An Audit for Radiotherapy Planning and Treatment Errors From a Low-Middle-Income Country Centre.

AIMS: To report the findings of an audit for radiotherapy errors from a low-middle-income country (LMICs) centre. This would serve as baseline data for radiotherapy error rates, their severity and causes, in such centres where modern error reporting and learning processes still do not exist. MATERIALS AND METHODS: A planned cross-sectional weekly audit of electronic radiotherapy charts at the radiotherapy planning and delivery step for all patients treated with curative intent was conducted. Detailed analysis was carried out to determine the step of origin of error, time and contributing factors. They were graded as per indigenous institutional (TMC) radiotherapy error grading (TREG) system and the contributing factors identified were prioritised using the product of frequency, severity and ease of detection. RESULTS: In total, 1005 consecutive radically treated patients' charts were audited, 67 radiotherapy errors affecting 60 patients, including 42 incidents and 25 near-misses were identified. Transcriptional errors (29%) were the most common type. Most errors occurred at the time of treatment planning (59.7%), with "plan information transfer to the radiation oncology information system" being the most frequently affected sub-step of the radiotherapy process (47.8%). More errors were noted at cobalt units (52/67; 77.6%) than at linear accelerators. Trend analysis showed an increased number of radiotherapy incidents on Fridays and near-misses on Mondays. Trend for increased radiotherapy errors noted in the evening over other shifts. On severity grading, most of the errors (54/60; 90%) were clinically insignificant (grade I/II). Inadequacies and non-adherence towards standard operating procedures, poor documentation and lack of continuing education were the three most prominent causes. CONCLUSION: Preliminary data suggest a vulnerability of LMIC set-up to radiotherapy errors and emphasises the need for the development of longitudinal prospective processes, such as voluntary reporting and a continued education system, to ensure robust and comprehensive safe practises on par with centres in developed countries.

Implementation of failure mode and effective analysis for high dose rate brachytherapy at Tata Memorial Hospital, Mumbai, India.

PURPOSE: To report our experience of failure mode and effective analysis for high dose rate brachytherapy of gynaecological cancer carried out in our hospital. MATERIALS AND METHODS: Failure mode and effective analysis process described in AAPM TG 100 was followed: a multidisciplinary team consisting of two physicians, physicists, dosimetrists, a medical resident, a nurse, and a secretary was formed. A weekly meeting was held for four months. A process tree was created based on the overview of the entire process, with the main branches as follows: procedure in the operating room, patient imaging, contouring, treatment planning, machine quality assurance and treatment delivery. Each team member assigned the risk probability numbers based on the predefined scoring system. For a particular failure mode, if the risk probability number assigned by one member differed from the other, the highest risk probability number was taken into consideration. RESULTS: The process tree consisted of 185 nodes, with risk probability numbers ranging from 1-220, with 77 possible failure modes. Four nodes were found with risk probability numbers greater than 200, which were considered for immediate process improvements. Twenty-four nodes were found to be with risk probability numbers ranging from 100 to 200. All 24 processes were considered for process improvement, out of which 12 were found effective and feasible, which includes failure nodes with high severity score at least 8. The processes with high-risk probability numbers (greater than 200) were reduced after the introduction of process improvements. For the other processes, standard procedures were modified. The common causes of failure, were found to be due to lack of attention, human error and work pressure. CONCLUSIONS: Failure mode and effective analysis is a useful tool that uses a systematic approach for quality management of a specific process.

Commissioning and validation of commercial deformable image registration software for adaptive contouring.

PURPOSE: To report the commissioning and validation of deformable image registration(DIR) software for adaptive contouring. METHODS: DIR (SmartAdapt®v13.6) was validated using two methods namely contour propagation accuracy and landmark tracking, using physical phantoms and clinical images of various disease sites. Five in-house made phantoms with various known deformations and a set of 10 virtual phantoms were used. Displacement in lateral, anterio-posterior (AP) and superior-inferior (SI) direction were evaluated for various organs and compared with the ground truth. Four clinical sites namely, brain (n = 5), HN (n = 9), cervix (n = 18) and prostate (n = 23) were used. Organs were manually delineated by a radiation oncologist, compared with the deformable image registration (DIR) generated contours. 3D slicer v4.5.0.1 was used to analyze Dice Similarity Co-efficient (DSC), shift in centre of mass (COM) and Hausdorff distances Hf95%/avg. RESULTS: Mean (SD) DSC, Hf95% (mm), Hfavg (mm) and COM of all the phantoms 1-5 were 0.84 (0.2) mm, 5.1 (7.4) mm, 1.6 (2.2) mm, and 1.6 (0.2) mm respectively. Phantom-5 had the largest deformation as compared to phantoms 1-4, and hence had suboptimal indices. The virtual phantom resulted in consistent results for all the ROIs investigated. Contours propagated for brain patients were better with a high DSC score (0.91 (0.04)) as compared to other sites (HN: 0.84, prostate: 0.81 and cervix 0.77). A similar trend was seen in other indices too. The accuracy of propagated contours is limited for complex deformations that include large volume and shape change of bladder and rectum respectively. Visual validation of the propagated contours is recommended for clinical implementation. CONCLUSION: The DIR algorithm was commissioned and validated for adaptive contouring.

Monte Carlo Investigation of Photon Beam Characteristics and its Variation with Incident Electron Beam Parameters for Indigenous Medical Linear Accelerator.

PURPOSE: A Monte Carlo model of a 6 MV medical linear accelerator (linac) unit built indigenously was developed using the BEAMnrc user code of the EGSnrc code system. The model was benchmarked against the measurements. Monte Carlo simulations were carried out for different incident electron beam parameters in the study. MATERIALS AND METHODS: Simulation of indigenously developed linac unit has been carried out using the Monte Carlo based BEAMnrc user-code of the EGSnrc code system. Using the model, percentage depth dose (PDD), and lateral dose profiles were studied using the DOSXYZnrc user code. To identify appropriate electron parameters, three different distributions of electron beam intensity were investigated. For each case, the kinetic energy of the incident electron was varied from 6 to 6.5 MeV (0.1 MeV increment). The calculated dose data were compared against the measurements using the PTW, Germany make RFA dosimetric system (water tank MP3-M and 0.125 cm3 ion chamber). RESULTS: The best fit of incident electron beam parameter was found for the combination of beam energy of 6.2 MeV and circular Gaussian distributed source in X and Y with FWHM of 1.0 mm. PDD and beam profiles (along both X and Y directions) were calculated for the field sizes from 5 cm × 5 cm to 25 cm × 25 cm. The dose difference between the calculated and measured PDD and profile values were under 1%, except for the penumbra region where the maximum deviation was found to be around 2%. CONCLUSIONS: A Monte Carlo model of indigenous linac (6 MV) has been developed and benchmarked against the measured data.

Acceptance criteria for flattening filter-free photon beam from standard medical electron linear accelerator: AERB task group recommendations.

Medical electron linear accelerators with the capability of generating unflat photon (flattening filter-free, FFF) beams are also available commercially for clinical applications in radiotherapy. However, the beam characteristics evaluation criteria and parameters are not yet available for such photon beams. Atomic Energy Regulatory Board (AERB) of India constituted a Task Group comprising experts from regulatory agency, advisory body/research and technical institutions, and clinical radiotherapy centers in the country to evolve and recommend the acceptance criteria for the flattening filter-free (FFF) photon beams. The Task Group thoroughly reviewed the literature and inputs of the manufactures/suppliers of the FFF linac and recommended a set of dosimetry parameters for evaluating the characteristics of the unflat photon beam. The recommendations included the evaluation of quality index, degree of unflatness, difference in percentage surface dose between flat and unflat photon beams, percentage depth dose at 10 cm depth, off-axis-ratios and radiation beam penumbra. The recommended parameters were evaluated for FFF photon beams generated by three different models of the linac, and it was observed that recommended evaluation methods are simple and easy to be implemented with the existing dosimetry and quality assurance infrastructure of the linac facilities of the radiotherapy departments. Recommendations were also made for periodic quality control check of the unflat photon beams and constancy evaluation in the beam characteristics.

Dose optimization in gynecological 3D image based interstitial brachytherapy using martinez universal perineal interstitial template (MUPIT) -an institutional experience.

The aim of this study was to evaluate the dose optimization in 3D image based gynecological interstitial brachytherapy using Martinez Universal Perineal Interstitial Template (MUPIT). Axial CT image data set of 20 patients of gynecological cancer who underwent external radiotherapy and high dose rate (HDR) interstitial brachytherapy using MUPIT was employed to delineate clinical target volume (CTV) and organs at risk (OARs). Geometrical and graphical optimization were done for optimum CTV coverage and sparing of OARs. Coverage Index (CI), dose homogeneity index (DHI), overdose index (OI), dose non-uniformity ratio (DNR), external volume index (EI), conformity index (COIN) and dose volume parameters recommended by GEC-ESTRO were evaluated. The mean CTV, bladder and rectum volume were 137 ± 47cc, 106 ± 41cc and 50 ± 25cc, respectively. Mean CI, DHI and DNR were 0.86 ± 0.03, 0.69 ± 0.11 and 0.31 ± 0.09, while the mean OI, EI, and COIN were 0.08 ± 0.03, 0.07 ± 0.05 and 0.79 ± 0.05, respectively. The estimated mean CTV D90 was 76 ± 11Gy and D100 was 63 ± 9Gy. The different dosimetric parameters of bladder D2cc, D1cc and D0.1cc were 76 ± 11Gy, 81 ± 14Gy, and 98 ± 21Gy and of rectum/recto-sigmoid were 80 ± 17Gy, 85 ± 13Gy, and 124 ± 37Gy, respectively. Dose optimization yields superior coverage with optimal values of indices. Emerging data on 3D image based brachytherapy with reporting and clinical correlation of DVH parameters outcome is enterprizing and provides definite assistance in improving the quality of brachytherapy implants. DVH parameter for urethra in gynecological implants needs to be defined further.

Investigation on the effect of sharp phantom edges on point dose measurement during patient-specific dosimetry with Rapid Arc.

The objective of this work was to investigate and quantify the effect of sharp edges of the phantom on the point dose measurement during patient-specific dosimetry with Rapid Arc (RA). Ten patients with carcinoma of prostate were randomly selected for this dosimetric study. Rapid Arc plans were generated with 6 MV X-rays in the Eclipse (v 8.6.14) with single arc (clockwise). Dosimetry verification plans were generated for two phantoms (cylindrical and rectangular). The cylindrical phantom was solid water (diameter 34 cm) and the rectangular phantom was a water phantom (25 cm × 25 cm × 10 cm). These phantoms were pre-scanned in computed tomography (CT) machine with cylindrical ionization chamber (FC65) in place. The plans were delivered with Novalis Tx linear accelerator with 6 MV X-rays for both the phantoms separately. The measured dose was compared with the planned dose for both the phantoms. Mean percentage deviation between measured and planned doses was found to be 4.19 (SD 0.82) and 3.63 (SD 0.89) for cylindrical and rectangular phantoms, respectively. No significant dosimetric variation was found due to the geometry (sharp edges) of the phantom. The sharp edges of the phantom do not perturb the patient specific Rapid Arc dosimetry significantly.

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.

Do all patients of breast carcinoma need 3-dimensional CT-based planning? A dosimetric study comparing different breast sizes.

Evaluation of dose distribution in a single plane (i.e., 2-dimensional [2D] planning) is simple and less resource-intensive than CT-based 3-dimensional radiotherapy (3DCRT) planning or intensity modulated radiotherapy (IMRT). The aim of the study was to determine if 2D planning could be an appropriate treatment in a subgroup of breast cancer patients based on their breast size. Twenty consecutive patients who underwent breast conservation were planned for radiotherapy. The patients were grouped in 3 different categories based on their respective chest wall separation (CWS) and the thickness of breast, as "small," "medium," and "large." Two more contours were taken at locations 5 cm superior and 5 cm inferior to the isocenter plane. Maximum dose recorded at specified points was compared in superior/inferior slices as compared to the central slice. The mean difference for small breast size was 1.93 (standard deviation [SD] = 1.08). For medium breas size, the mean difference was 2.98 (SD = 2.40). For the large breasts, the mean difference was 4.28 (SD = 2.69). Based on our dosimetric study, breast planning only on the single isocentric contour is an appropriate technique for patients with small breasts. However, for large- and medium-size breasts, CT-based planning and 3D planning have a definite role. These results can be especially useful for rationalizing treatment in busy oncology centers.

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.

Patient specific dosimetry for intensity-modulated radiotherapy delivered with first helical tomotherapy in India--our initial experience of 50 patients.

Two-dimensional (2D) treatment verifications were performed for fifty patients planned and treated with helical tomotherapy (HT). The treatment verification consisted of an extended dose range (EDR2) film measurement as well as point dose measurements made with an A1SL ion chamber. The agreement between the calculated and the measured film dose distributions was evaluated with the gamma parameter calculated (3 mm and 3%). Good agreement was found between measured and calculated distributions with dose parameter registration using reference marks. The mean percent discrepancy for the point dose measurements was -0.4 (SD 1.4) for the high dose, low dose-gradient region. Thus the treatment verification results confirmed the safe delivery of the planned dose to the patient with helical tomotherapy.

Dosimetric evaluation of rectum and bladder using image-based CT planning and orthogonal radiographs with ICRU 38 recommendations in intracavitary brachytherapy.

The purpose is to compare CT-based dosimetry with International Commission on Radiation Units and Measurements (ICRU 38) bladder and rectum reference points in patients of carcinoma of uterine cervix treated with intracavitary brachytherapy (ICA). Twenty-two consecutive patients were evaluated. Orthogonal radiographs and CT images were acquired and transferred to PLATO planning system. Bladder and rectal reference points were identified according to ICRU 38 recommendations. Dosimetry was carried out based on Manchester system. Patient treatment was done using (192)Iridium high dose rate (HDR) remote after-loading machine based on the conventional radiograph-based dosimetry. ICRU rectal and bladder point doses from the radiograph plans were compared with D(2), dose received by 2 cm(3) of the organ receiving maximum dose from CT plan. V(2), volume of organ receiving dose more than the ICRU reference point, was evaluated. The mean (+/-standard deviation) volume of rectum and bladder was 60 (+/-28) cm(3) and 138 (+/-41) cm(3) respectively. The mean reference volume in radiograph and CT plan was 105 (+/-7) cm(3) and 107 (+/-7) cm(3) respectively. It was found that 6 (+/-4) cm(3) of rectum and 16 (+/-10) cm(3) of bladder received dose more than the prescription dose. V(2) of rectum and bladder was 7 (+/-1.7) cm(3) and 20.8 (+/-6) cm(3) respectively. Mean D(2) of rectum and bladder was found to be 1.11 (+/-0.2) and 1.56 (+/-0.6) times the mean ICRU reference points respectively. This dosimteric study suggests that comparison of orthogonal X-ray-based and CT-based HDR ICA planning is feasible. ICRU rectal point dose correlates well with maximum rectal dose, while ICRU bladder point underestimates the maximum bladder dose.

Dosimetric comparison of conventional radiograph- and three-dimensional computed tomography-based planning using dose volume indices for partial breast intraoperative implants.

AIMS: The dosimetric outcomes of radiograph- and computed tomography-based plans generated with various optimisation strategies were compared using dose volume indices for partial breast intraoperative implants. MATERIALS AND METHODS: Eighteen patients with early stage breast cancer underwent conventional orthogonal radiograph and computed tomography to generate dosimetric data. Catheter reconstruction and delineation of the lumpectomy cavity, the planning target volume (PTV) and the ipsilateral breast were carried out on computed tomography images. For each patient, geometrically optimised plans (P(xray) and P(CT)) were generated using the active loading length based on the PTV defined from radiographs (PTV(xray)) and computed tomography (PTV(CT)). The plan P(CT) was further optimised graphically and yielded P(graphical). Plans were compared using the coverage index (CI), the external volume index (EI), the dose homogeneity index (DHI), the overdose volume index (OI) and the conformal index (COIN). RESULTS: The mean CI of the lumpectomy cavity estimated from P(xray), P(CT) and P(graphical) was 0.80, 0.82 and 0.92, respectively. The corresponding values for PTV(CT) were 0.69, 0.71 and 0.85. P(graphical) showed an increase in CI by 23 and 19% with respect to P(xray) and P(CT) (P<0.001 in all) with a decrease in DHI from 0.81 to 0.71 (P<0.001) and increase in OI from 0.041 to 0.087 (P<0.001). The EI was highest in P(xray) (mean 44 cm(3)) as compared with 25 cm(3) in P(CT) and 30 cm(3) in P(graphical). A significant improvement in COIN was observed in P(graphical) (mean 0.68) compared with P(xray) (0.48) and P(CT) (0.58) (P<0.001). CONCLUSIONS: Objective dosimetric evaluation on three-dimensional computed tomography confirms its superiority over conventional two-dimensional radiograph-based planning in terms of a reduction in normal breast irradiated with the prescription dose and improvement in conformity. Interactive graphical optimisation based on the target volume in computed tomography further improves conformity with a reduction in dose homogeneity. The use of dose volume indices allows the comparison of different plans and can be used as a tool to correlate dosimetric with clinical outcome.

Cosmesis, late sequelae and local control after breast-conserving therapy: influence of type of tumour bed boost and adjuvant chemotherapy.

AIMS: To study the influence of various factors affecting cosmetic outcome and late sequelae in a large cohort of women treated with breast-conserving therapy. MATERIALS AND METHODS: Between 1980 and 2000, 1022 pathological stage I/II breast cancer patients underwent breast-conserving therapy. On the basis of the type of tumour bed boost they received after whole breast radiotherapy, these women were assigned to three groups: (A) low dose rate (LDR) brachytherapy of 15-20 Gy (n=383); (B) high dose rate (HDR) brachytherapy of 10 Gy (optimised) in a single fraction (n=153); (C) electron beam 15 Gy/six fractions (n=460). Systemic adjuvant therapy was given to 757 women, of whom 570 received adjuvant chemotherapy. RESULTS: Cosmesis at the last follow-up was good or excellent in 77% of women. Post-radiation worsening of cosmesis was observed in 11.5% of women and was similar in the three boost groups. Moderate to severe late breast sequelae were observed in 22% of women in group B, which was significantly higher than the 12% in group A (P=0.002) and 9% in group C (P=0.0001). The actuarial 5-year local control rate was 91% and was 90, 92 and 93% in groups A, B and C, respectively. Tumour size (P=0.049) and adjuvant chemotherapy (P=0.04) were the significant factors affecting cosmetic outcome on univariate analysis. On multivariate analysis, adjuvant chemotherapy was the only factor leading to worsening in the cosmetic outcome, with P=0.03 (hazard ratio 1.65 [95% confidence interval 1.05-2.59]). CONCLUSION: The type of tumour bed boost did not have a significant effect on the worsening of cosmetic outcome. However, there were significantly more late breast sequelae in women treated with single fraction HDR implants. Chemotherapy had an adverse effect on the cosmetic outcome, but the late breast sequelae and local control rates were comparable.

Commissioning of motorized wedge for the first equinox-80 telecobalt unit and implementation in the Eclipse 3D treatment planning system.

A new model of the telecobalt unit (TCU), Theratron Equinox-80, (MDS Nordion, Canada) equipped with a single 60 degree motorized wedge (MW), four universal wedges (UW) for 15 degrees, 30 degrees, 45 degrees and 60 degrees have been evaluated. MW was commissioned in Eclipse (Varian, Palo Alto, USA) 3D treatment planning system (TPS). The profiles and central axis depth doses (CADD) were measured with Wellhofer blue water phantom for MW and the measured data was commissioned in Eclipse. These profiles and CADD for MW were compared with UW in a homogeneous phantom generated in Eclipse for various field sizes. The dose was also calculated in the same phantom at 10 cm depth. For the particular MW angle and the respective open and MW beam weights, the dose was measured for a field size of 10 cm x 10 cm in a MEDTEC water phantom at 10 cm depth with a 0.13 cc thimble ion chamber (Scanditronix Wellhofer, Uppsala, Sweden) and a NE electrometer (Nuclear Enterprises, UK). Measured dose with ion chamber was compared with the TPS calculated dose. MW angle verification was also done on the Equinox for four angles (15 degrees, 30 degrees, 45 degrees and 60 degrees). The variation in measured and calculated dose at 10 cm depth was within 2%. The measured and the calculated wedge angles were in good agreement within 2 degrees. The motorized wedges were successfully commissioned in Eclipse for four wedge angles.

Intensity modulated radiotherapy dosimetry with ion chambers, TLD, MOSFET and EDR2 film.

Purpose of this study was to report in a together our experience of using ion chambers, TLD, MOSFET and EDR2 film for dosimetric verification of IMRT plans delivered with dynamic multileaf collimator (DMLC). Two ion chambers (0.6 and 0.13 CC) were used. All measurements were performed with a 6MV photon beam on a Varian Clinac 6EX LINAC equipped with a Millennium MLC. All measurements were additionally carried out with (LiF:Mg,TI) TLD chips. Five MOSFET detectors were also irradiated. EDR2 films were used to measure coronal planar dose for 10 patients. Measurements were carried out simultaneously for cumulative fields at central axis and at off-axis at isocenter plane (+/- 1, and +/- 2 cm). The mean percentage variation between measured cumulative central axis dose with 0.6 cc ion chamber and calculated dose with TPS was -1.4% (SD 3.2). The mean percentage variation between measured cumulative absolute central axis dose with 0.13 cc ion chamber and calculated dose with TPS was -0.6% (SD 1.9). The mean percentage variation between measured central axis dose with TLD and calculated dose with TPS was -1.8% (SD 2.9). A variation of less than 5% was found between measured off-axis doses with TLD and calculated dose with TPS. For all the cases, MOSFET agreed within +/- 5%. A good agreement was found between measured and calculated isodoses. Both ion chambers (0.6 CC and 0.13 CC) were found in good agreement with calculated dose with TPS.

Evaluation of radiograph-based interstitial implant dosimetry on computed tomography images using dose volume indices for head and neck cancer.

Conventional radiograph-based implant dosimetry fails to correlate the spatial dose distribution on patient anatomy with lack in dosimetry quality. Though these limitations are overcome in computed tomography (CT)-based dosimetry, it requires an algorithm which can reconstruct catheters on the multi-planner CT images. In the absence of such algorithm, we proposed a technique in which the implanted geometry and dose distribution generated from orthogonal radiograph were mapped onto the CT data using coordinate transformation method.Radiograph-based implant dosimetry was generated for five head and neck cancer patients on Plato Sunrise treatment planning system. Dosimetry was geometrically optimized on volume, and dose was prescribed according to the natural prescription dose. The final dose distribution was retrospectively mapped onto the CT data set of the same patients using coordinate transformation method, which was verified in a phantom prior to patient study. Dosimetric outcomes were evaluated qualitatively by visualizing isodose distribution on CT images and quantitatively using the dose volume indices, which includes coverage index (CI), external volume index (EI), relative dose homogeneity index (HI), overdose volume index (OI) and conformal index (COIN).The accuracy of coordinate transformation was within ±1 mm in phantom and ±2 mm in patients. Qualitative evaluation of dosimetry on the CT images shows reasonably good coverage of target at the expense of excessive normal tissue irradiation. The mean (SD) values of CI, EI and HI were estimated to be 0.81 (0.039), 0.55 (0.174) and 0.65 (0.074) respectively. The maximum OI estimated was 0.06 (mean 0.04, SD = 0.015). Finally, the COIN computed for each patient ranged from 0.4 to 0.61 (mean 0.52, SD = 0.078).The proposed technique is feasible and accurate to implement even for the most complicated implant geometry. It allows the physicist and physician to evaluate the plan both qualitatively and quantitatively. Dose volume indices derived from CT data set are useful for evaluating the implant and comparing different brachytherapy plans. COIN index is an important tool to assess the target coverage and sparing of normal tissues in brachytherapy.

Peripheral dose from uniform dynamic multileaf collimation fields: implications for sliding window intensity-modulated radiotherapy.

The increase in the number of monitor units in sliding window intensity-modulated radiotherapy, compared with conventional techniques for the same target dose, may lead to an increase in peripheral dose (PD). PD from a linear accelerator was measured for 6 MV X-ray using 0.6 cm3 ionization chamber inserted at 5 cm depth into a 35 cm x 35 cm x 105 cm plastic water phantom. Measurements were made for field sizes of 6 cm x 6 cm, 10 cm x 10 cm and 14 cm x 14 cm, shaped in both static and dynamic multileaf collimation (DMLC) mode, employing strip fields of fixed width 0.5 cm, 1.0 cm, 1.5 cm, and 2.0 cm, respectively. The effect of collimator rotation and depth of measurement on peripheral dose was investigated for 10 cm x 10 cm field. Dynamic fields require 2 to 14 times the number of monitor units than does a static open field for the same dose at the isocentre, depending on strip field width and field size. Peripheral dose resulting from dynamic fields manifests two distinct regions showing a crest and trough within 30 cm from the field edge and a steady exponential fall beyond 30 cm. All dynamic fields were found to deliver a higher PD compared with the corresponding static open fields, being highest for smallest strip field width and largest field size; also, the percentage increase observed was highest at the largest out-of-field distance. For 6 cm x 6 cm field, dynamic fields with 0.5 cm and 2 cm strip field width deliver PDs 8 and 2 times higher than that of the static open field. The corresponding factors for 14 cm x 14 cm field were 15 and 6, respectively. The factors by which PD for DMLC fields increase, relative to jaws-shaped static fields for out-of-field distance beyond 30 cm, are almost the same as the corresponding increases in the number of monitor units. Reductions of 20% and 40% in PD were observed when the measurements were done at a depth of 10 cm and 15 cm, respectively. When the multileaf collimator executes in-plane (collimator 90 degrees) motion, peripheral dose decreases by as much as a factor of 3 compared with cross-plane data. The knowledge of PD from DMLC field is necessary to estimate the increase in whole-body dose and the likelihood of radiation induced secondary malignancy.

Performance evaluation of a dedicated computed tomography scanner used for virtual simulation using in-house fabricated CT phantoms.

Comprehensive tests on single slice CT scanner was carried out using in-house fabricated phantoms/test tools following AAPM recommended methods to independently validate the auto-performance test (APT) results. Test results of all the electromechanical parameters were found within the specified limits. Radiation and sensitivity profile widths were within ± 0.05 cm of the set slice thickness. Effective energy corresponding to nominal kVp of 80, 110 and 130 were 49.99, 55.08 and 59.48 keV, respectively. Percentage noise obtained by APT was 1.32% while the independently measured value was 0.38%. Observed contrast resolutions by independent method at 0.78% and 12% contrast difference were 4 mm and 1.25 mm (= 4 lp/cm) respectively. However, high contrast resolution (limiting spatial resolution) by APT at 50, 10 and 2% MTF levels were 9, 12.5 and 14.1 lp/cm respectively. Difference in calculated and measured CT numbers of water, air, teflon, acrylic, polystyrene and polypropylene were in the range of 0 to 24 HU, while this difference was 46 and 94 HU in case of nylon and bakelite respectively. The contrast scale determined using CT linearity phantom was 1.998×10(-4) cm(-1)/CT number. CT dose index (CTDI) and weighted CTDI (CTDI(w)) measured at different kVp for standard head and body phantoms were smaller than manufacturer-specified and system-calculated values and were found within the manufacturer-specified limit of ± 20%. Measured CTDIs on surface (head: 3.6 cGy and body: 2.6 cGy) and at the center (3.3 cGy, head; and 1.2 cGy, body) were comparable to reported values of other similar CT scanners and were also within the industry-quoted CTDI range. Comprehensive QA and independent validation of APT results are necessary to obtain baseline data for CT virtual simulation.