimized by MCO according to an a-priori assigned priority list. The main criteria for planning approval were a dose ≥95% of the Dp to at least 95% of the planning target volume (PTV), V33.2 (PTV) ≥ 95%, a dose less than the Dp to the hottest cubic centimeter (V35 ≤ 1 cm3) of rectum, bladder, penile bulb, and urethral planning risk volume (ie, urethra plus 3 mm isotropically), and V32 ≤ 5%, V28 ≤ 10%, and V18 ≤ 35% to the rectum. Such dose-volume metrics, plus some efficiency and deliverability metrics, were used for the comparison of mCycle versus manual plans. ResultsmCycle plans improved target dose coverage, with V33.2 (PTV) passing on average (±1 SD) from 95.7% (±1.0%) for manual plans to 97.5% (±1.3%) for mCycle plans (P < .001), and rectal dose sparing, with significantly reduced V32, V28, and V18 (P ≤ .004). Although at an equivalent number of segments, mCycle plans consumed moderately more monitor units (+17%) and delivery time (+9%) (P < .001), whereas they were generally faster (–19%) in terms of optimization times (P < .019). No significant differences were found for the passing rates of locally normalized γ (3 mm, 3%) (P = .059) and γ (2 mm, 2%) (P = .432) deliverability metrics. ConclusionsIn the offline setting, mCycle proved to be a trustable solution for automated planning of PC-SBRT on the 1.5 T MR-linac. mCycle integration in the online workflow will free the user from the challenging online-optimization task.

Effect of the timing of hydrogel spacer placement on prostate and rectal dosimetry of low-dose-rate brachytherapy implants.

PurposeTo verify the dose sparing effect of hydrogel spacer (SpaceOAR™) on rectal dosimetry for prostate brachytherapy, and to determine whether prostate and rectal dosimetry was affected by the time gap between hydrogel spacer injection and brachytherapy dosimetry.Material and methodsThe 103Pd brachytherapy dosimetry of 174 consecutive intermediate- and high-risk patients injected with hydrogel was compared with a dosimetry of 174 contemporaneous patients without hydrogel injections. Of the SpaceOAR™ patients, 91 had hydrogel injected upon completion of brachytherapy implant, while the remaining 83 patients had hydrogel placed prior to external beam radiation therapy (EBRT), followed 2-10 weeks later by brachytherapy. Brachytherapy implants were either planned with the prostate undistorted by any hydrogel or planned with hydrogel in place. Dosimetry of the prostate and tissues at risk was determined from CT imaging on the day of brachytherapy implant.ResultsSpaceOAR™ significantly reduced mean and maximum rectal doses as well as rectal wall V50, but there was a statistically significant reduction of planning target volume (PTV) D90 to 121.1% of the prescribed dose in hydrogel patients compared to 123.3% in the non-hydrogel patients. Rectal dosimetry was similar between patients injected with hydrogel after brachytherapy and those with spacer injected prior to EBRT. However, patients who had hydrogel placed prior to EBRT had statistically significantly higher dosimetry indices of PTV and urethra relative to those with spacer placed at the completion of brachytherapy.ConclusionsThere was a significant rectal dose sparing in the cohort with hydrogel spacer compared to a reference group without spacer injection. The rectal dose sparing effect was similar in the sub-group of patients injected with hydrogel prior to EBRT and the sub-group injected with hydrogel at the conclusion of brachytherapy.

Biological Effective Dose (EBD) in Analysis of Rectal Dose in Prostate Cancer Patients Undergoing a Combination Therapy of EBRT and LDR With Hydrogel Spacer Insertion

To evaluate rectal wall doses sparing in prostate cancer patients undergoing combination therapy of external beam radiotherapy (EBRT) and low-dose-rate (LDR) brachytherapy with hydrogel spacer insertion. Fifty-five patients were enrolled in this retrospective study, in which 25 patients were applied with injection of polyethylene-glycol hydrogel and others without the insertion as control. Patient was prescribed with EBRT in dose of 45Gy to primary tumor site and nodal regions over 25 fractions, and LDR in dose of 100Gy on prostate. Two weeks after spacer placement, patient underwent CT scan for VMAT planning. Intraoperative prostate seed (Pd-103) implant (PSI) was performed two weeks after EBRT completed. Doses to the target and rectal wall were examined in each plan. BED calculation was applied in evaluation of sum dose of the rectal wall in each patient according to AAPM TG-137. On average, 1.04cm interspace between prostate and rectum was achieved by spacer insertion. Rectal wall was defined as 0.4cm thickness inside of rectum contour, and the volume was in average of 13.9cm3 in patients with spacer and 12.6cm3 in those without spacer. Target dose coverage was not seen in significant difference in VMAT and PSI plans between two groups of the patients. In VMAT plan, rectal wall doses were in average of 44.8Gy, 43.6Gy, 42.3Gy, 40.0Gy and 28.8Gy to the volume of 0.1, 0.5, 1, 2 and 5cm3, respectively, in patients with no spacer. In patients with spacer, the doses reduced significantly in average of 42.7Gy, 39.9Gy, 37.7Gy, 34.2Gy and 24.5Gy to rectal volume of 0.1, 0.5, 1, 2 and 5cm3, respectively (p<0.01 or <0.05 in t-test). In postimplant dosimetry, in patient without spacer, rectal wall doses were in average of 129.1Gy, 84.2Gy, 64.8Gy, 43.8Gy and 15.3Gy to the volume of 0.1, 0.5, 1, 2 and 5cm3, respectively. In patient with spacer, the doses decreased to 46.3Gy, 35.3Gy, 28.8Gy, 21.1Gy and 9.3Gy to rectal wall volume of 0.1, 0.5, 1, 2 and 5cm3, respectively. Dose decrease in the volume 0.1 - 5cm3 was significant in patients with spacer, compared with those without insertion (p<0.01 or <0.05 in t-test). Calculation of BED was performed in order to evaluate sum dose of the rectal wall in combination of both VMAT and PSI plans. In patient without spacer, BEDsum was in average of 126.9Gy, 106.7Gy, 84.3Gy and 45.5Gy to rectal volume of 0.5, 1, 2 and 5cm3, respectively. In contrast, BEDsum was seen in decrease of 20% to 39% in patient with spacer, in which was 76.8Gy, 68.2Gy, 57.2Gy and 36.2Gy to the rectal volume of 0.5, 1, 2 and 5cm3, respectively. Insertion of hydrogel spacer significantly improved rectal dose sparing, about 20% to 39%, in prostate cancer patient in EBRT and LDR combination therapy.

Effectiveness of rectal displacement devices in managing prostate motion: asystematic review.

To determine whether rectal displacement devices (RDDs) have aprostate-stabilizing effect during prostate external beam radiotherapy (EBRT). Asystematic literature search using the PubMed database from January1, 2000 to December30th, 2019 was conducted. The effect of RDDs on inter- and intra-fractional prostate displacements was extracted. From 356 articles identified via the PubMed database and hand search, 21articles were included in the systematic review. There was no randomized study. Twelve studies evaluated the role of the endorectal balloon (ERB) in managing prostate motion. Four studies reported the effect of hydrogel spacer on prostate motion. Four studies examined the effect of the rectal retractor (RR) on intra-fractional prostate motion, and only one study assessed the impact of ProSpare (Nottinghamshire, UK) in reducing prostate motion. Using an ERB significantly reduces intra-fractional prostate motion. This prostate-stabilizing effect of the ERB can translate into reduced planning target volume (PTV) margins and additional rectal dose sparing. Even with an ERB in place, inter-fractional prostate displacements are seen. As aconsequence, ERB application does not obviate daily verification; however, this is not acrucial topic because pretreatment imaging is always done nowadays. As compared with ERB, the hydrogel spacer significantly reduces rectal dose and toxicity without influencing prostate immobilization. The RR can increase prostate and rectal inter- and intra-fractional stability without aclear influence on the reduction of rectal toxicity. Finally, it is unclear whether ProSpare is asuitable device reducing prostate motion. Further study will be required to clarify whether the prostate-stabilizing effects of the ERB and RR can result in asafe reduction of PTV margins and further sparing of organs at risks, especially the rectum.

Maximizing rectal dose sparing with hydrogel: A retrospective planning study.

External beam radiation therapy for prostate cancer can result in urinary, sexual, and rectal side effects, often impairing quality of life. A polyethylene glycol‐based product, SpaceOAR© hydrogel (SOH), implanted into the connective tissue between the prostate gland and rectum can significantly reduce the dose received by the rectum and hence risk of rectal toxicity. The optimal way to manage the hydrogel and rectal structures for plan optimization is therefore of interest. In 13 patients, computerized tomography (CT) scans were taken pre‐ and post‐SpaceOAR© implant. A prescription of 60 Gy in 20 fractions was planned on both scans. Six treatment plans were produced per anonymized dataset using either a structure of rectum plus the hydrogel, termed composite rectum wall (CRW), or rectal wall (RW) as an inverse optimization structure and intensity modulated radiotherapy (IMRT) or volumetric modulated arc therapy (VMAT) as a treatment technique. Dose‐volume histogram metrics were compared between plans to determine which optimization structure and treatment technique offered the maximum rectal dose sparing. RW structures offered a statistically significant decrease in rectal dose over CRW structures, whereas the treatment technique (IMRT vs VMAT) did not significantly affect the rectal dose. There was improvement seen in bladder and penile bulb dose when VMAT was used as a treatment technique. Overall, treatment plans using the RW optimization structure offered the lowest rectal dose while VMAT treatment technique offered the lowest bladder and penile bulb dose.

A Monte Carlo study of I-125 prostate brachytherapy with gold nanoparticles: dose enhancement with simultaneous rectal dose sparing via radiation shielding

We investigate via Monte Carlo simulations a new 125I brachytherapy treatment technique for high-risk prostate cancer patients via injection of Au nanoparticle (AuNP) directly into the prostate. The purpose of using the nanoparticles is to increase the therapeutic index via two synergistic effects: enhanced energy deposition within the prostate and simultaneous shielding of organs at risk from radiation escaping from the prostate. Both uniform and non-uniform concentrations of AuNP are studied. The latter are modeled considering the possibility of AuNP diffusion after the injection using brachy needles. We study two extreme cases of coaxial AuNP concentrations: centered on brachy needles and centered half-way between them.Assuming uniform distribution of 30 mg g−1 of AuNP within the prostate, we obtain a dose enhancement larger than a factor of 2 to the prostate. Non-uniform concentration of AuNP ranging from 10 mg g−1 and 66 mg g−1 were studied. The higher the concentration in a given region of the prostate the greater is the enhancement therein. We obtain the highest dose enhancement when the brachytherapy needles are coincident with AuNP injection needles but, at the same time, the regions in the tail are colder (average dose ratio of 0.7). The best enhancement uniformity is obtained with the seeds in the tail of the AuNP distribution. In both uniform and non-uniform cases the urethra and rectum receive less than 1/3 dose compared to an analog treatment without AuNP.Remarkably, employing AuNP not only significantly increases dose to the target but also decreases dose to the neighboring rectum and even urethra, which is embedded within the prostate. These are mutually interdependent effects as more enhancement leads to more shielding and vice-versa. Caution must be paid since cold spot or hot spots may be created if the AuNP concentration versus seed position is not properly distributed respect to the seed locations.

Radiosensitization of Prostate With Simultaneous Rectal Dose Sparing Via Radiation Trapping by Gold Nanoparticles in I-125 Prostate Brachytherapy

Radiosensitization of Prostate With Simultaneous Rectal Dose Sparing Via Radiation Trapping by Gold Nanoparticles in I-125 Prostate Brachytherapy

Autologous fat transfer as prostate-rectal spacer: Technique description and early results

Purpose: Several attempts have been made to increase the distance between the prostate and the rectum through injection of different synthetic compounds, generating space between organs. To report an original technique to increase the distance between the rectum and the prostate, by autologous fat implantation into the rectoprostatic space, with the aim of providing physical dosimetry protection and rectal dose sparing. Methods: We prospectively evaluated twelve patients subjected to autologous fat implantation as recto-prostatic spacer subsequently receiving prostate either radical (n = 6), or salvage brachytherapy for local recurrence after external beam radiation therapy (EBRT) (n = 6). Standard permanent prostate brachytherapy seed implantation was performed through transperineal approach and under transrectal ultrasonography (TRUS) and template guidance. Prescribed D90 dose for Iodine - 125 monotherapy was 140 - 160 Gy, reduced by 30% for rescue cases to obtain a Rectum V100 under 1 cc. Results: Lipo-transfer was completed in all 12 patients. Control CT scan at 1 month showed average distances of: 10.7 mm (range) (2.8 - 15.9 mm), 7.6 (1.8 - 11.6 mm) and 6.8 (4.2 - 8.3) mm at prostate base, middle and apex, respectively. Shortest separation distance observed was at apex and midline, while largest was observed the sides and at seminal vesicles level. Control CT at 3 months showed average distances of 9.6 mm (1.9 - 14.6 mm), 6.3 mm (1.8 - 10.2 mm) and 5.4 mm (3.8 - 7.2 mm) at prostate base, middle and apex, respectively. Most complications were minor. Conclusion: Autologous fat transfer is a feasible and simple procedure for experienced practitioners with low complication rates, which allows dose escalation to the prostate.

SU‐E‐T‐280: Reconstructed Rectal Wall Dose Map‐Based Verification of Rectal Dose Sparing Effect According to Rectum Definition Methods and Dose Perturbation by Air Cavity in Endo‐Rectal Balloon

Purpose:Dosimetric effect and discrepancy according to the rectum definition methods and dose perturbation by air cavity in an endo‐rectal balloon (ERB) were verified using rectal‐wall (Rwall) dose maps considering systematic errors in dose optimization and calculation accuracy in intensity‐modulated radiation treatment (IMRT) for prostate cancer patients.Methods:When the inflated ERB having average diameter of 4.5 cm and air volume of 100 cc is used for patient, Rwall doses were predicted by pencil‐beam convolution (PBC), anisotropic analytic algorithm (AAA), and AcurosXB (AXB) with material assignment function. The errors of dose optimization and calculation by separating air cavity from the whole rectum (Rwhole) were verified with measured rectal doses. The Rwall doses affected by the dose perturbation of air cavity were evaluated using a featured rectal phantom allowing insert of rolled‐up gafchromic films and glass rod detectors placed along the rectum perimeter. Inner and outer Rwall doses were verified with reconstructed predicted rectal wall dose maps. Dose errors and extent at dose levels were evaluated with estimated rectal toxicity.Results:While AXB showed insignificant difference of target dose coverage, Rwall doses underestimated by up to 20% in dose optimization for the Rwhole than Rwall at all dose range except for the maximum dose. As dose optimization for Rwall was applied, the Rwall doses presented dose error less than 3% between dose calculation algorithm except for overestimation of maximum rectal dose up to 5% in PBC. Dose optimization for Rwhole caused dose difference of Rwall especially at intermediate doses.Conclusion:Dose optimization for Rwall could be suggested for more accurate prediction of rectal wall dose prediction and dose perturbation effect by air cavity in IMRT for prostate cancer.This research was supported by the Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT &amp; Future Planning (MSIP) (Grant No. 200900420)

First clinical implementation of the Capri applicator

This study was to assess the Capri applicator for patients with endometrial cancer undergoing high‐radiation dose treatments following external‐beam radiation therapy. The Capri applicator is an inflatable vaginal cylinder with multiple channels. It is used to tailor the dose distribution to an asymmetric vaginal disease, and better spare organs at risk. Five patients with high‐risk endometrial cancer were selected for this study. The patients were treated with a high dose of radiation using the Capri applicator: daily fraction of 7 Gy was prescribed for a total dose of 21 Gy. The treatment plans included radiobiological parameters such as equivalent uniform dose (EUD), normal tissue complication probability (NTCP), and tumor control probability (TCP). Based on the dose‐volume histograms (DVH), we also calculated four quality factors: conformity index (CI), dose homogeneity index (DHI), dose nonuniformity index (DNR), and overdose index (OI). The TCP values range from 82.26% to 95.92%. Very low values of NTCP were observed for the bladder and rectum. The EUDs to organs at risk ranged from 4.65 Gy to 18.22 Gy for the bladder, and from 3.41 Gy from to 6.56 Gy for the rectum. The mean CI was 1.05(SD=0.0008). The mean DNR was 0.10(range0.0−0.295,SD=0.100). The mean OI was 0.019(SD=0.028). The DHIs were in the range of 1.0−0.754(mean0.886,SD=0.116). The use of a multichannel vaginal cylinder may not only help cover extensive vaginal disease, but also reduce the dose to the rectum. This dosimetric analysis shows that rectal doses could be reduced using a multichannel cylinder. However, the dose delivered to the bladder based on EUD calculation may be higher than that obtained with other methods. Each patient must be evaluated independently to determine if a multichannel treatment is appropriate. Clinical followup will show whether this rectal dose sparing translates into a real toxicity improvement.PACS number: 3.6.96.0

Role of step size and max dwell time in anatomy based inverse optimization for prostate implants

In high dose rate (HDR) brachytherapy, the source dwell times and dwell positions are vital parameters in achieving a desirable implant dose distribution. Inverse treatment planning requires an optimal choice of these parameters to achieve the desired target coverage with the lowest achievable dose to the organs at risk (OAR). This study was designed to evaluate the optimum source step size and maximum source dwell time for prostate brachytherapy implants using an Ir-192 source. In total, one hundred inverse treatment plans were generated for the four patients included in this study. Twenty-five treatment plans were created for each patient by varying the step size and maximum source dwell time during anatomy-based, inverse-planned optimization. Other relevant treatment planning parameters were kept constant, including the dose constraints and source dwell positions. Each plan was evaluated for target coverage, urethral and rectal dose sparing, treatment time, relative target dose homogeneity, and nonuniformity ratio. The plans with 0.5 cm step size were seen to have clinically acceptable tumor coverage, minimal normal structure doses, and minimum treatment time as compared with the other step sizes. The target coverage for this step size is 87% of the prescription dose, while the urethral and maximum rectal doses were 107.3 and 68.7%, respectively. No appreciable difference in plan quality was observed with variation in maximum source dwell time. The step size plays a significant role in plan optimization for prostate implants. Our study supports use of a 0.5 cm step size for prostate implants.

Dosimetric Comparison of VMAT, IMRT and Proton Therapy for Post- Prostatectomy Radiation Therapy for Prostate Cancer

Purpose/Objectives: Radiation therapy (RT) for the treatment of prostate cancer after radical prostatectomy (RP) is widely accepted. With technological advances, radiation treatment technique and dosimetry may vary. The purpose of this study is to quantify dosimetric difference of proton therapy versus VMAT and IMRT focusing on bladder and rectal dose sparing and target coverage. American Society for Therapeutic Radiation Oncology (ASTRO) 52nd Annual Meeting October 31 - November 4, San Diego, CA

TH‐C‐AUD C‐09: Efficacy of Virtual Reality Simulation for Noncoplanar Prostate IMRT — a Peek of Future RTP System

Purpose: To evaluate the efficacy of virtual reality simulation for noncoplanar prostate IMRT optimization process. Method and Materials: 14 prostate IMRT cases, randomly selected from current clinic, were re‐simulated using a pioneer virtual reality simulation (VRS) system for noncoplanar beam arrangement. Using 3D stereoscopic technology, the VRS system displays the virtual treatment environment on a DTI™ real 3D LCD screen without aid of gaggles. With the virtual patient loaded to the screen, one can simulate the treatment by realistically manipulating the gantry and table. An optimal beam projection is suggested when the inclusion of vital organ was reduced in the volumetric beam eye‐view. When exploring a noncoplanar beam position, the anti‐collision function effectively alerted the mechanical limitations. In comparing with original seven evenly‐spread coplanar beams, in the resultant new setting all beams were rotated more anteriorly, while two anterior oblique beams were tilted 20–30° inferiorly. All IMRT plans were calculated with similar modulated intensity level and number of segments. To standardize the endpoint for comparison, PTV D95 was normalized to 45 Gy. Results: VRS created non‐coplanar beam arrangements were proved clinically to be deliverable beam arrangement without risk of collision. Dose homogeneity in new plans were improved, indicated by 2.2% reduction in global maximal dose and 2.0% in high dose tail (D5) of PTV. Enhancement of rectal dose sparing was suggested by 5.7% and 3.7% lower values for rDmean and rD10cc respectively. Less inclusion of bladder in BEV of the two anterior noncoplanar oblique beams translated a 14% and 5.1% reduction in bladder mean dose and bD30cc respectively. Conclusion: The VRS effectively concluded deliverable noncoplanar beam arrangements for prostate IMRT with resultant improvement of dose characteristics. Further development of VRS may present future RTPS.

SU-FF-T-110: Collimator Angle Effect in Optimizing Prostate IMSR Plans Using 3-Mm MLC

Purpose: To evaluating the effect of two common collimator angles in optimizing intensity modulated stereotactic body radiosurgery (IMSR) plans. Methods and Materials: 8 localized prostate carcinoma cases were selected. Treatment plans were performed with Brainlab iPlan RT Dose™ v3.0.1 platform on 2 mm CT data sets at supine position. Each plan consists of seven Novalis™ 6 MV photon beams, symmetrically spread along the patient axial plane. The maximum dimension of PTV's was 6.3 cm or less, which fits the 3–mm MLC portion. Contours defining the rectum were drawn anatomically as extending from the sigmoid flexure to the anal verge. For each case, two IMRT plans were calculated with identical parameters, except collimator angles were set at 0°, when MLC travel parallel to the rectum (test group), and 270° for the control. Beamlet size max was 4 mm. The goal dose of 75.6 Gy was normalized to D99.5 (ranged from D99.2 to D99.7) of PTV. Results: Dose homogeneity shows significantly superior in the test group, indicated by a lower maximal dose (mean 81.95 Gy vs. 83.45 Gy, p<0.01). An improved rectal dose sparing was also suggested in the test group with lowered rD10cc (56.48 Gy vs. 58.23 Gy, p=0.04). Discussion and conclusions: The longitudinal resolution along the travel direction of MLC routinely sets as 4 mm or greater in clinic, which creates an unsymmetrical resolution during IMRT optimization with 3–mm MLC's. Dose gradient changes are significant along the cross section of PTV/rectal junction (PRJ). When a finer calculation steps used perpendicularly, a better overall dose optimization was indicated. Although a better gantry clearance with a 270° collimator angle, it may not be the optimal setting in all prostate IMRT as clearly implied in this study.

The Impact of Varying Volumes in Rectal Balloons on Rectal Dose Sparing in Conformal Radiation Therapy of Prostate Cancer

The Impact of Varying Volumes in Rectal Balloons on Rectal Dose Sparing in Conformal Radiation Therapy of Prostate Cancer

Rectal dose sparing with a balloon catheter and ultrasound localization in conformal radiation therapy for prostate cancer.

To compare the rectal wall and bladder volume in the high dose region with or without the use of a balloon catheter with both three-dimensional (3D)-conformal and intensity modulated radiation therapy (CRT, IMRT) approaches in the treatment of prostate cancer. Five patients with a wide range of prostate volumes and treated with primary external beam radiation therapy for localized prostate cancer were selected for analysis. Pinnacle treatment plans were generated utilizing a 3D conformal six-field design and an IMRT seven coplanar-field plan with a novel, three-step optimization and with ultrasound localization. Separate plans were devised with a rectal balloon deflated or air inflated with and without inclusion of the seminal vesicles (SV) in the target volume. The prescription dose was 76Gy in 38 fractions of 2Gy each. Cumulative dose-volume histograms (DVHs) were analyzed for the planning target volume (PTV), rectal wall, and bladder with an inflated (60cc air) or deflated balloon with and without SV included. The volumes of rectal wall and bladder above 60, 65, and 70Gy with each treatment approach were evaluated. Daily balloon placement was well-tolerated with good patient positional reproducibility. Inflation of the rectal balloon in all cases resulted in a significant decrease in the absolute volume of rectal wall receiving greater than 60, 65, or 70Gy. The rectal sparing ratio (RSR), consisting of a structure's high dose volume with the catheter inflated, divided by the volume with the catheter deflated, was calculated for each patient with and without seminal vesicle inclusion for 3D-CRT and IMRT. For 3D-CRT, RSRs with SV included were 0.59, 0.59, and 0.56 and with SV excluded were 0.60, 0.58, and 0.54 at doses of greater than 60, 65, and 70Gy, respectively. Similarly, for IMRT, the mean RSRs were 0.59, 0.59, and 0.63 including SV and 0.71, 0.66, and 0.67 excluding SV at these same dose levels, respectively. Averaged over all conditions, inflation of the rectal balloon resulted in a significant reduction in rectal volume receiving > or =65Gy to a mean ratio of 0.61 (P=0.01) or, in other words, a mean fractional high dose rectal sparing of 39%. There was a slight overall increase to 1.13 in the relative volume of bladder receiving at least 65Gy; however, this was not significant (P=0.6). Use of an endorectal balloon with a non-image-guided 3D-CRT plan produced about as much rectal dose sparing as a highly conformal, image-guided IMRT approach without a balloon. However, inclusion of a balloon with IMRT produced further rectal sparing still. These results indicate that use of a rectal balloon with a 3D-CRT plan incorporating typical treatment margins will produce significant high dose rectal sparing that is comparable to that achieved by a highly conformal IMRT with ultrasound localization. Further sparing is achieved with the inclusion of a balloon catheter in an IMRT plan. Thus, in addition to a previously reported advantage of prostate immobilization, the use of a rectal displacement balloon during daily treatment results in high dose rectal wall sparing during both modestly and highly conformal radiotherapy. Such sparing could assist in controlling and limiting rectal toxicity during increasingly aggressive dose escalation.