V Subramani

Normal tissue complication probability of fibrosis in radiotherapy of breast cancer: accelerated partial breast irradiation vs conventional external-beam radiotherapy.

AIMS Radiotherapy forms an integral part of breast-conserving treatment in early-stage breast cancer. Subcutaneous fibrosis of the treated breast is an important late effect in whole-breast irradiation. The aim of this study was to compare the normal tissue complication probability (NTCP) for radiation-induced fibrosis in treated breast using accelerated partial-breast irradiation (APBI) vs conventional treatment. MATERIALS AND METHODS Ten postoperative early-stage breast cancer patients (T1N0M0) were included in this dosimetric analysis. APBI treatment was planned using conformal radiotherapy technique and conventional treatment plans included two tangential portals. All the APBI treatment plans were made with five non-coplanar beams with 6 MV photons. The prescription dose was 38 Gy in 10 fractions for the APBI treatments and 50 Gy in 25 fractions, followed by a boost dose of 16 Gy in 8 fractions, for the conventional treatments. We used Lymans relative-seriality model and the breast fibrosis NTCP model fitting parameters for the study. RESULTS The equivalent uniform dose (EUD) was 30.09 Gy and 50.79 Gy in APBI and conventional treatment, respectively. The mean NTCP values for ipsilateral breast fibrosis in APBI and conventional treatment were 0.51 and 25.66%, respectively. Using the paired t-test, a statistically significant difference was seen in the breast fibrosis NTCP values for APBI vs conventional treatment (P < 0.001). CONCLUSIONS APBI reduces the ipsilateral breast fibrosis compared to conventional whole-breast treatment in early-stage breast cancer.

Adenoid Cystic Carcinoma of the Trachea Treated with PET-CT Based Intensity Modulated Radiotherapy

Primary tumors of the trachea are rare and are usually malignant in adults and benign in children. Adenoid cystic carcinoma, which is of salivary gland origin, account for about one thirds of adult primary tracheal tumors. A 49-year-old gentleman presented to us after undergoing a pneumonectomy elsewhere. Computed tomography scan of the thorax at our hospital showed a residual disease in the primary site, size of which was same as that in the preoperative scan. Because there was a compromised respiratory reserve due to pneumonectomy we decided to keep the radiation dose to the remaining lung as low as possible. We treated him by positron emission tomography-computed tomography (PET-CT) directed intensity modulated radiation therapy to a dose of 60 Gy in 30 Fractions over 6 weeks on a linear accelerator. PET helped in exact localization of the target on the planning CT. He tolerated the treatment very well. PET-CT done 1 year posttreatment showed no residual disease. Presently he is disease free with good pulmonary reserve.

Static versus dynamic intensity-modulated radiotherapy: Profile of integral dose in carcinoma of the nasopharynx

This study is aimed to evaluate the impact of static and dynamic intensity modulated radiotherapy (IMRT) delivery techniques planned with Eclipse TPS on the integral dose to the healthy normal tissue surrounding the tumor-bearing area and to the volume receiving doses < 5 Gy in patients with carcinoma nasopharynx treated with Simultaneous Integrated Boost IMRT (SIB-IMRT). Ten patients with carcinoma nasopharynx were chosen for this dosimetric study. IMRT plans were generated with 6X using dynamic multileaf collimator (DMLC) and static multileaf collimator (SMLC) with 5, 10 and 15 intensity levels (L). Integral dose, volume receiving 5 Gy, number of monitor units (MU) is compared against DMLC. The mean difference in the MU delivered per fraction between 5, 10 and 15 L SMLC and DMLC was −13.25% (P < 0.001, with paired t test), −11.82% (P < 0.001) and −10.81% (P < 0.001), respectively. The mean difference in the integral dose with 5, 10 and 15 L compared to DMLC was −2.96% (P < 0.001), −2.67% (P = 0.016) and −0.39% (P = 0.430), respectively. However, the difference in low-dose volume (V5Gy) was statistically insignificant with mean difference of 0.60% (P = 0.23), 1.18% (P = 0.017) and 1.70% (P = 0.078), respectively for 5, 10 and 15 L compared to DMLC. Our results show that while choosing the IMRT delivery technique using conventional MLC the concerns about integral dose and volume receiving very low doses such as 5 Gy can be ignored.

MAGAT gel dosimetry for its application in small field treatment techniques

Purpose of this work is to present the role of in-house manufactured MAGAT gel for treatment verification in small field dosimetric techniques such as Gammaknife (GK) and intensity-modulated radiation therapy (IMRT). Magnetic resonance imaging (MRI) is one of the most extensively used imaging technique for polymer gel dosimetry hence we used this method for gel evaluation. Different MR scanners and MRI sequences were used in this study for obtaining calibration plot between R2 and absorbed dose. An experimental plan was created for Gammaknife and IMRT. The prepared gel was filled in spherical glass phantom and in-house designed human head shape phantom for verification purpose. We used 8 TE values for all the imaging sequences for two reasons. Firstly it is sufficient enough to give good signal to noise ratio. Second considering the enormous scanning time involved in multiple spin echo sequence. MATLAB based in-house programs were used for R2 estimation and dose comparison. The isodose comparison with MAGAT gel showed reasonable agreement for both Gammaknife and IMRT techniques.

Normal tissue complication probability: Does simultaneous integrated boost intensity-modulated radiotherapy score over other techniques in treatment of prostate adenocarcinoma

AIM The main objective of this study was to analyze the radiobiological effect of different treatment strategies on high-risk prostate adenocarcinoma. MATERIALS AND METHODS Ten cases of high-risk prostate adenocarcinoma were selected for this dosimetric study. Four different treatment strategies used for treating prostate cancer were compared. Conventional four-field box technique covering prostate and nodal volumes followed by three-field conformal boost (3D + 3DCRT), four-field box technique followed by intensity-modulated radiotherapy (IMRT) boost (3D + IMRT), IMRT followed by IMRT boost (IMRT + IMRT), and simultaneous integrated boost IMRT (SIBIMRT) were compared in terms of tumor control probability (TCP) and normal tissue complication probability (NTCP). The dose prescription except for SIBIMRT was 45 Gy in 25 fractions for the prostate and nodal volumes in the initial phase and 27 Gy in 15 fractions for the prostate in the boost phase. For SIBIMRT, equivalent doses were calculated using biologically equivalent dose assuming the alpha/beta ratio of 1.5 Gy with a dose prescription of 60.75 Gy for the gross tumor volume (GTV) and 45 Gy for the clinical target volume in 25 fractions. IMRT plans were made with 15-MV equispaced seven coplanar fields. NTCP was calculated using the Lyman-Kutcher-Burman (LKB) model. RESULTS An NTCP of 10.7 +/- 0.99%, 8.36 +/- 0.66%, 6.72 +/- 0.85%, and 1.45 +/- 0.11% for the bladder and 14.9 +/- 0.99%, 14.04 +/- 0.66%, 11.38 +/- 0.85%, 5.12 +/- 0.11% for the rectum was seen with 3D + 3DCRT, 3D + IMRT, IMRT + IMRT, and SIBIMRT respectively. CONCLUSIONS SIBIMRT had the least NTCP over all other strategies with a reduced treatment time (3 weeks less). It should be the technique of choice for dose escalation in prostate carcinoma.

SU-F-P-46: Comparative Study Between Two Normalization Prescriptions for Accelerated Partial Breast Irradiation: A Dosimetric Study

PURPOSE To compare the Accelerated Partial Breast Irradiation (APBI) plan with the normalized basal dose points and 5mm box prescription. METHODS Five patients of APBI were planned twice in Oncentra Master planning TPS (Version 4.3) using TG-43 algorithm. The number of catheters for all the patients was 10 to 16 and implant plane 2 to 3. For planning all catheters were reconstructed. Source loading was done as per HR-CTV contoured. The HR-CTV volume range was from 75cc to 182cc. Plans were normalized in two methods. First all plans were normalized on Basal dose points (PlanA) and second all the plan were normalized on 5mm box (PlanB). The prescription dose (PD) was 35Gy in 10 fractions. All the plans were completely based on normalization and without optimization. Plan evaluation was based on certain parameters coverage Index (CI), dose homogeneity index (DHI), conformity index (COIN), over dose volume index (OI). RESULTS The average and median of CI for planA was 0.835 and 0.8154, for planB 0.82 and 0.799 respectively. The median and average of DHI for planA was 0.66 and 0.6062, for planB 0.67 and 0.62 respectively. The range of COIN for planA and planB was from 0.58 to 0.65 respectively. The range of OI was from 0.083 to 0.169 for planA and planB. The treatment time in planA was in average 1.13 times more than planB as V150% of HR_CTV in planA was 4-6% more. The ipsilateral lung was getting 30% of PD which was 0.6% to 3.5%. CONCLUSION Treatment Planning should be individualized based on implants characteristics. Planning with prescription to basal dose points should be preferred to 5mm box prescription, in order to achieve better DHI and less treatment time.

SU‐E‐T‐636: Impact of Different Intensity Levels On Step and Shoot IMRT Plan Quality and Deliverability

PURPOSE To evaluate the impact of different intensity-levels on step and shoot IMRT plan quality and deliverability. METHODS Five previously treated patients of carcinoma rectum (pre-operative) were studied. Planning target volume (PTV) and organ at risk (OAR) i.e. bladder and bowel were contoured. Step and shoot IMRT plans (6MV, 45Gy/25fraction prescribed at 95% isodose) were created in Eclipse TPS for Varian CL2300C/D linear accelerator at 300MU/min. During optimization, dose volume constraints and priorities were kept constant and only intensity-levels were varied as follows: 5, 10 and 15. Plan quality was analyzed in terms of maximum and mean doses of PTV, coverage index (CI=PTV covered by prescription dose/PTV), heterogeneity index (HI)=D5 /D9 5 (D5 =dose received by 5% of PTV and D9 5 =dose received by 95% of PTV), OAR mean-doses and normal tissue integral-dose (NTID) (liter-Gray). Total monitor units (MUs) required to deliver a plan was noted. Deliverability of treatment plans were verified with I-matriXX array and compared with TPS dose-plane using gamma index of 3% dose difference and 3mm distance to agreement (DTA) criteria. RESULTS Maximum dose to PTV was 5219.06±54.55cGy, 5071.30±49.01cGy and 5053.26±71.85cGy for 5, 10 and 15 intensity-levels, respectively. Mean dose to the PTV was 4655.02±27.29cGy, 4664.00±23.29cGy and 4659.98±22.01cGy. The CI of PTV was 0.8573±0.07, 0.9661±0.02 and 0.9686±0.01 and HI of PTV was 1.0800±0.02, 1.0701±0.01, and 1.0677±0.01 for 5, 10 and 15 intensity-levels. Mean dose to bladder was 3491.88±365.15cGy, 3513.98±387.04cGy and 3501.93±380.91cGy. Bowel mean dose was 1352.05±365.20cGy, 1369.65±378.76cGy and 1369.53±375.30cGy. NTID (liter-Gray) was 144.64±11.88, 145.77±12.85 and 145.60±12.73. Total MU required to deliver a plan was 1060±307, 1081±309 and 1061±303. Gamma pass rate was 99.65±0.35%, 99.81±0.16% and 99.87±0.12%. CONCLUSION PTV coverage and homogeneity increases with increase in intensity-level. Plan quality was better at intensity-level 5 in terms of OAR mean-doses, NTID and total MU. Gamma pass rates were improved with increase of intensity-level.

Digital Filtering Techniques to Reduce Image Noise and Improve Dose Resolution in X-Ray CT Based Normoxic Gel Dosimetry

Gel dosimetry is a promising technique for the implementation of 3D dose verification within the radiation therapy clinic, since it is the only methodology which provides comprehensive 3D dose measurement of conformal treatments such as Intensity Modulated Radiation Therapy (IMRT), Stereotactic radiosurgery (SRS) and Stereotactic Radiotherapy (SRT), and volumetric arc therapies (VMAT). In polymer gel dosimetry, monomer molecules polymerize and are fixed in the gel matrix when exposed to radiation. The spatial dose information thus can be obtained by gel imaging. Gel imaging methods which are currently under investigation are MRI, optical computed tomography and x-ray computed tomography. While promising, x-ray CT images of irradiated polymer gel exhibit low contrast due to small change in density that occurs during polymerization. The response of CT contrast to dose is weak and noise reduction is critical in order to achieve adequate dose resolution in gel dosimetry using x-ray computed tomography. Ideally, image noise is minimized using high tube current, long scan times and high number of image averages [Hilts et al 2005]. Digital image filtering is an effective method in reducing image noise while maintaining accurate spatial dose information. Generally it is performed in either frequency domain or spatial domain. In a previous work, several spatial filters were applied to Stereotactic Radiosurgery (SRS) irradiated polymer gel image [Hilts and Duzenli 2004]. In their study it was found that the two highest performing filters were the adaptive mean (Wiener) and smallest univalue segment assimilating nucleus (SUSAN). In another study a new method of signal to noise ratio (SNR) enhancement by 2D two-point maximum entropy regularization method (TPMEM) was investigated [Jirasek 2006]. However, a comprehensive examination of the best performing filters from these two studies has not been undertaken. Here, we test the Weiner and TPMEM digital filters on X-ray CT image of the irradiated PAGAT gel in order to reduce noise and improve dose resolution. This work builds off of previous studies and evaluates and compares the highest-performing filters from past individual works [Hilts and Duzenli 2004, Jirasek et al 2006].

SU‐E‐T‐587: Whole IMRT, Hybrid IMRT and 3D Conformal Plan a Dosimetric Comparison for Large Target

PURPOSE To dosimetrically compare the whole-IMRT, hybrid-IMRT (combination of IMRT and 3D-CRT) and 3D-conformal radiotherapy (3D- CRT) plans for larger targets. METHODS Five previously treated patients of carcinoma cervix with para-aortic lymph-nodes (target length 33-34cm) were selected. PTV-P (PTV-Primary), PTV-PA (PTV-para-aortic) and organ at risks (OARs) were defined. Three plans were generated using Eclipse TPS for Varian CL2300C/D linear accelerator using 6MV photon beam. Three plans were: (i) Whole-IMRT: IMRT for both PTV-P and PTV-PA (ii) Hybrid-IMRT: IMRT for PTV-P and 3D-CRT for PTV-PA (iii) 3D-CRT: 3D-CRT for both PTV-P and PTV-PA. Prescription dose for PTV-P is 50.4Gy and PTV-PA is 45Gy in 28 fractions. Coverage index (CI=Target volume covered by prescription dose/Target volume), mean doses to bladder, rectum and bowel were used for plan comparison by using DVH. Integral dose (liter-Gray) to normal tissue (i.e., patient volume minus PTV-P and PTV-PA) and total monitor units (MUs) required to deliver a plan was also noted. RESULTS The CI for PTV-P is 0.98±0.20, 0.96±0.09, and 0.95±0.01 for Whole-IMRT, Hybrid-IMRT and 3D-CRT plan and for PTV- PA is 0.98±0.01, 0.98±0.01, and 0.97±0.20. Maximum doses to PTV-P are 5660.85±90.85cGy, 5640.35±70.35cGy and 5813.80±97.40cGy. Maximum doses to PTV-PA are 5000.60±109.10cGy, 5079.85±20.25cGy and 5092.25±19.75cGy. Mean doses to the bladder are 3810±225.80cGy, 3842.10±182.70cGy and 5204±98.25cGy for Whole-IMRT, Hybrid-IMRT and 3D-CRT plan, respectively. Mean doses to rectum are 3955.35±324.95cGy, 3971.15±354.15cGy and 4741.20±371.60cGy. Mean doses to bowel are 2623.35±320.85cGy, 2855.30±371.05cGy and 3011.7±433.80cGy. Average MUs required to deliver one fraction is 1285±87, 1585±186, 485±46 for Whole-IMRT, Hybrid-IMRT and 3D-CRT plans, respectively. Higher integral doses to normal tissue were observed for whole-IMRT (267.60±76 liter-Gy) followed by hybrid-IMRT (259.20±53 liter-Gy) and 3D-CRT (186.30±33 liter-Gy). CONCLUSIONS Whole-IMRT is useful for larger targets compared to hybrid-IMRT in terms of dose conformity, lesser MUs and reduced critical organ doses with little compromise on integral dose, where 3D-CRT sacrificed the OAR sparing.

SU‐E‐T‐268: Evaluation of Photoneutron Contamination in Elekta Synergy‐S High‐Energy Linear Accelerator and Indigenous Novel Solution: The AIIMS Experience

PURPOSE The photoneutron contamination problem was encountered due to laminated barrier wall and short maze. The purpose of this study was to report our experience in evaluating the photoneutron contamination during radiation safety survey and solution. METHODS The photoneutron contamination measurement was carried out in Elekta Synergy-S high-energylinear accelerator for 15MV beam. A NE Neutron survey meter and for photon, Victoreen and RADOS survey meters were used. The laminated barrier wall composed of 37cm steel with 30cm concrete both side and short maze length of 5 meter. During safety survey, higher photoneutron levels for 15MV X-rays at treatment room door found. The effect of photoneutron contamination as function of neutron shielding materials of wood, polyethylene and boron and thickness, distance, locations and directions to the control console at distance upto 7 meter were investigated for 4 gantry angles at locations of treatment room entry doors namely door1(A), door2(B), console(C), conduit (D) and above-ceiling(G) for 15MV. RESULTS The initial safety survey showed that neutron level of 47mR/h and photon leakage of 3.2mR/hr at the treatment entry room door1. The neutron values could bring down to the level of acceptance at the treatment entry door2, but the photon values are not acceptable. Therefore, 30cm concrete wall block was made at the location of door2 and another bend was taken. Finally, treatment entrance room door was made using 3cm polyethylene neutron shielding materials in order to achieve the both neutron contamination and photon leakage within the acceptable levels. CONCLUSIONS The neutron sliding-door is operated manually in finger-push by technologist for day-to-day usage. This simple solution is cost effective and increases the patient throughput. This study underlines that one needs to take appropriate safety measures prior to facility design whenever the space constraints situations arises for high energy linear accelerator.