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• W2039414198 abstract "Purpose: The aim of this study was to evaluate the feasibility of treating the pelvic lymphatic regions during prostate intensity-modulated radiotherapy (IMRT) with respect to our routine acceptance criteria.Methods and Materials: A series of 10 previously treated prostate patients were randomly selected and the pelvic lymphatic regions delineated on the fused magnetic resonance/computed tomography data sets. A targeting progression was formed from the prostate and proximal seminal vesicles only to the inclusion of all pelvic lymphatic regions and presacral region resulting in 5 planning scenarios of increasing geometric difficulty. IMRT plans were generated for each stage for two accelerator manufacturers. Dose volume histogram data were analyzed with respect to dose to the planning target volumes, rectum, bladder, bowel, and normal tissue. Analysis was performed for the number of segments required, monitor units, “hot spots,” and treatment time.Results: Both rectal endpoints were met for all targets. Bladder endpoints were not met and the bowel endpoint was met in 40% of cases with the inclusion of the extended and presacral lymphatics. A significant difference was found in the number of segments and monitor units with targeting progression and between accelerators, with the smaller beamlets yielding poorer results. Treatment times between the 2 linacs did not exhibit a clinically significant difference when compared.Conclusions: Many issues should be considered with pelvic lymphatic irradiation during IMRT delivery for prostate cancer including dose per fraction, normal structure dose/volume limits, planning target volumes generation, localization, treatment time, and increased radiation leakage. We would suggest that, at a minimum, the endpoints used in this work be evaluated before beginning IMRT pelvic nodal irradiation. Purpose: The aim of this study was to evaluate the feasibility of treating the pelvic lymphatic regions during prostate intensity-modulated radiotherapy (IMRT) with respect to our routine acceptance criteria. Methods and Materials: A series of 10 previously treated prostate patients were randomly selected and the pelvic lymphatic regions delineated on the fused magnetic resonance/computed tomography data sets. A targeting progression was formed from the prostate and proximal seminal vesicles only to the inclusion of all pelvic lymphatic regions and presacral region resulting in 5 planning scenarios of increasing geometric difficulty. IMRT plans were generated for each stage for two accelerator manufacturers. Dose volume histogram data were analyzed with respect to dose to the planning target volumes, rectum, bladder, bowel, and normal tissue. Analysis was performed for the number of segments required, monitor units, “hot spots,” and treatment time. Results: Both rectal endpoints were met for all targets. Bladder endpoints were not met and the bowel endpoint was met in 40% of cases with the inclusion of the extended and presacral lymphatics. A significant difference was found in the number of segments and monitor units with targeting progression and between accelerators, with the smaller beamlets yielding poorer results. Treatment times between the 2 linacs did not exhibit a clinically significant difference when compared. Conclusions: Many issues should be considered with pelvic lymphatic irradiation during IMRT delivery for prostate cancer including dose per fraction, normal structure dose/volume limits, planning target volumes generation, localization, treatment time, and increased radiation leakage. We would suggest that, at a minimum, the endpoints used in this work be evaluated before beginning IMRT pelvic nodal irradiation. IntroductionOver the past decade or so we have experienced a relative decrease in radiotherapy treatment volume for prostate cancer. Target volumes are smaller as whole pelvic radiotherapy (WPRT) is used less and margins are smaller with the common use of daily localization techniques. The use of intensity-modulated radiotherapy (IMRT) for treatment (Tx) of these relatively small targets, surrounded by dose limiting structures, has become relatively routine. However, a randomized trial by Roach et al. (1Roach III, M. DeSilvio M. Lawton C. et al.Phase III trial comparing whole-pelvis versus prostate-only radiotherapy and neoadjuvant versus adjuvant combined androgen suppression: Radiotherapy Oncology Group 9413.JCO. 2003; 21: 1904-1911Crossref PubMed Scopus (571) Google Scholar) suggests that the irradiation of the pelvic lymphatic regions may be a benefit for a certain subset of men with prostate cancer. Critical structure sparing is a primary concern when treating prostate cancer and the addition of the nodal regions places larger volumes of bladder and bowel in the treatment area that may result in unwanted complications. The use of IMRT for these treatments appears obvious but presents additional challenges to be considered. Nutting et al. (2Nutting C.M. Convery D.J. Cosgrove V.P. et al.Reduction of small and large bowel irradiation using an optimized intensity-modulated pelvic radiotherapy technique in patients with prostate cancer.Int J Radiat Oncol Biol Phys. 2000; 48: 649-656Abstract Full Text Full Text PDF PubMed Scopus (198) Google Scholar) had previously shown a reduction of dose to the organs-at-risk (OAR) within the pelvis, including bowel, when using IMRT vs. conventional three-dimensional conformal radiotherapy (3D–CRT) delivery. Sanguinetti et al. (3Sanguinetti G. Cavey M.L. Endres E.J. et al.Is IMRT needed to spare the rectum when pelvic lymph nodes are part of the initial treatment volume for prostate cancer?.Int J Radiat Oncol Biol Phys. 2006; 64: 151-160Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar) also evaluated the effects on target coverage and OARs with emphasis on the rectum when pelvic lymphatics are included in the treatment of prostate cancer with conventional methods and IMRT. Ashman et al. (4Ashman J.B. Zelefsky M.J. Hunt M.S. et al.Whole pelvic radiotherapy for prostate cancer using 3D conformal and intensity-modulated radiotherapy.Int J Radiat Oncol Biol Phys. 2005; 63: 765-771Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar) have recently made comparisons between patients treated with WPRT using 3D–CRT and IMRT with respect to observed clinical morbidity and dosimetric parameters. In this work we evaluate the dosimetric impact of lymphatic inclusion on our routine IMRT prostate plan acceptance criteria as well as treatment delivery time and leakage radiation.Methods and materialsTen prostate cancer patients with nonmetastatic lymph node negative disease were randomly selected for this study. All men underwent noncontrast computed tomography (CT) (PQ5000, Philips Medical Systems, Cleveland, OH) and magnetic resonance imaging (MRI) (0.23T open MRI scanner, Philips Medical Systems, Cleveland, OH) simulation with a full bladder and empty rectum. The target and normal tissue volumes were defined on the fused MR/CT data sets. The prostate, proximal seminal vesicles, distal seminal vesicles, periprostatic/peri seminal vesicle lymph nodes, external iliac lymph nodes, proximal obturator, proximal internal iliac nodes, presacral/perirectal nodes were all outlined separately. The proximal and distal seminal vesicles are defined separately because the proximal seminal vesicles receive the full dose, whereas the distal seminal vesicles are given the same dose as the lymph nodes. The rectum was defined as the rectal wall and its contents from the anal verge to the sigmoid flexure superiorly. The bladder volume was defined as the entire bladder and its contents. Bowel was conservatively delineated for each patient to include all space that could potentially be occupied by bowel. The potential bowel space is the area between the pelvic nodal regions, beginning at the sigmoid flexure inferiorly from just above the rectum and extending superiorly to 1 cut above the most superior lymph nodes outlined. A more detailed description of the target and normal tissue volume definitions has been described elsewhere (5Pollack A. Hanlon A.L. Horwitz E.M. et al.Dosimetry and preliminary acute toxicity in the first 100 men treated for prostate cancer on a randomized hypofractionation dose escalation trial.Int J Radiat Oncol Biol Phys. 2006; 64: 518-526Abstract Full Text Full Text PDF PubMed Scopus (216) Google Scholar, 6Buyyounouski M.K. Horwitz E.M. Price Jr, R.A. et al.Prostate IMRT.in: Bortfield T. Rupert Schmidt-Ullrich R. de Neve W. Wazer D.E. IMRT handbook: concepts & clinical applications. Springer-Verlag, Heidelberg2006: 391-410Google Scholar).The clinical target volume (CTV) for each group was determined by using a combination of sub volumes (i.e., CTV1, CTV2, etc.). The CTV for Group 1 included the prostate and proximal seminal vesicles (CTV1). For Group 2, the distal seminal vesicles (CTV2) were also included. Group 3 further adds the periprostatic and peri-seminal vesicle lymph nodes (CTV3). Group 4 (extended lymphatics) includes the external iliac, proximal obturator, and proximal internal iliac nodes (CTV4). Group 5 is the most comprehensive treatment and also includes the presacral/perirectal lymph nodes (CTV5). Groups 2 to 5 represent a progression of potential treatment volumes for high-risk patients, with Group 5 comprehensively including potential lymph node metastasis sites—the presacral/presciatic lymph nodes to S3 and the perirectal lymph nodes below that to the level of the seminal vesicles (7Shih H.A. Harisinghani M. Zietman A.L. et al.Mapping of nodal disease in locally advanced prostate cancer: Rethinking the clinical target volume for pelvic nodal irradiation based on vascular rather than bony anatomy.Int J Radiat Oncol Biol Phys. 2005; 63: 1262-1269Abstract Full Text Full Text PDF PubMed Scopus (92) Google Scholar, 8Ganswindt U. Paulsen F. Corvin S. et al.Intensity-modulated radiotherapy for high risk prostate cancer based on sentinel node SPECT imaging for target volume definition.BMC Cancer. 2005; 5: 91Crossref PubMed Scopus (30) Google Scholar, 9Murray S.K. Breau R.H. Guha A.K. Gupta R. Spread of prostate carcinoma to the perirectal lymph node basin: Analysis of 112 rectal resections over a 10-year span for primary rectal adenocarcinoma.Am J Surg Pathol. 2004; 28: 1154-1162Crossref PubMed Scopus (34) Google Scholar). This progression, illustrated in Figs. 1a to 1e, resulted in 5 different planning scenarios of increasing geometric difficulty.Volume expansion to define PTV3 and PTV4 is somewhat problematic because prostate motion is independent of the lymph nodes; prostate motion corrected using transabdominal ultrasound may result in a shift of the dose distribution away from the lymph nodes, assuming the isocenter remains aligned with the bony anatomy. Thus, PTV3 and PTV4 should use a larger margin than PTV1. However, this would lead to compromise in the treatment of the prostate in terms of achieving high target doses and sparing of the bladder and rectum. We are using the same margins for all PTVs (8 mm everywhere and 5 mm posteriorly), although we have found that sometimes 6 mm lateral margins for PTV4 are necessary to limit bladder and bowel dose. Since lateral interfraction prostate displacement is typically small, this seems reasonable. Because of the generous definition of CTV5 and its extremely complex geometry, no additional margin was used for this targeting group in this study.Intensity-modulated radiotherapy plans were generated for each targeting group (associated with each PTV) with the goal of meeting our routine clinical prostate plan acceptance criteria (5Pollack A. Hanlon A.L. Horwitz E.M. et al.Dosimetry and preliminary acute toxicity in the first 100 men treated for prostate cancer on a randomized hypofractionation dose escalation trial.Int J Radiat Oncol Biol Phys. 2006; 64: 518-526Abstract Full Text Full Text PDF PubMed Scopus (216) Google Scholar). These criteria include 95% of the planning target volume (PTV95) receiving at least the prescription dose. The rectal volume receiving ≥65 Gy (R65) should not be more than 17% and the volume receiving ≥40 Gy (R40) should not be more than 35%. The bladder volume receiving ≥65 Gy (B65) should not be more than 25%, and the volume receiving ≥40 Gy (B40) should not be more than 50%. The 50% isodose line should fall within the rectal contour and the 90% isodose line should not exceed half the diameter of the rectum on any CT slice. Additionally, the distance between the posterior edge of the CTV and the prescription isodose line should be between 3 and 8 mm on each axial slice. A 40-Gy bowel limit of 150 cc was based on experience gained from pelvic irradiation for rectal cancer (10Baglan K.L. Frazier R.C. Yan D. et al.The dose-volume relationship of acute small bowel toxicity from concurrent 5-FU-based chemotherapy and radiotherapy for rectal cancer.Int J Radiat Oncol Biol Phys. 2002; 52L: 176-183Abstract Full Text Full Text PDF Scopus (220) Google Scholar) as well as dose levels discussed in the treatment of anal and gynecologic diseases (11Roeske J.C. Lujan A. Rotmensch J. et al.Intensity-modulated whole pelvic irradiotherapy in patients with gynecologic malignancies.Intern J Radiat Oncol Biol Phys. 2000; 48 (1613–1621)Abstract Full Text Full Text PDF PubMed Scopus (311) Google Scholar, 12Portelance L. Chao K.S.C. Grigsby P.W. et al.Intensity-modulated radiotherapy (IMRT) reduces small bowel, rectum, and bladder doses in patients with cervical cancer receiving pelvic and para-aortic irradiation.Int J Radiat Oncol Biol Phys. 2001; 51: 261-266Abstract Full Text Full Text PDF PubMed Scopus (380) Google Scholar, 13Adli M. Mayr N.A. Kaiser H.S. et al.Does prone positioning reduce small bowel dose in pelvic radiation with intensity-modulated radiotherapy for gynecological cancer?.Int J Radiat Oncol Biol Phys. 2003; 57: 230-238Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar, 14Mundt A.J. Lujan A.E. Rotmensch J. et al.Intensity-modulated whole pelvic radiotherapy in women with gynecologic malignancies.Int J Radiat Oncol Biol Phys. 2005; 63: 354-361Abstract Full Text Full Text PDF PubMed Scopus (187) Google Scholar, 15Milano M.T. Jani A.B. Farrey K.J. et al.Intensity-modulated radiotherapy (IMRT) in the treatment of anal cancer: Toxicity and clinical outcome.Int J Radiat Oncol Biol Phys. 2003; 57: 230-238Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar). This is likely to be conservative because of the lower dose per fraction to this region and the absence of concurrent chemotherapy.Plans were generated for delivery on 2 accelerators. The first accelerator was a 10 MV Siemens Primus (Siemens Medical Systems, Concord, CA) using a minimum beamlet size of 10 × 10 mm2. The second accelerator was a 10MV Varian Ex 21 (Varian Medical Systems, Palo Alto, CA) utilizing a minimum beamlet size of 10 × 5 mm2 with the collimator rotated to 90 degrees (16Price Jr, R.A. Paskalev K. McNeeley S.W. Ma C.-M. Elongated beamlets: A simple technique for segment and MU reduction for sMLC IMRT delivery on accelerators utilizing 5 mm leaf widths.Phys Med Biol. 2005; 50: N235-N242Crossref PubMed Scopus (2) Google Scholar). By using the leaf width (5 mm) as the short side of the beamlet and rotating the collimator, we are matching the resolution of a 5 × 5 mm2 beamlet in the direction perpendicular to the prostate-rectum interface. In addition, using a step size of 10 mm results in fewer MLC positions and a faster treatment delivery. The increased area of the elongated beamlet results in increased output and fewer segments and monitor units (MU). This MU reduction also results in a decrease in the amount of accelerator head leakage.All plans were generated using the Corvus inverse planning system, version 5.0 (NOMOS Corp., Cranberry, PA) by the same experienced planner (R.P.) using 5 intensity levels per beam. The step-and-shoot delivery method was used throughout. Each plan was started using 6 beam directions progressing to 9 as needed. In addition, regions for dose constraint were used for all plans (17Price Jr, R.A. Murphy S. McNeeley S.W. et al.A method for increased dose conformity and segment reduction for sMLC delivered IMRT treatment of the prostate.Int J Radiat Oncol Biol Phys. 2003; 57: 843-852Abstract Full Text Full Text PDF PubMed Scopus (31) Google Scholar). This technique is based on the idea that all regions that are not designated as target, or some normal or critical structure, are combined as 1 region during the IMRT planning process. This region (termed “tissue” for this planning system) is controlled by a single set of dose constraints. However, the volume of tissue is far greater than that of the structures of interest. Furthermore, tissue is given the lowest priority during the optimization process. The addition of the dose-conforming regions allows more partial volume input data and provides extra control over the dose distribution in the regions of interest with respect to plans run without the regions. By designating subsets of tissue as concentric regions around the target(s) and defining each region’s dose constraints, an increased measure of control over the dose gradient outside the target boundaries is realized resulting in increased dose conformity and decreased treatment time.Dose volume histogram (DVH) data were analyzed with respect to dose to the PTVs, rectum, bladder, bowel, and normal tissue. Additional analysis was performed for the number of segments required, MU, “hot spots” (i.e., unacceptable high-dose regions), and treatment time. Normal distributions of the endpoints were evaluated with the Shapiro-Wilk test of normality. For the 4 more complicated targeting groups, comparisons were made to Group 1 by paired t tests (normally distributed variables) or by Wilcoxon signed rank tests (non-normally distributed variables). The McNemar Exact Test (MET) was used to evaluate endpoints with respect to meeting or failing each criterion. This test is based on the number of patients that meet the criterion on 1 plan and fail on another and uses exact binomial probabilities because of the small sample size.ResultsAll plans were normalized such that 95% of the prostate and proximal seminal vesicle PTV received 76 Gy. The remaining PTVs for each group received at least 56 Gy to 95% of their volumes. Table 1 contains the results for all endpoints studied resulting from plans generated for the Siemens Primus unit (10 × 10 mm2 minimum beamlet size). Included are the mean values for all 10 patients and all 5 targeting groups as well as the respective ranges and statistical significance (p-values). All endpoints except the bowel volume variables were normally distributed. Statistical significance was calculated for all groups with respect to Group 1. Since the conformity index is not defined for multiple co-joined targets with differing prescription doses and the fact that our target volume(s) changes with targeting progression, we have decided to evaluate the volume of normal tissue (NT) receiving some reference dose as a function of targeting progression. The reference dose used was our 56 Gy (NT56) nodal target prescription dose. Table 2 shows the results for all endpoints studied resulting from plans generated for the Varian Ex 21 unit (10 × 5 mm2 minimum beamlet size). Values for p and NT are defined as in Table 1. Table 3 contains the results from a head-to-head comparison for plans generated for the 2 accelerators. In this table, statistical significance is calculated between the results for the 2 linacs within each targeting group.Table 1Average results from plans generated with a minimum beamlet size of 10 × 10 mm210 × 10 mm2Group 1Group 2Group 3Group 4Group 5R65 (%)11.911.811.611.310.8 Range (%)7.0–16.67.4–15.87.6–16.28.1–15.06.4–14.7 p0.7910.6400.3210.082R40 (%)27.128.829.029.835.4 Range (%)19.5–33.919.8–36.319.7–36.022.9–36.621.0–43.7 p0.0710.0390.011<0.005B65 (%)19.921.623.827.830.2 Range (%)7.7–35.87.3–39.410.8–50.314.2–43.020.1–49.2 p0.0180.017<0.005<0.005B40 (%)36.839.847.870.972.0 Range (%)15.9–63.017.6–75.421.4–84.961.6–85.959.1–81.0 p0.126<0.005<0.005<0.005Bowel-65 (cc)0.30.71.37.39.1 Range (cc)0.0–2.50.0–2.40.0–7.30.4–27.90.4–62.9 p0.2500.063<0.005<0.005Bowel-40 (cc)3.99.214.0210.8209.3 Range (cc)0.0–16.30.0–36.30.0–68.848.4–382.857.7–358.5 p0.016<0.005<0.005<0.005MU10131047120515211701 Range855–1210873–12861053–13331296–17431533–1935 p0.170<0.005<0.005<0.005Segments545576117145 Range40–7843–7956–9089–136130–163 p0.788<0.005<0.005<0.005Tx time (min)9.09.112.619.323.9 Range (min)6.6–12.97.1–13.09.2–14.914.7–22.421.5–26.9 p0.788<0.005<0.005<0.005Max dose (%)118.1118.5118.7119.0120.2 Range (%)114.9–119.9117.5–119.4116.7–120.4115.9–122.1117.8–121.4 p0.9860.645<0.005<0.005NT56 (cc)337356409780920 Range (cc)253–447256–449284–509612–980679–1170 p0.122<0.005<0.005<0.005Abbreviations: Max = maximum; MU = monitor units; Tx = treatment.For reference, primary limits are as follows: R65 ≤17%, R40 ≤35%, B65 ≤25%, B40 ≤50%, Bowel40 ≤150 cc. Open table in a new tab Table 2Average results from plans generated with a minimum beamlet size of 10 × 5 mm210 × 5 mm2Group 1Group 2Group 3Group 4Group 5R65 (%)10.310.610.310.410.2 Range (%)6.3–15.26.8–14.46.1–14.46.3–14.06.4–13.2 p0.2850.8410.5880.799R40 (%)23.224.525.126.130.5 Range (%)14.2–30.815.9–34.015.6–34.817.0–33.418.3–37.9 p0.0730.031<0.005<0.005B65 (%)19.120.023.126.929.2 Range (%)7.3–36.18.2–37.410.4–45.715.3–40.517.3–47.6 p0.122<0.005<0.005<0.005B40 (%)35.138.947.867.770.4 Range (%)13.8–64.717.9–73.721.7–86.158.8–81.861.3–83.9 p0.012<0.005<0.005<0.005Bowel-65 (cc)0.30.70.94.45.9 Range (cc)0.0–1.50.0–2.70.0–3.20.0–20.10.0–35.1 p0.0310.0630.008<0.005Bowel-40 (cc)3.38.313.6199.1197.1 Range (cc)0.0–15.50.0–34.30.0–63.036.2–340.249.2–322.6 p0.0160.016<0.005<0.005MU15151516148423142252 Range1309–17811248–1836846–18702138–24672072–2340 p0.9860.645<0.005<0.005Segments189203262628725 Range137–324141–291174–351489–737640–792 p0.338<0.005<0.005<0.005Tx time (min)9.810.412.221.325.4 Range (min)7.9–11.58.2–13.79.4–15.312.2–25.822.4–27.7 p0.2670.007<0.005<0.005Max dose (%)117.0116.9117.0117.5119.9 Range (%)114.7–119.5114.3–118.9114.4–119.5114.5–121.8117.2–126.3 p0.8860.9820.5540.019NT56 (cc)316331381735859 Range (cc)216–417224–421266–485573–892631–1071 p0.021<0.005<0.005<0.005Abbreviations as in Table 1.For reference, primary limits are as follows: R65 ≤17%, R40 ≤35%, B65 ≤25%, B40 ≤50%, Bowe140 ≤150 cc. Open table in a new tab Table 3Results of head-to-head comparison of plans generated for the two different minimum beamlet sizes and accelerator manufacturersGroup 1Group 2Group 3Group 4Group 5R65 (10 × 10) %11.911.811.611.310.8R65 (10 × 5) %10.310.610.310.410.2 p0.0040.0050.0050.0060.016R40 (10 × 10) %27.128.829.029.835.4R40 (10 × 5) %23.224.525.126.130.5 p<0.005<0.005<0.005<0.005<0.005B65 (10 × 10) %19.921.623.827.830.2B65 (10 × 5) %19.120.023.126.929.2 p0.1130.0780.1440.2300.225B40 (10 × 10) %36.839.847.870.972.0B40 (10 × 5) %35.138.947.867.770.4 p0.0200.0710.9500.0040.104Bowel-65 (10 × 10) cc0.30.71.37.39.1Bowel-65 (10 × 5) cc0.30.70.94.45.9 p1.0000.7500.688<0.0050.232Bowel-40 (10 × 10) cc3.99.214.0210.8209.3Bowel-40 (10 × 5) cc3.38.313.6199.1197.1 p0.1250.1020.461<0.005<0.005MU (10 × 10)10131047120515211701MU (10 × 5)15151516148423142252 p<0.005<0.0050.007<0.005<0.005Segments (10 × 10)545576117145Segments (10 × 5)189203262628725 p<0.005<0.005<0.005<0.005<0.005Tx time (min) (10 × 10)9.09.112.619.323.9Tx time (min) (10 × 5)9.810.412.221.325.4 p0.1220.0410.6970.031<0.005Max% (10 × 10)118.1118.5118.7119.0120.2Max% (10 × 5)117.0116.9117.0117.5119.9 p0.1030.006<0.005<0.0050.621NT56 (cc) (10 × 10)337356409780920NT56 (cc) (10 × 5)316331381735859 p<0.005<0.005<0.005<0.005<0.005Abbreviations as in Table 1.For reference, primary limits are as follows: R65 ≤17%, R40 ≤35%, B65 ≤25%, B40 ≤50%, Bowel40 ≤150 cc. Open table in a new tab Figure 2a illustrates the rectal results for all patients when prescribing to targeting Group 4 (extended lymphatics). It can be seen that our clinical criteria are met in almost all cases. This is to be expected in that the lymphatic regions involved are not directly adjacent to the rectum. It is seen that the smaller leaf width MLC plans, particularly for the R40 endpoint, result in lower percentages of rectum being irradiated indicating increased dose conformity. The bladder endpoints for all patients, illustrated in Fig. 2b, are not met in the majority of cases. The geometry of targeting Group 4 with the extended lymphatics bounding the bladder laterally, and the associated PTVs, make it impractical to limit the bladder to our clinical criteria in all cases. The paired t-test evaluates statistical significance by comparing the mean values between groups for a given endpoint. We also evaluated endpoints on a case-by-case basis with respect to meeting or failing to meet our criteria using McNemar’s Exact Test (MET). This test indicates a highly significant trend in failing to meet the B40 criteria with the addition of the extended lymphatics (p = 0.008). Attempts to increase the bladder constraints further during optimization resulted in the unacceptable high-dose regions known as hot spots. It should be noted that the values indicated for hot spots represent a single point in the dose matrix defined by voxels with sides approximately 0.9 mm in length. If we evaluate the dose distribution for clinically meaningful high-dose regions, at least 2 cm2 on an individual CT slice for example, the listed values decrease on average by approximately 5%. For example, an indicated hot spot of 118% would typically represent a clinically significant hot spot of 113%. These hot spots are always found within the prostate PTV. Given that there is some overlap between the prostate PTV and critical structures efforts are made to ensure that the high-dose regions do not fall in these overlap areas if possible.Fig. 2(a) Rectal endpoint results for all patients with the inclusion of the extended lymphatics (TG4). The 40- and 65-Gy limits are delineated as well as the results for both multileaf collimator (MLC) sizes (v indicates 5 mm leaf widths and s indicates 10-mm leaf widths). (b) Bladder endpoint results for all patients with the inclusion of the extended lymphatics (TG4). The 40- and 65-Gy limits are delineated as well as the results for both MLC sizes (v indicates 5-mm leaf widths and s indicates 10-mm leaf widths). (c) Rectal endpoint results for all patients with the inclusion of the presacral lymphatics (TG5). The 40 and 65 Gy limits are delineated as well as the results for both MLC sizes (v indicates 5-mm leaf widths and s indicates 10-mm leaf widths). (d) Bladder endpoint results for all patients with the inclusion of the presacral lymphatics (TG5). The 40- and 65-Gy limits are delineated as well as the results for both MLC sizes (v indicates 5-mm leaf widths and s indicates 10-mm leaf widths).View Large Image Figure ViewerDownload (PPT)The affects on our rectal endpoints with the addition of the presacral region is illustrated in Fig. 2c. The high-dose limit is consistently met. The low-dose limit is not met in the majority of cases (MET p = 0.03 for the 10 × 10 mm2 beamlets; the smaller beamlets showed no significant trend). The addition of the presacral region, bounding a portion of the rectum laterally and posteriorly, results in a tube-like dose distribution. Maintaining the R40 limit was not always possible. As expected the bladder endpoints are not met in the majority of cases as illustrated in Fig. 2d (MET for B40, p = 0.008 for both linacs). The plans that met the B65 endpoints are consistent with those for targeting Group 4 indicating the major contributing factor to increased bladder dose being the addition of the extended lymphatics.Figure 3 illustrates the results for all plans with respect to our bowel criteria. On average we were unable to meet the 40 Gy cut point with the addition of targeting Groups 4 and 5. Additionally these groups exhibit statistical significance with respect to failing this endpoint on a case-by-case basis (MET p = 0.016, both groups, both linacs). There is little change in the results between plans for these groups indicating the addition of the extended lymphatics as being the major contributor to increased dose to bowel. It is of note that the smaller MLC leaf widths resulted in a decreased volume of bowel being irradiated indicating increased target conformity.Fig. 3Bowel-40 endpoint results for all patients and all targeting groups. The 150-cc limit is delineated as well as the results for both MLC sizes (minimum beamlet sizes are listed and correspond to linac vendor).View Large Image Figure ViewerDownload (PPT)We have chosen to treat all targets, regardless of the intended dose, with a single IMRT plan. The prostate prescription of 76 Gy delivered in 38 fractions assures approximately 2.0 Gy per fraction is being delivered. However, the prescription dose for the distal seminal vesicles and all nodal groups is 56 Gy in 38 fractions resulting in a gradient between approximately 1.5 Gy and 2.0 Gy per fraction. This prescription was derived with the aid of biologically equivalent dose calculations and is equivalent to 49 to 50 Gy delivered at 2.0 Gy per fraction. An alternative approach would be to use a series of “cone down” IMRT plans to maintain 2.0 Gy per fraction for all targets. We do not believe the dosimetric results will differ from those found in this study.DiscussionIn this" @default.
• W2039414198 created "2016-06-24" @default.
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• W2039414198 creator A5030553105 @default.
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• W2039414198 date "2006-10-01" @default.
• W2039414198 modified "2023-10-16" @default.
• W2039414198 title "Impact of pelvic nodal irradiation with intensity-modulated radiotherapy on treatment of prostate cancer" @default.
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