Which is the most optimal technique to spare hippocampus?-Dosimetric comparisons of SCRT, IMRT, and tomotherapy.

AIMS: To evaluate current focal high precision radiotherapy (RT) techniques to spare hippocampi most optimally, in view of mounting clinical evidence to preserve neurocognition. MATERIALS AND METHODS: Computed tomography/magnetic resonance imaging (CT/MRI) datasets of 10 patients with benign/low-grade brain tumors, treated with focal conformal RT were replanned with helical tomotherapy (Tomo), intensity-modulated radiotherapy (IMRT) with high definition multileaf collimator (HD-MLC), and forward planning stereotactic conformal radiotherapy (SCRT). The primary planning objective was to encompass 99% of planning target volume (PTV) by 95% of prescribed dose (54 Gy/30#). Assessments included target coverage (TC), homogeneity index (HI), and maximum (max) and minimum (min) dose. Hippocampal dose was assessed with mean, maximum, minimum, median dosem and various dose levels. RESULTS: Mean V 95 for PTV coverage in Tomo, IMRT, and SCRT were 99.7, 99.4, and 98.3%, respectively. PTV coverage was significantly better in Tomo and IMRT compared to SCRT (P = 0.03). Tomotherapy (HI ≤ 0.06) and IMRT (HI ≤ 0.06) plans were more homogenous than SCRT (HI > 0.7) (P = 0.00). Right hippocampus mean dose with Tomo (20Gy) was 18.5% less than SCRT (30 Gy); but for left hippocampus, difference decreased to 3.3% (Tomo-32.2Gy and SCRT-34Gy). At 30% dose level, 9% more volume of right hippocampus was treated in IMRT and 20% in SCRT when compared to Tomo plan. At 80% dose, 6 and 12% more volumes were treated with IMRT and SCRT, respectively, in comparison to Tomo plan. For left hippocampus all three techniques were comparable. CONCLUSION: Tomotherapy and Linear accelerator (LINAC)-based IMRT achieved significantly better PTV coverage than forward planned SCRT. Tomo as compared to SCRT and IMRT plans showed trend towards significant sparing of the contralateral hippocampus, in eccentrically located tumors.

Electron arc therapy for bilateral chest wall irradiation: treatment planning and dosimetric study.

AIMS: The treatment of patients with synchronous bilateral breast cancer is a challenge. We present a report of dosimetric data of patients with bilateral chest walls as the target treated with electron arc therapy. MATERIALS AND METHODS: Ten consecutive patients who had undergone electron arc therapy to the bilateral chest wall for breast cancer were analysed. After positioning and immobilisation, patients underwent computed tomography scans from the neck to the upper abdomen. Electron arc plans were generated using the PLATO RTS (V1.8.2 Nucletron) treatment planning system. Electron energy was chosen depending upon the depth and thickness of the planning target volume (PTV). For all patients, the arc angle ranged between 80 and 280° (start angle 80°, stop angle 280°). The homogeneity index, coverage index and doses to organs at risk were evaluated. The patient-specific output factor and thermoluminescence dosimetry (TLD) measurements were carried out for all patients. The total planned dose to the PTV was 50Gy/25 fractions/5 weeks. RESULTS: The mean PTV (± standard deviation) was 568.9 (±116)cm(3). The mean PTV coverage was 89 (±5.8)% of the prescribed dose. For the right lung, the mean values of D(1) and D(10) were 46 (±7.6) and 30 (±9)Gy, respectively. For the left lung, the mean values of D(1) and D(10) were 45 (±7) and 27 (±8)Gy, respectively. For the heart, the mean values of D(1), D(5) and D(10) were 21 (±15), 13.5 (±12) and 9 (±9)Gy, respectively. The mean values of TLD at various pre-specified locations on the chest wall surface were 1.84, 1.82, 1.82, 1.89 and 1.78Gy, respectively CONCLUSION: The electron arc technique for treating the bilateral chest wall is a feasible and pragmatic technique. This technique has the twin advantages of adequate coverage of the target volume and sparing of adjacent normal structures. However, compared with other techniques, it needs a firm quality assurance protocol for dosimetry and treatment delivery.

Commissioning and comprehensive quality assurance of commercial 3D treatment planning system using IAEA Technical Report Series-430.

The purpose of this work is to report the results of commissioning and to establish a quality assurance (QA) program for commercial 3D treatment planning system (TPS) based on IAEA Technical Report Series 430. Eclipse v 7.3.10, (Varian Medical Systems, Palo Alto, CA, U.S.A.) TPS was commissioned for a Clinac 6EX (Varian Medical Systems, Palo Alto, CA, USA) linear accelerator. CT images of a phantom with various known in-homogeneities were acquired. The images were transferred to TPS and tested for various parameters related to patient data acquisition, anatomical modeling, plan evaluation and dose calculation. Dosimetric parameters including open, asymmetric and wedged shaped fields, oblique incidence, buildup region behavior and SSD dependence were evaluated. Representative clinical cases were tested for MU calculation and point doses. The maximum variation between the measured and the known CT numbers was 20 +/- 11.7 HU (1 SD). The results of all non-dosimetric tests were found within tolerance, however expansion at the sharp corners was found distorted. The accuracy of the DVH calculations depends on the grid size. TPS calculations of all the dosimetric parameters were in good agreement with the measured values, however for asymmetric open and wedged fields, few points were found out of tolerance. Smaller grid size calculation showed better agreement of dose calculation in the build-up region. Independent tests for MU calculation showed a variation within +/-2% (relative to planning system), meanwhile variation of 3.0% was observed when the central axis was blocked. The test results were in agreement with the tolerance specified by IAEA TRS 430. A subset of the commissioning tests has been identified as a baseline data for an ongoing QA program.

A treatment planning study comparing volumetric arc modulation with RapidArc and fixed field IMRT for cervix uteri radiotherapy.

PURPOSE: A treatment planning study was performed to evaluate the performance of the novel volumetric modulated single arc radiotherapy on cervix uteri cancer patients. Conventional fixed field IMRT was used as benchmark. METHODS AND MATERIALS: CT datasets of eight patients were included in the study. Plans were optimised with the aim to assess organs at risk and healthy tissue sparing while enforcing highly conformal target coverage. Planning objectives for PTV were: maximum significant dose lower than 52.5 Gy and minimum significant dose higher than 47.5 Gy. For organs at risk, the median and maximum doses were constrained to be lower than 30 (rectum), 35 (bladder) and 25 Gy (small bowel) and 47.5 Gy; additional objectives were set on various volume thresholds. Plans were evaluated on parameters derived from dose volume histograms and on NTCP estimates. Peripheral doses at 5, 10 and 15 cm from the PTV surface were recorded to assess the low-level dose bath. The MU and delivery time were scored to measure expected treatment efficiency. RESULTS: Both RapidArc and IMRT resulted in equivalent target coverage but RapidArc had an improved homogeneity (D(5%)-D(95%) = 3.5 +/- 0.6 Gy for RapidArc and 4.3 +/- 0.8 Gy for IMRT) and conformity index (CI(90%) = 1.30 +/- 0.06 for RapidArc and 1.41 +/- 0.15 for IMRT). On rectum the mean dose was reduced by about 6 Gy (10 Gy for the rectum fraction not included in the PTV). Similar trends were observed for the various dose levels with reductions ranging from approximately 3 to 14.4 Gy. For the bladder, RapidArc allowed a reduction of mean dose ranging from approximately 4 to 6Gy and a reduction from approximately 3 to 9 Gy w.r.t. IMRT. Similar trends but with smaller absolute differences were observed for the small bowel and left and right femur. NTCP calculations on bladder and rectum confirmed the DVH data with a potential relative reduction ranging from 30 to 70% from IMRT to RapidArc. The healthy tissue was significantly less irradiated in the medium to high dose regions (from 20 to 30 Gy) and the integral dose reduction with RapidArc was about 12% compared to IMRT. Concerning peripheral dose, the relative difference between IMRT and RapidArc was of 9 +/- 2%, 43 +/- 11% and 36 +/- 5% at 5, 10 and 15 cm from the PTV surface, respectively. The MU/Gy from RapidArc was 245 +/- 17 corresponding to an expected average beam on time of 73 +/- 10 s per fractions of 2 Gy. IMRT plans presented higher values with an average of MU/Gy = 479 +/- 63. CONCLUSION: RapidArc was investigated for cervix uteri cancer showing significant improvements in organs at risk and healthy tissue sparing with uncompromised target coverage leading to better conformal avoidance of treatments w.r.t. conventional IMRT. This, in combination with the confirmed short delivery time, can lead to clinically significant advances in the management of this highly aggressive cancer type. Clinical protocols are now advised to evaluate prospectively the potential benefit observed at the planning level.

Performance characterization of siemens primus linear accelerator under small monitor unit and small segments for the implementation of step-and-shoot intensity-modulated radiotherapy.

Implementation of step-and-shoot intensity-modulated radiotherapy (IMRT) needs careful understanding of the accelerator start-up characteristic to ensure accurate and precise delivery of radiation dose to patient. The dosimetric characteristic of a Siemens Primus linear accelerator (LA) which delivers 6 and 18 MV x-rays at the dose rate of 300 and 500 monitor unit (MU) per minutes (min) respectively was studied under the condition of small MU ranging from 1 to 100. Dose monitor linearity was studied at different dose calibration parameter (D1_C0) by measuring ionization at 10 cm depth in a solid water phantom using a 0.6 cc ionization chamber. Monitor unit stability was studied from different intensity modulated (IM) groups comprising various combinations of MU per field and number of fields. Stability of beam flatness and symmetry was investigated under normal and IMRT mode for 20×20 cm(2) field under small MU using a 2D Profiler kept isocentrically at 5 cm depth. Inter segment response was investigated form 1 to 10 MU by measuring the dose per MU from various IM groups, each consisting of four segments with inter-segment separation of 2 cm.In the range 1-4 MU, the dose linearity error was more than 5% (max -32% at 1 MU) for 6 MV x-rays at factory calibrated D1_C0 value of 6000. The dose linearity error was reduced to -10.95% at 1 MU, within -3% for 2 and 3 MU and ±1% for MU ≥4 when the D1_C0 was subsequently tuned at 4500. For 18 MV x-rays, the dose linearity error at factory calibrated D1_C0 value of 4400 was within ±1% for MU ≥3 with maximum of -13.5 observed at 1 MU. For both the beam energies and MU/field ≥4, the stability of monitor unit tested for different IM groups was within ±1% of the dose from the normal treatment field. This variation increases to -2.6% for 6 MV and -2.7% for 18 MV x-rays for 2 MU/field. No significant variation was observed in the stability of beam profile measured from normal and IMRT mode. The beam flatness was within 3% for 6 MV x-rays and more than 3% (Max 3.5%) for 18 MV x-rays at lesser irradiation time ≤3 MU. The beam stability improves with the increase in irradiation time. Both the beam energies show very good symmetry (≤2%) at all irradiation time.For all the three segment sizes studied, the nonlinearity was observed at smaller MU/segment in both the energies. When the MU/segment is ≥4, all segment size shows fairly linear relation with dose/MU. The smaller segment size shows larger nonlinearity at smaller MU/segment and become more linear at larger MU/segment. Based on our study, we conclude that the Primus LA from Siemens installed at our hospital is ideally suited for step-and-shoot IMRT preferably for radiation ON time ≥4MU per segment.