M. J. Follwell1, W. J. Morris1, M. Vitali1, R. Shaffer2, K. James1, T. Pickles1, K. Otto1. 1BC Cancer Agency, Vancouver, BC, Canada, 2Royal Surrey County ...
Proceedings of the 53rd Annual ASTRO Meeting
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Volumetric Modulated Arc Therapy for Intraprostatic Maximal Simultaneous Boost (IMAX): Dosimetric Analyses, Safety and Early Clinical Outcomes
M. J. Follwell1, W. J. Morris1, M. Vitali1, R. Shaffer2, K. James1, T. Pickles1, K. Otto1 1
BC Cancer Agency, Vancouver, BC, Canada, 2Royal Surrey County Hospital - NHS Foundation Trust, Surrey, United Kingdom
Purpose/Objective(s): To assess the dosimetric quality, safety and early efficacy of hypofractionated radiotherapy incorporating a simultaneous intraprostatic boost (SIB) for prostate cancer. Materials/Methods: Ten patients diagnosed with intermediate-risk prostate cancer (NCCN risk stratification) were treated with volumetric modulated arc therapy, optimized and delivered with RapidArc in the first phase of an SIB dose escalation study. The Clinical target volume (CTV), excluding the prostatic urethra, was the prostate contour on CT images; the planning target volume (PTV) was generated by adding a 7mm margin in all but the posterior direction where 5mm was employed. The minimum dose to the PTV was 70 Gy (28 x 2.5Gy; nominal isocenter dose = 73.7Gy). A SIB was planned to deliver an additional 7.4Gy (10% of the nominal isocenter dose; total dose = 81.1 Gy or 28 x 2.9Gy) to as much of the CTVas possible while meeting predetermined dose constraints for rectum, bladder neck, femoral heads and sparing the prostatic urethra. Plan quality was assessed by means of dose volume histogram (DVH) analysis. Early toxicity was scored according to the modified RTOG/SOMA scale at the end of treatment, and 6 weeks and 6 months following treatment. Results: DVH analyses summarized the quality of the delivered radiotherapy plans by fulfilling planning objectives for target coverage and organ at risk sparing. The mean PTV V70 was 99.1 ± 0.5%. The CTV V70 was 100.0% in all cases and the mean CTV V81.1Gy was 93.8 ± 1.4%. All planning objectives were met for rectum, urethra, bladder and femur. Concerning acute toxicity, from study entry to 6 months follow-up no patient had experienced Grade 3 gastrointestinal toxicity, while one patient had experienced Grade 2 and one patient Grade 1; one patient experienced Grade 3 genitourinary toxicity, while one experienced Grade 2 and seven experienced Grade 1. The median PSA was reduced from 10.6 ± 3.9 mg/L prior to radiation therapy to 0.92 ± 2.0 mg/L at 6 months follow-up. Conclusions: We confirm our planning study and demonstrate that RapidArc is able to deliver hypofractionated IGRT with a SIB to 81.1 Gy to more than 90% of the CTV, excluding the prostatic urethra. In brief follow up of the first 10 patients so treated, no safety concerns have arisen, and further dose escalation of the SIB is planned. Author Disclosure: M.J. Follwell: None. W.J. Morris: None. M. Vitali: None. R. Shaffer: None. K. James: None. T. Pickles: None. K. Otto: None.
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Reduction of Daily Imaging Dose Associated with Prostate Planar kV IGRT
A. M. Block, J. Luce, J. Y. Lin, R. Garza, J. C. Roeske Loyola University Medical Center, Maywood, IL Purpose/Objective(s): IGRT is the means by which the high degree of accuracy and precision required for modern radiotherapy techniques is achieved. Several recent articles in the popular press have patients concerned that too much radiation exposure, such as from daily kilo-voltage (kV) images or therapeutic doses, puts them at risk for serious adverse effects. In our clinic, daily kV images are used for patient setup verification and repositioning. The aim of this study is to determine if the kV IGRT dose may be reduced by manipulating image parameters while producing high quality images. Materials/Methods: In our clinic, image parameters (kVp, mA, and time) are based on predefined standardized protocols. To estimate the dose delivered near the surface for a kV IGRT protocol, an ion chamber was placed 0.5 cm beneath the surface of a phantom made from solid water material. Using these standard parameters, anterior and lateral kV images were taken, and the measurements from the ion chamber were converted to absorbed dose. Image parameters were then chosen to reduce the image dose by decreasing the mAs and varying the kVp. Ion chamber measurements were subsequently obtained and compared with the nominal values. Using the various imaging parameters, anterior and lateral kV images were acquired on a pelvic RANDO phantom. To quantitatively compare the pelvic images taken with different parameters, a program was written to calculate a metric known as the contrast-to-noise ratio (CNR) for each image. CNR represents the difference between a high contrast object (such as bone or fiducial markers) and the background, divided by the standard deviation (noise) of each of these regions of interest. Thus, CNR provides a quantitative evaluation of image quality. CNR values were then compared based on the imaging parameters, as well as the absorbed doses that were required to obtain each image. Results: The absorbed dose per anterior image using standardized parameters was 0.18 cGy. The associated CNR was 2.24. By decreasing the mAs, the anterior image dose could be reduced by 39% (0.11 cGy/image) with only a 2% reduction in the CNR (2.19). For the lateral images, the absorbed dose was 1.51 cGy/image. The associated CNR was 4.24. Reducing the mAs by 49% decreased the dose near the surface to 0.77 cGy/image, while maintaining a CNR that decreased by only 1% (4.21). Conclusions: By altering the imaging parameters, the absorbed dose from anterior and lateral kV imaging may be greatly reduced without sacrificing image quality. Future studies will involve applying these altered parameters clinically. Author Disclosure: A.M. Block: None. J. Luce: None. J.Y. Lin: None. R. Garza: None. J.C. Roeske: None.
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Lost In Translation: Interference Between Electromagnetic Transponders and In Vivo Prostate MOSFET Dosimeters
G. Gejerman1, M. Winburn2, E. Mullokandov2, V. Lanteri1, M. Goldstein1, S. Rome1, T. Chun2, S. Levey1, A. Siegel1, M. Esposito1 1
Hackensack University Medical Center, Hackensack, NJ, 2New Jersey Urology, Saddle Brook, NJ
Purpose/Objective(s): To investigate the potential interference between Calypso electromagnetic transponders and Sicel MOSFET in vivo dosimeters (DVS) when used together for prostate IGRT.
S439
I. J. Radiation Oncology d Biology d Physics
S440
Volume 81, Number 2, Supplement, 2011
Materials/Methods: Ten patients with prostate cancer were implanted with 3 Calypso beacons and 2 DVS dosimeters. Over a 9 week IGRT course, their daily DVS radiation dose was measured and the percent difference between measured and planned fractional dose was calculated for each dosimeter. The daily Calypso isocenter offset, intertransponder distance errors, and geometric/ rotational errors were measured. To test for interference, the DVS and Calypso measurements were compared to those of 10 patients implanted with DVS and Visicoil fiducial markers and 10 patients implanted with Calypso only, treated during the same period. Results: The DVS measurements differed from their expected dose more frequently in those patients who also had Calypso beacons: Variability in expected dose .5% occurred in 30% of measurements, .7% in 14%, and .10% in 4%. DVS measurements were less variable in those who had Visicoil fiducial markers: Variability in expected dose .5% occurred in 12% of measurements, .7% in 5%, and .10% in 1%. The difference in DVS dose variability with and without Calypso beacons was statistically significant: Variability in expected dose .5% = 0.004, .7% = 0.021, and .10% = 0.021. No difference in Calypso isocenter offset or geometric/ rotational errors was found with or without DVS dosimeters. Conclusions: Our analysis demonstrates that while Calypso transponder localization is not affected by the simultaneous use of DVS in vivo dosimeters, statistically significant differences in DVS dose measurement were encountered. DVS dosimeter measurements should be interpreted with caution when simultaneous electromagnetic transponders are used. Author Disclosure: G. Gejerman: None. M. Winburn: None. E. Mullokandov: None. V. Lanteri: None. M. Goldstein: None. S. Rome: None. T. Chun: None. S. Levey: None. A. Siegel: None. M. Esposito: None.
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Inclusion of Clinical Risk Factors into NTCP Modeling of Late Rectal Toxicity after High Dose Radiotherapy for Prostate Cancer
T. Rancati1, C. Fiorino2, V. Vavassori3, G. Fellin4, F. Mauro5, E. Cagna6, G. Girelli7, R. Valdagni1 Fondazione IRCCS - Istituto Nazionale dei Tumori, Milan, Italy, 2Istituto Scientifico San Raffaele, Milan, Italy, 3HumanitasGavazzeni, Bergamo, Italy, 4Ospedale Santa Chiara, Trento, Italy, 5Villa Maria Cecilia, Lugo di Romagna, Italy, 6Ospedale Sant’Anna, Como, Italy, 7Ospedale ASL 9, Ivrea, Italy 1
Purpose/Objective(s): To fit a normal tissue complication probability (NTCP) model including clinical risk factors to late rectal toxicities (tox) after radiotherapy (RT) for prostate cancer. Materials/Methods: Rectal dose-volume histograms (DVHs) and clinical data of patients (pts) enrolled in the AIROPROS 0102 prospective multicenter trial were considered. All pts were treated in supine position mostly with 3 or 4-field techniques: ICRU doses were between 64 and 81.6 Gy (1.8-2 Gy/fr). Minimum follow-up was 36 mos. Pts were considered as bleeders if showing G2-G3 late rectal bleeding (lrb) within 36 mos after the end of RT. G2 lrb was scored if bleeding occurred .2 times/week ; G3 if pts reported daily bleeding or if pts received any blood transfusions and/or laser coagulations. G1 late fecal incontinence (linc) was scored if unintentional stool discharge was ‘‘sometimes’’ experienced, G2 linc was scored if unintentional stool discharge was ‘‘often’’ experienced or if pts sporadically used sanitary pads; G3 if pts reported daily unintentional stool discharge or use of sanitary pad .2 times/week. 4 endpoints were considered: G2-G3 lrb, G3 lrb, chronic linc (C_INC: persistence of . = G1 after any G2-G3 event) and mean linc (M_INC: average score during 3-year after RT, M_INC = .1 was chosen as the relevant endpoint). 669 and 506 pts were available for lrb for linc analysis respectively. The Logit model with DVH reduced to the equivalent uniform dose (EUD) was considered for NTCP fit. The model was extended by fitting a double D50 (a D50 for pts with the clinical risk factor and a D50 for pts without the clinical predisposing feature, Peeters 06). The ratio of the two D50s is the dose-modifying factor (Dmd) for the selected clinical risk factor. Maximum likelihood analysis (MINUIT software) was used for the fitting procedure. Results: Previous analysis (Fellin 09) underlined that abdominal surgery (surg) was a risk factor for lrb and this was reflected in a D50 = 82.7Gy for pts with surg and D50 = 88.4Gy for pts without surg (Dmd = 1.07) for G2-G3 lrb (51/669 tox) and in Dmd = 1.11 for G3 lrb (33/669 tox, D50 = 91.7Gy for pts without surg). When fitting linc, two risk factors were considered separately: surg and previous diseases of the colon (colon). When C_INC (16/506 tox) was considered Dmd(surg) = 2 and Dmd(colon) = 2.35, while when M_INC was taken into account (22/506 tox) Dmd(surg) = 1.38 and Dmd(colon) = 1.55. Conclusions: Inclusion of predisposing clinical features helps to improve NTCP estimation, further work is needed in order to reduce parameter uncertainties, especially when rare tox are considered (e.g. linc). More severe dose constraints should therefore be used during RT optimization when dealing with pts with a history of abdominal surgery and/or with previous diseases of the colon. Author Disclosure: T. Rancati: None. C. Fiorino: None. V. Vavassori: None. G. Fellin: None. F. Mauro: None. E. Cagna: None. G. Girelli: None. R. Valdagni: None.
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Comparative Study Of Permanent Prostate Brachytherapy In Patients With And Without Median Lobe Hyperplasia: Evaluation Of Dosimetric Factors, Acute Urinary Toxicity And Biochemical Outcomes
P. P. Amin, S. Vyas, M. Naslund University of Maryland Medical Center, Baltimore, MD Purpose/Objective(s): Median Lobe Hyperplasia (MLH) is a known relative contraindication for permanent prostate brachytherapy (PPB). We wanted to evaluate the effects of MLH with regards to acute toxicity and outcome and compared them to patients without MLH, treated during the same time-period. Materials/Methods: In our center, we had 26 patients who had a prominent median lobe out of 89 patients who received PPB between Jan’ 2007 to Dec’ 2009. The patients having MLH were carefully implanted by judicious placement of seeds, limiting the dose to the bladder neck area. We retrospectively analyzed these patients along with randomly selected 36 patients who did not have MLH and underwent PPB during the same period. Their dosimetric parameters, acute urinary toxicity and outcomes were comparatively evaluated.
