Mean Bladder Dose Research Articles

Volumetric-modulated arc therapy in postprostatectomy radiotherapy patients: A planning comparison study

The purpose of this study was to compare postprostatectomy planning for volumetric-modulated arc therapy (VMAT) with both single arc (SA) and double arcs (DA) against dynamic sliding window intensity-modulated radiotherapy (IMRT). Ten cases were planned with IMRT, SA VMAT, and DA VMAT. All cases were planned to achieve a minimum dose of 68Gy to 95% of the planning target volume (PTV) and goals to limit rectal volume >40Gy to 35% and >65Gy to 17%, and bladder volumes >40Gy to 50% and >65Gy to 25%. Plans were averaged across the 10 patients and compared for mean dose, conformity, homogeneity, rectal and bladder doses, and monitor units. The mean dose to the clinical target volume and PTV was significantly higher (p<0.05) for SA compared with DA or IMRT. The homogeneity index was not significantly different: SA = 0.09; DA = 0.08; and IMRT = 0.07. The rectal V40 was lowest for the DA plan. The rectal V20 was significantly lower (p<0.05) for both the VMAT plans compared with IMRT. There were no significant differences for bladder V40 or rectal and bladder V65. The IMRT plans required 1400MU compared with 745 for DA and 708 for SA. This study shows that for equivalent dose coverage, SA and DA VMAT plans result in higher mean doses to the clinical target volume and PTV. This greater dose heterogeneity is balanced by improved low-range rectal doses and halving of the monitor units.

The effect of anterior proton beams in the setting of a prostate-rectum spacer

Studies suggest that anterior beams with in vivo range verification would improve rectal dosimetry in proton therapy for prostate cancer. We investigated whether prostate-rectum spacers would enhance or diminish the benefits of anterior proton beams in these treatments. Twenty milliliters of hydrogel was injected between the prostate and rectum of a cadaver using a transperineal approach. Computed tomography (CT) and magnetic resonance (MR) images were used to generate 7 uniform scanning (US) and 7 single-field uniform dose pencil-beam scanning (PBS) plans with different beam arrangements. Pearson correlations were calculated between rectal, bladder, and femoral head dosimetric outcomes and beam arrangement anterior scores, which characterize the degree to which dose is delivered anteriorly. The overall quality of each plan was compared using a virtual dose-escalation study. For US plans, rectal mean dose was inversely correlated with anterior score, but for PBS plans there was no association between rectal mean dose and anterior score. For both US and PBS plans, full bladder and empty bladder mean doses were correlated with anterior scores. For both US and PBS plans, femoral head mean doses were inversely correlated with anterior score. For US plans and a full bladder, 4 beam arrangements that included an anterior beam tied for the highest maximum prescription dose (MPD). For US plans and an empty bladder, the arrangement with 1 anterior and 2 anterior oblique beams achieved the highest MPD in the virtual dose-escalation study. The dose-escalation study did not differentiate beam arrangements for PBS. All arrangements in the dose-escalation study were limited by bladder constraints except for the arrangement with 2 posterior oblique beams. The benefits of anterior proton beams in the setting of prostate-rectum spacers appear to be proton modality dependent and may not extend to PBS.

Is there a role for an external beam boost in cervical cancer radiotherapy?†

Objectives: Some patients are medically unfit for or averse to undergoing a brachytherapy boost as part of cervical cancer radiotherapy. In order to be able to definitively treat these patients, we assessed whether we could achieve a boost plan that would mimic our brachytherapy plans using external beam radiotherapy.Methods: High dose rate brachytherapy plans of 20 patients with stage IIB cervical cancer treated with definitive chemoradiotherapy were included in this study. Patients had undergone computer tomography (CT) simulations with tandem and ovoids in place. Point “A” dose was 600–700 cGy. We attempted to replicate the boost dose distribution from brachytherapy plans using intensity-modulated radiotherapy (Varian Medical Systems, Palo Alto, CA, USA), volumetric modulated arc therapy (Rapid Arc, Varian Medical Systems, Palo Alto, CA, USA), or TomoTherapy (Accuray, Inc., Sunnyvale, CA, USA) with the brachytherapy 100% isodose line as our target. Target coverage, normal tissue dose, and brachytherapy point doses were compared with ANOVA. Two-sided p-values ≤0.05 were considered significant.Results: External beam plans had excellent planning target volume (PTV) coverage, with no difference in mean PTV V95% among planning techniques (range 98–100%). External beam plans had lower bladder Dmax, small intestine Dmax, and vaginal mucosal point dose than brachytherapy plans, with no difference in bladder point dose, mean bladder dose, mean small intestine dose, or rectal dose. Femoral head dose, parametria point dose, and pelvic sidewall point dose were higher with external beam techniques than brachytherapy.Conclusions: External beam plans had comparable target coverage and potential for improved sparing of most normal tissues compared to tandem and ovoid brachytherapy.

Impact of rectal balloon‐filling materials on the dosimetry of prostate and organs at risk in photon beam therapy

The use of rectal balloon in radiotherapy of prostate cancer is shown to be effective in reducing prostate motion and minimizing rectal volume, thus reducing rectal toxicity. Air‐filled rectal balloon has been used most commonly, but creates dose perturbation at the air‐tissue interface. In this study, we evaluate the effects of rectal balloon‐filling materials on the dose distribution to the target and organs at risk. The dosimetric impact of rectal balloon filling was studied in detail for a typical prostate patient, and the general effect of the balloon filling was investigated from a study of ten prostate patients covering a wide range of anterior–posterior and left–right separations, as well as rectal and bladder volumes. Hounsfield units (HU) of the rectal balloon filling was changed from −1000 HU to 1000 HU at an interval of 250 HU, and the corresponding changes in the relative electron density (RED) was calculated. For each of the HU of the rectal balloon filling, a seven‐field IMRT plan was generated with 6 MV and 15 MV photon beams, respectively. Dosimetric evaluation was performed with the AAA algorithm for inhomogeneity corrections. A detailed study of the rectal balloon filling shows that the GTV, PTV, rectal, and bladder mean dose decreased with increasing values of RED in the rectal balloon. There is significant underdosage in the target volume at the rectum–prostate interface with an air‐filled balloon as compared to that with a water‐filled balloon for both 6 MV and 15 MV beams. While the dosimetric effect of the rectal balloon filling is reduced when averaged over ten patients, generally an air‐filled balloon results in lower minimum dose and lower mean dose in the overlap region (and possibly the PTV) compared to those produced by water‐filled or contrast‐filled balloons. Dose inhomogeneity in the target volume is increased with an air‐filled rectal balloon. Thus a water‐filled or contrast‐filled rectal balloon is preferred to an air‐filled rectal balloon in EBRT of prostate treatment.PACS numbers: 87.55.D‐, 87.55.de, 87.55.dk, 87.55.Gh, 87.55.kd

Dosimetric and cost comparison of first fraction imaging versus fractional re-imaging on critical organ dose in vaginal cuff brachytherapy

PurposeVaginal cylinder high-dose-rate (HDR) brachytherapy is currently one of the most common procedures performed in the treatment of early-stage endometrial cancer. However, current recommendations by the American Brachytherapy Society regarding fractional re-imaging and dose calculation for organs at risk for HDR vaginal cuff brachytherapy are not well defined. In this study, we aim to compare a fractional re-imaging approach using computed tomographic (CT) scans prior to each fraction with a first fraction imaging-only approach with respect to bladder, rectal, and bowel dosimetry. Methods and MaterialsNineteen endometrial cancer patients undergoing vaginal cuff brachytherapy for endometrial cancer were imaged with CT scanning prior to every HDR fraction (fractional re-imaging [FRI]). Dose to the bowel, bladder, and rectum were calculated and compared with the estimated dose if imaging and planning were done only on the first fraction (first fraction imaging [FFI]). ResultsIn the analysis of FFI versus FRI, we observed mean bladder doses of 8.34 Gy vs 8.33 Gy (P = .98), mean rectal doses of 12.19 Gy versus 12.14 Gy (P = .81), and mean bowel doses of 2.82 Gy versus 2.76 Gy (P = .81). The FFI approach underestimated the FRI doses to the bladder, rectum, and bowel by 20% or more in 11%, 5%, and 29% of patients, respectively. Cost analysis revealed an estimated $663.06, or a 35% savings per patient treated with FFI. ConclusionsThere is no statistically significant difference in the mean dose to the bladder, rectum, or bowel in patients undergoing HDR vaginal cuff brachytherapy with a first fraction imaging scheme versus a fractional re-imaging scheme. These results indicate that fractional re-imaging is not necessary except in patients whose estimated dose to critical organs is near the maximum limit.

Secondary Radiation Doses of Intensity Modulated Radiation Therapy in Patients With Lung and Liver Cancer

than for IMRT (p<0.001, respectively). VSB30 and VSB40 were significantly reduced in the IMRT (p<0.001, respectively). In middle and low rectal cancer, IMRT significantly increased VSB15 and VSB20 (increasing volume: middle38 50 cc, 35 31 cc, low 38 39 cc, 37 31 cc) whereas reduced only VSB40 compared with 3D-CRT (reducing volume: middle12 12 cc, low11 12 cc). In upper rectal cancer, however, IMRT reduced VSB30 and VSB40 (reducing volume: 24 19 cc, 13 7.5 cc), and increased only VSB20 compared with 3D-CRT (Increasing volume: 26 24.8 cc). The mean dose of bladder between two plans did not differ significantly. Conclusions: There is no difference target coverage between 3D-CRT and IMRT. Although the SB volume irradiated high dose was significantly reduced with IMRT, low dose of it increased with IMRT. In aspect of small bowel sparing, IMRT can be used more effectively in upper rectal cancer. AuthorDisclosure:H. Lee:None. J. Kim:None. I. Cho:None. S. Park:None. S. Yoon: None. Y. Kim: None. S. Song: None. S. Lee: None. E. Choi: None. S. Ahn: None.

Implementation of HybridArc treatment technique in preoperative radiotherapy of rectal cancer: dose patterns in target lesions and organs at risk as compared to helical Tomotherapy and RapidArc

PurposeHybridArc is a novel treatment technique blending aperture-enhanced optimized arcs with discrete IMRT-elements, allowing selection of arcs with a set of static IMRT-beams. This study compared this new technique to helical Tomotherapy, and RapidArc, in preoperative radiotherapy of rectal cancer.Material and methodsTwelve rectal cancer patients treated consecutively with Tomotherapy Hi-Art II system were simulated with HybridArc and RapidArc. Treatment plans were designed to deliver homogeneous dose of 46.0Gy to mesorectum and draining lymph nodes, with a simultaneous-integrated-boost to the primary tumor up to a total dose of 55.2Gy. Planning objectives were 95% of prescribed dose to 95% of PTVs, while minimizing the volume of small bowel receiving more than 15Gy (V15) and the mean bladder dose. Dose gradient towards simultaneous-integrated-boost (GI), calculated by dividing the volume receiving more then 52.4Gy (95% of PTV55.2Gy)to the volume of PTV55.2Gy, was kept below 1.5. Mean beam-on time and amount of MUs were also analyzed.ResultsPTV swere adequately covered by all plans. Significant advantage was found for Tomotherapy in sparing small bowel (V15 = 112.7cm3SD73.4cm3) compared to RapidArc (133.4cm3SD75.3cm3) and HybridArc (143.7cm3SD74.4cm3) (p < 0.01). The mean bladder dose was better with RapidArc (20.6GySD2.2Gy) compared to HybridArc (24.2Gy SD4.3Gy) and Tomotherapy (23.0GySD4.7Gy) (p < 0.01). The mean beam-on time was significantly lower (p < 0.01) for HybridArc (2.7min SD0.8) and RapidArc (2.5min SD0.5) compared to Tomotherapy (11.0min SD0.7). The total amount of MUs was significantly (p < 0.01) lower for RapidArc (547SD44)compared to HybridArc (949 SD153).ConclusionsHybridArc is a feasible solution for preoperative RT with a simultaneous-integrated-boost in rectal cancer patients. It achieved similar PTV coverage with significant lower beam-on time, but less efficient in sparing small bowel and bladder compared to Tomotherapy and RapidArc. The added value of HybridArc is that the treatment modality can be implemented on every LINAC equipped with Dynamic-Conform-Arc and IMRT treatment techniques, while maintaining the same QA-schemes.

SU-E-T-597: Influence of Smoothing Parameters on Dynamic IMRT Plan Quality and Deliverability.

To study the impact of different smoothing parameters on IMRT plan quality and deliverabilityMethods: Five previously treated patients of carcinoma cervix were chosen. Planning target volume (PTV) and organ at risk (OAR) i.e. bladder and rectum were contoured. In each case, five different dynamic IMRT plans with 6MV photon beam were created in eclipse TPS for Varian 2300C/D linear accelerator. During optimization, dose volume constraints and priorities were kept constant and smoothing parameters were varied as follows: 10/5, 40/30 (TPS default value), 80/60, 100/80 and 200/150 in x/y direction. Total dose was 5040cGy in 28 fractions and prescribed at 95% isodose. Plan quality was analyzed by means of coverage index (CI=PTV covered by prescription dose/PTV), OAR mean doses and total monitor units (MUs) required to deliver a plan. In each case, deliverability of treatment plans were verified with I'matriXX ion-chamber array and compared with TPS dose-plane using gamma index of 3% dose difference and 3mm distance to agreement criteria. The CI values were 0.9435±0.032, 0.9418±0.034, 0.9380±0.041, 0.9330±0.047 and 0.8681±0.072 for 10/5, 40/30, 80/60, 100/80 and 200/150 in x/y direction. PTV dose maximum decreases with the increase of smoothing parameters and values were 5724.38±106.08 5723.30±131.60, 5708.44±1 16.74, 5697.92±116.82 and 5587.50±189.50cGy. The bladder mean doses were 4027.46±630.40, 3821.62±420.62, 3819.58±427.08, 3813.42±435.02 and 3814.78±438.0cGy. Rectum mean doses were 3839.88±466.02, 3835.52±473.18, 3837.52±472.88, 3839.10±471.20 and 3918.94±469.76cGy. Similarly, Total MUs were 1588±205, 1573±214, 1513±274, 1456±335 and 1219±68. Gamma pass rate increases with the increase of smoothing parameters and values were 99.16±0.21%, 99.07±0.19%, 99.24±0.28%, 99.29±0.29% and 99.75±0.15%. When smoothing parameters decreased below TPS default value, plan quality increases, but deliverability decreases. If smoothing parameters increased above TPS default value, deliverability increases but plan quality decreases. Total MU decreases with the increase of smoothing parameters. Therefore, it's a trade-off between plan quality and deliverability which needs to be justified clinically.

A study of segment weight optimization with the CMS XiO step‐and‐shoot IMRT technique for prostate cancer

The aim of this study was to compare IMRT optimization in the CMS XiO radiotherapy treatment planning system, with and without segment weight optimization. Twenty‐one prostate cancer patients were selected for this study. All patients were initially planned with step‐and‐shoot IMRT (S‐IMRT). A new plan was then created for each patient by applying the segment weight optimization tool (SWO‐IMRT). Analysis was performed on the (SWO‐IMRT) and (S‐IMRT) plans by comparing the total number of segments, monitor units, rectal and bladder dose. The study showed a statistically significant reduction in the total number of segments (mean: 25.3%; range: 16.8%–31.1%) with SWO‐IMRT as compared to S‐IMRT (p<0.0001). Similarly, a mean reduction of 3.8% (range: 0.4%–7.7%) in the total MU was observed with SWO‐IMRT (p<0.0001). The study showed an average rectal dose decrease of 13.7% (range: 7.9%–21.4%) with SWO‐IMRT (p<0.0001). We also observed a statistically significant reduction of 26.7% (range: 16.0%–41.4%; p < 0.0001) in the mean dose to the posterior one‐third rectum and an overall reduction in mean bladder dose of 2.2% (range: 0.1%–6.1%) for SWO‐IMRT (p<0.0001). This study shows that the segment weight optimization method significantly reduces the total number of segments and the dose to the rectum for IMRT prostate cancer. It also resulted in fewer monitor units for most of the prostate cases observed in this study.PACS numbers: 85.55.ne; 87.55.de; 87.55.kd

Comparative Toxicity and Dosimetric Profile of Whole-Pelvis Versus Prostate Bed-Only Intensity-Modulated Radiation Therapy After Prostatectomy

To assess whether whole-pelvis (WP) intensity modulated radiation therapy (IMRT) for prostate cancer (PCa) after prostatectomy is associated with increased toxicity compared to prostate-bed only (PB) IMRT. All patients (n= 67) undergoing postprostatectomy IMRT to 70.2 Gy at our institution from January 2006 to January 2009 with minimum 12-month follow-up were divided into WP (n= 36) and PB (n= 31) comparison groups. WP patients received initial pelvic nodal IMRT to 45 Gy. Pretreatment demographics, bladder and rectal dose-volume histograms, and maximum genitourinary (GU) and gastrointestinal (GI) toxicities were compared. Logistic regression models evaluated uni- and multivariate associations between pretreatment demographics and toxicities. Pretreatment demographics including age and comorbidities were similar between groups. WP patients had higher Gleason scores, T stages, and preoperative prostate-specific antigen (PSA) levels, and more WP patients underwent androgen deprivation therapy (ADT). WP minimum (Dmin) and mean bladder doses, bladder volumes receiving more than 5 Gy (V5) and V20, rectal Dmin, and PB bladder and rectal V65 were significantly increased. Maximum acute GI toxicity was Grade 2 and was increased for WP (61%) vs. PB (29%) patients (p= 0.001); there was no significant difference in acute Grade ≥2 GU toxicity (22% WP vs. 10% PB; p= 0.193), late Grade ≥2 GI toxicity (3% WP vs. 0% PB; p= 0.678), or late Grade ≥2 GU toxicity (28% WP vs. 19% PB; p= 0.274) with 25-month median follow-up (range, 12-44 months). On multivariate analysis, long-term ADT use was associated with Grade ≥2 late GU toxicity (p= 0.02). Despite dosimetric differences in irradiated bowel, bladder, and rectum, WP IMRT resulted only in clinically significant increased acute GI toxicity in comparison to that with PB IMRT, with no differences in GU or late GI toxicity.

SU-E-T-748: Comparison of Three Treatment Planning Techniques for Adjuvant Radiation of the Bladder

Purpose: To compare the bowel and rectal dose distribution in patients who have had a prior cystectomy receiving pelvic node irradiation using 3D conformal photon therapy (3DCT), intensity modulated radiotherapy (IMRT) or intensity modulated proton therapy (IMPT) planning technique. Methods: We performed treatment planning for five patients generating plans with IMPT, IMRT, and 3DCT. The planning target volume (PTV) is nodal region including a 4 mm expansion for setup errors, and dosimetric uncertainty. The normal tissues of concern are rectum and combined small and large bowel. The dosimetric requirements for PTV includes a total prescription dose (PD) of 50.4 Gy for each plan with 95% of PD covers 98% of PTV volume (V95%=98%) and the maximum dose not to exceed 107% of PD. Bowels and rectum dose constraints is D30% ≤ 40 Gy and the maximum dose not exceed 105% and 107% of PD for rectum and bladder respectively. For evaluation, each plan is normalized to PTV coverage criteria and V35% and V65%, V95% and mean dose are compared for bowels and rectum. Results: The V95% for both IMPT and IMRT plans for rectum and bladder are much lower than 3D CT plans. The mean doses for bladder and rectum is 9.9 and 6.8 Gy for IMPT plans as compared to 23.7 Gy and 12.4 Gy for IMRT plans. These doses are higher for the 3DCT plan with the values of 32.3 and 28.2 Gy. Conclusions: With equivalent target coverage, both IMPT and IMRT improve normal tissue sparing compared to 3DCT. The IMPT technique results in plans with better PTV coverage and lower dose to organ at risk as compared to plans with IMRT and 3DCT.

Individualized margins for prostate patients using a wireless localization and tracking system

This study investigates the dosimetric benefits of designing patient‐specific margins for prostate cancer patients based on 4D localization and tracking. Ten prostate patients, each implanted with three radiofrequency transponders, were localized and tracked for 40 fractions. “Conventional margin” (CM) planning target volumes (PTV) and PTVs resulting from uniform margins of 5 mm (5M) and 7 mm (7M) were explored. Through retrospective review of each patient's tracking data, an individualized margin (IM) design for each patient was determined. IMRT treatment plans with identical constraints were generated for all four margin strategies and compared. The IM plans generally created the smallest PTV volumes. For similar PTV coverage, the IM plans had a lower mean bladder (rectal) dose by an average of 3.9% (2.5%), 8.5% (5.7%) and 16.2 % (9.8%) compared to 5M, 7M and CM plans, respectively. The IM plan had the lowest gEUD value of 23.8 Gy for bladder, compared to 35.1, 28.4 and 25.7, for CM, 7M and 5M, respectively. Likewise, the IM plan had the lowest NTCP value for rectum of 0.04, compared to 0.07, 0.06 and 0.05 for CM, 7M and 5M, respectively. Individualized margins can lead to significantly reduced PTV volumes and critical structure doses, while still ensuring a minimum delivered CTV dose equal to 95% of the prescribed dose.PACS numbers: 87.53.Kn, 87.55.D

Relationship Between Pelvic Organ-at-Risk Dose and Clinical Target Volume in Postprostatectomy Patients Receiving Intensity-Modulated Radiotherapy

To investigate dose-volume consequences of inclusion of the seminal vesicle (SV) bed in the clinical target volume (CTV) for the rectum and bladder using biological response indices in postprostatectomy patients receiving intensity-modulated radiotherapy (IMRT). We studied 10 consecutive patients who underwent prostatectomy for prostate cancer and subsequently received adjuvant or salvage RT to the prostate fossa. The CTV to planning target volume (PTV) expansion was 7 mm, except posterior expansion, which was 5 mm. Two IMRT plans were generated for each patient, including either the prostate fossa alone or the prostate fossa with the SV bed, but identical in all other aspects. Prescription dose was 68.4 Gy in 1.8-Gy fractions prescribed to ≥95% PTV. With inclusion of the SV bed in the treatment volume, PTV increased and correlated with PTV-bladder and PTV-rectum volume overlap (Spearman ρ 0.91 and 0.86, respectively; p < 0.05). As a result, the dose delivered to the bladder and rectum was higher (p < 0.05): mean bladder dose increased from 11.3 ± 3.5 Gy to 21.2 ± 6.6 Gy, whereas mean rectal dose increased from 25.8 ± 5.5 Gy to 32.3 ± 5.5 Gy. Bladder and rectal equivalent uniform dose correlated with mean bladder and rectal dose. Inclusion of the SV bed in the treatment volume increased rectal normal tissue complication probability from 2.4% to 4.8% (p < 0.01). Inclusion of the SV bed in the CTV in postprostatectomy patients receiving IMRT increases bladder and rectal dose, as well as rectal normal tissue complication probability. The magnitude of PTV-bladder and PTV-rectal volume overlap and subsequent bladder and rectum dose increase will be higher if larger PTV expansion margins are used.

Potential for dose escalation in the postprostatectomy setting with intensity-modulated radiation therapy: a dosimetric study using EORTC consensus guidelines for target volume contours

PurposeRadiation therapy (RT) is delivered as adjuvant and salvage therapy after radical prostatectomy for prostate cancer. Interest in dose escalation in this setting may necessitate more advanced RT techniques, such as intensity modulation. This study was designed to compare intensity-modulated radiation therapy (IMRT) versus 3-dimensional conformal radiation therapy (3DCRT) planning. MethodsTwenty patients were identified who received post--radical prostatectomy RT with 4-field, 3DCRT plans to 68.4 Gy. Contours were revised to comply with the European Organization for Research and Treatment of Cancer consensus guidelines. The IMRT plans with 5 versus 9 coplanar fields were compared for 10 patients. Then the 9-field IMRT plans were compared to 3DCRT in all 20 patients. Differences in dose-volume histogram values were evaluated using 2-tailed paired t tests. Cone beam computed tomographic images were analyzed to evaluate rectum doses in the treatment position during the RT course. The IMRT plans to 72.0 Gy were compared to 3DCRT to 68.4 Gy to be able to consider potential use in dose escalation. ResultsThe 9-field IMRT plans (vs 3DCRT) improved bladder mean dose and volume receiving 65 Gy or more (V65), as well as rectum mean dose (31.6 Gy vs 36.1 Gy; P < .001), volume receiving 75% or more of the prescription dose (24.4% vs 31.0%; P < .001), and V65 (10.5% vs 20.0%; P < .001). Advantages of IMRT were at the cost of small increases in maximum point doses delivered to the bladder and rectum. Cone beam computed tomographic images (n = 132) were analyzed for 8 patients; rectum mean dose and V65 were also improved by IMRT on these scans. IMRT allowed increasing dose to 72.0 Gy with similar bladder and rectum mean doses, V65, and V40 compared to 3DCRT to a total dose of 68.4 Gy. ConclusionsThe IMRT improves dosimetric parameters for the rectum and bladder, which may allow dose escalation after radical prostatectomy. Future studies should determine whether these advantages translate into improved clinical outcomes for prostate cancer patients.

Potential for Dose-escalation in the Post-prostatectomy Setting with Intensity Modulated Radiation Therapy: A Dosimetric Study using EORTC Consensus Guidelines for Target Volume Contours

Radiation therapy (RT) is delivered after radical prostatectomy (RP) for prostate cancer (PC). Although intensity modulated radiation therapy (IMRT) has become standard in definitive RT for PC, dosimetric data in support of post-RP IMRT are limited. This study was designed to quantify benefits of IMRT versus 3D conformal RT (3DCRT) with respect to dose sparing of rectum and bladder. Motivated by the desire to deliver higher radiation doses and to quantify the dosimetric impact of IMRT, we retrospectively analyzed images in order to: 1) identify a preferred IMRT beam arrangement; 2) evaluate the dosimetric advantages of IMRT compared to 3DCRT; and 3) assess the variation of dosimetric parameters during the RT course using conebeam CT (CBCT) images. 20 patients who received post-RP RT at our institution were identified. All patients were treated with 4-field 3DCRT plans (68.4 Gy to a minimum of 95% coverage). A single observer delineated all volumes to comply with EORTC consensus guidelines. The CTV was expanded 7 mm, except 5 mm posteriorly, to create the planning target volume (PTV). To identify optimal IMRT beam arrangements, 10 patients were planned with both 5- and 9-field, coplanar IMRT plans. Then, 9-field IMRT plans (72.0 Gy) were completed for the remaining patients. Differences of DVH values between IMRT and 3DCRT plans were evaluated using 2-tailed paired t-tests. CBCT data was used to evaluate rectal volume and dose throughout treatment. The 9-field IMRT plans (vs. 3DCRT) improved mean bladder dose values (31.23 Gy vs. 34.89 Gy, p = 0.003) and V65Gy (15.4% vs. 21.4%, p = 0.06). Additionally, 9-field IMRT reduced rectal endpoints: mean (31.58Gy vs. 36.14 Gy, p = 6.8E-7), V75% (24.4% vs. 31.0%, p = 0.0005), and V65Gy (10.5% vs. 20.0%, p = 1.08E-8). Advantages of IMRT were at the cost of small increases in maximum dose delivered to the bladder (73.3 Gy vs. 71.3 Gy, p = 0.0004) and the rectum (72.8Gy vs. 71.1 Gy, p = 0.001) compared to 3DCRT. The 9-field and 5-field IMRT plans had comparable PTV coverage. 9-field IMRT produced minimal improvements in mean rectal doses, but had lower maximum dose to 1 cc volume (71.58 Gy vs. 73.18 Gy, p = 0.003). CBCT images (n = 132) were analyzed for 8 patients; rectum mean dose and V65 were also improved by IMRT on these scans. IMRT allowed dose escalation to 72.0 Gy while maintaining similar bladder and rectum mean doses, V65, and V40 compared to 3DCRT to 68.4 Gy. IMRT improves dosimetric parameters for the rectum and bladder, which may reduce toxicity and allow dose-escalation after radical prostatectomy. Future studies should determine whether these dosimetric improvements translate into improved clinical outcomes for PC patients.

Prostate Treatment using Intensity Modulated Arc Therapy: Clinical Experience and Comparison with DMLC IMRT

Prostate Treatment using Intensity Modulated Arc Therapy: Clinical Experience and Comparison with DMLC IMRT

SU‐FF‐T‐108: Evaluation of IMRT Plans Between Linac and Helical Tomotherapy Based On Dose Volume Indices and Normal Tissue Sparing in Prostate Cancer Patients

Purpose: The objective of this study was to evaluate and quantify the dosimetric gains of linac‐based sliding window IMRT technique versus Helical Tomotherapy (HT) in terms of Conformity Index (CI), Homogeneity Index (HI), and normal‐tissue sparing for prostate cancer patients Methods and Material: Eight locally advanced, non metastatic, prostate cancer patients were selected for this study. Volume delineation was done on contrast enhances CT scans with 5mm slice thickness Target volumes included PTV74 (prostate and seminal vesicle) and PTV55 (pelvic lymph nodal regions) and Organs at Risk (OARs) were contoured. The PTV74 volume ranged from 79.8 cc to 177.85 cc (mean 121.02 ± 28.9 cc), while PTV55 ranged from 431.08 cc to 941.52 cc (mean 677.78 ± 185.06 cc). The prescription doses were 74 Gy/37♯ and 55 Gy/37♯ to 95% of PTV74 and PTV55 respectively. Results: Mean CI ± SD for PTV74 were 0.69 ± 0.06 and 0.77 ± 0.06 with IMRT and HT respectively (statistically significant, p=0.025). The mean CI for PTV55 were 0.43 ± 0.04 and 0.57 ± 0.07 with IMRT and HT respectively (p=0.21). Mean HI ± SD for PTV74 were 0.07 ± 0.02 and 0.04 ± 0.01 for IMRT and HT plans respectively (p=0.012) while for PTV55 mean HI were 0.2 ± 0.09 and 0.06 ± 0.02 for IMRT and HT respectively (p=0.012). HI was statistically superior with HT than with IMRT for both PTVs. OARs sparing in HT was statistically superior in rectum (p=0.025), Femoral Heads (pr=0.012, Pl=0.017) and statistically non significant difference in mean dose of bladder (p=0.18), small bowel (p=0.33) compare to conventional IMRT. Conclusion: Helical Tomotherapy plans revealed better Conformity Indices and Homogeneity Indices compared to sliding window IMRT technique for PTV74. Overall, the HT plans also spared the OARs significantly more than the IMRT plans except the small bowel.

SU‐FF‐T‐35: A Monte Carlo Study for Evaluation of Tissue Heterogeneity Effect of Inversely Optimized Intracavitary High Dose Rate Brachytherapy Plan

Purpose: To investigate tissue heterogeneity effect of an intracavitary high dose rate brachytherapy plan which is optimized by the inverse planning option. Method and Materials: A CT and VOI based brachytherapy plan for an intra‐uterine and cervix cancer treatment was made using inverse planning simulated‐annealing (IPSA) module provided by Plato TPS. The microSelectron‐HDR Ir‐192 afterloader and 3 catheters (2 ovoids and 1 tandem) were used for the treatment. For a verification of the plan result, EGSnrc based dosxyznrc Monte Carlo code was employed. Patient CT data was converted to egsphant data by the ctcreate code and each source position and weight from the plan data was used for a Monte Carlo simulation as well. Ir‐192 source was approximately modeled by 3.6 mm length and 0.6 mm diameter and it was assumed to be located at the center of the catheters, while parallel to the z‐axis at each position because of restricted freedom of source modeling. The microSelectron‐HDR Ir‐192 spectrum data could be obtained from the EGSnrc code, however, number of history was determined empirically based on known air‐kerma and dose rate value. Conventional point A and B doses and DVHs of PTV, bladder and rectum were evaluated for a quantitative comparison. Results: Significant decrease of dose at point A and B more than 10% was observed in the Monte Carlo simulation. PTV coverage by 100% isodose surface was degraded while rectum and bladder mean dose was reduced up to 15%. Resultant dose rate decrease was also observed. Conclusion: Conventional brachytherapy dose calculation method could not appropriately consider tissue heterogeneity effect and inverse planning optimization could invoke serious erroneous result. Therefore inverse planning optimization should be carefully implemented for a HDR brachytherapy planning.

Dosimetric Impact and Theoretical Clinical Benefits of Fiducial Markers for Dose Escalated Prostate Cancer Radiation Treatment

To assess the impact of fiducial markers and daily kilovoltage imaging (FM-kV) on dose-volume histogram (DVH) parameters and normal tissue complication probabilities (NTCPs) for the rectum and bladder during prostate cancer radiotherapy. Two different setup scenarios were compared for 20 patients treated with three-dimensional conformal radiotherapy (3D-CRT) for localized prostate cancer to a total dose of 76 Gy: a traditional setup with planning target volume (PTV) margins associated with skin mark alignment vs. another setup using FM-kV. Various DVH parameters were compared, including Radiation Therapy Oncology Group (RTOG) dose-volume constraints for the rectum and bladder. Analysis of NTCPs was also performed according to the Lyman model. With the traditional setup, 85% of patients had rectal V70(Gy) >25% compared with 45% with FM-kV. Moreover, 30% of patients with traditional setup vs. 5% with FM-kV did not fulfill at least 3 RTOG constraint parameters for the rectum. Mean rectal and bladder dose were 4.7 Gy and 6.7 Gy less, respectively, with FM-kV. The NTCP for the rectum was 11.5% with the traditional setup and 9% with FM-kV. This indicates that with FM-kV, the prescription dose could be increased by 2.1 Gy while keeping the same level of late rectal toxicity as with the traditional setup. Use of FM-kV is an efficient way of lowering the proportion of patients not fulfilling RTOG rectal and bladder dose-volume constraints. The results of the NTCP analysis suggest that the PTV margin reduction allowed by FM-kV should decrease the rate of late rectal toxicities or may allow moderate dose escalation.

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.