Kj Maria Das

Performance evaluation of respiratory motion‐synchronized dynamic IMRT delivery

The purpose of this study was to evaluate the capabilities of DMLC to deliver the respiratory motion‐synchronized dynamic IMRT (MS‐IMRT) treatments under various dose rates. In order to create MS‐IMRT plans, the DMLC leaf motions in dynamic IMRT plans of eight lung patients were synchronized with the respiratory motion of breathing period 4 sec and amplitude 2 cm (peak to peak) using an in‐house developed leaf position modification program. The MS‐IMRT plans were generated for the dose rates of 100 MU/min, 400 MU/min, and 600 MU/min. All the MS‐IMRT plans were delivered in a medical linear accelerator, and the fluences were measured using a 2D ion chamber array, placed over a moving platform. The accuracy of MS‐IMRT deliveries was evaluated with respect to static deliveries (no compensation for target motion) using gamma test. In addition, the fluences of gated delivery of 30% duty cycle and non‐MS‐IMRT deliveries were also measured and compared with static deliveries. The MS‐IMRT was better in terms of dosimetric accuracy, compared to gated and non‐MS‐IMRT deliveries. The dosimetric accuracy was observed to be significantly better for 100 MU/min MS‐IMRT. However, the use of high‐dose rate in a MS‐IMRT delivery introduced dose‐rate modulation/beam hold‐offs that affected the synchronization between the DMLC leaf motion and target motion. This resulted in more dose deviations in MS‐IMRT deliveries at the dose rate of 600 MU/min. PACS numbers: 87.53.kn, 87.56.N‐

Utility of single photon emission computed tomography perfusion scans in radiation treatment planning of locally advanced lung cancers

Purpose: Lung perfusion scan provides a map of the spatial distribution of lung perfusion. This can be used to design radiation portals to spare functional lung (FL), potentially reducing lung toxicity. The purpose of this study was to assess the utility of lung perfusion single photon emission computed tomography (SPECT) in treatment planning for lung cancer patients. Materials and Methods: Radiotherapy treatment planning computed tomography (CT) scans and SPECT scans of 11 patients of lung cancer suitable for external radiotherapy were co-registered. Conventional treatment plans (anatomic plan) and plans with FL information (functional plan) was generated. The difference in dose volume parameters (V20, V30 and mean lung doses) due to these two plans were compared using Bland-Altman plots. Results: Functional plans produced a more favorable plan compared with anatomic plan in all except three cases. FL V20 values and FL mean lung dose were reduced for all patients by an average of 5.45 Gy and 7.72 Gy respectively which were statistically significant. Conclusions: Lung perfusion scans provide functional information which is not provided by CT scans. SPECT-guidance aids in reducing the dose delivered to highly perfused regions which could reduce the incidence of pneumonitis.

Dosimetric effect of multileaf collimator leaf width in intensity-modulated radiotherapy delivery techniques for small- and large-volume targets

The purpose of this study was to evaluate the dosimetric effect of the leaf width of a multileaf collimator (MLC) in intensity-modulated radiotherapy (IMRT) delivery techniques for small- and large-volume targets. We retrospectively selected previously treated 5 intracranial and 5 head-neck patients for this study to represent small- (range, 18.37-72.75 cc; mean, 42.99 cc) and large-volume (range, 312.31-472.84 cc; mean, 361.14 cc) targets. A 6-MV photon beam data was configured for Brianlab m3 (3 mm), Varian Millennium 120 (5 mm) and Millennium 80 (10 mm) MLCs in the Eclipse treatment-planning system. Sliding window and step-shoot IMRT plans were generated for intracranial patients using all the above-mentioned MLCs; but due to the field size limitation of Brainlab MLC, we used only 5-mm and 10-mm MLCs in the head-and-neck patients. Target conformity, dose to the critical organs and dose to normal tissues were recorded and evaluated. Although the 3-mm MLC resulted in better target conformity (mean difference of 7.7% over 5-mm MLC and 12.7% over 10-mm MLC) over other MLCs for small-volume targets, it increased the total monitor units of the plans. No appreciable differences in terms of target conformity, organ at risk and normal-tissue sparing were observed between the 5-mm and 10-mm MLCs for large-volume targets. The effect of MLC leaf width was not quantifiably different in sliding window and step and shoot techniques. In addition, we observed that there was no additional benefit to the sliding-window (SW) technique when compared to the step-shoot (SS) technique as a result of reduction of MLC leaf width.

Electron beam therapy at extended SSDs: an analysis of output correction factors for a Mitsubishi linear accelerator

The effects of extended source-to-surface distance (SSD) on the electron beam dose profiles were evaluated for various electron beam energies--6, 9, 12, 15 and 20 MeV-and the accuracy of various output correction methods was analysed on a Mitsubishi linear accelerator using a radiation field analyser (RFA). The dose fall-off region of the central axis depth-dose curves was nearly independent for SSDs up to 120 cm where as in the build-up region, a marginal reduction of surface dose was observed, particularly for lower energies and for smaller field sizes. Effective SSDs and virtual source distances were evaluated for field sizes ranging from 5 x 5 to 15 x 15 cm2 for various energies. Curve fitting was done with the measured outputs with various equations and coefficients were evaluated. The accuracy of the derived output correction factors by effective SSD, virtual source distance and curve-fit methods was assessed by evaluating correlation coefficients between the calculated and the measured values. The correlation coefficient was best with the linear-quadratic equation followed by the effective SSD method and the virtual source method. The output correction based on the linear-quadratic equation showed the best estimate of electron beam output at extended SSDs with an accuracy well within +/- 1%. The rapid reduction of dose due to the applicator-scattered component at d(max) point with an extended SSD was significant for the 5 x 5 cm2 applicator and lower energies.

Changes in salivary flow rates in head and neck cancer after chemoradiotherapy

BACKGROUND Changes in salivary flow rate were studied in head and neck (H and N) cancer patients who, after receiving moderately accelerated radiotherapy (RT) and concurrent chemotherapy (CT), were free of disease at 1 year. MATERIALS AND METHODS Between July 2003 and July 2005, saliva estimation was performed for 36 patients of locally advanced (AJCC stages III and IV) squamous cell carcinoma of the H and N. RT, moderately accelerated (70 Gy/35 fx/6 weeks) along with concurrent weekly cisplatin at 35 mg/m 2 (capped at 50 mg) with standard hydration and anti-emetic cover, was planned using conventional planning on telecobalt or 6 MV photons. The saliva flow rate was estimated for 5 min at rest (unstimulated) and after using lemon drops (stimulated) for the next 5 min at baseline (pre-treatment), 3, 6 and 12 months following treatment. RESULTS The median follow-up of this study was 29 months. Compared with baseline, by 3 months, a significant reduction in unstimulated (0.35 ml/min and 0.10 ml/min) and stimulated (0.97 ml/min and 0.28 ml/min) salivary flow rate was observed, respectively. This continued to decrease further till 6 months (0.06 ml/min and 0.17 ml/min) and, by 12 months, a minimal and non-significant recovery was observed in both unstimulated (0.08 ml/min) and stimulated salivary flow rates (0.22 ml/min), respectively. CONCLUSIONS Salivary flow rates fall to a fourth of the baseline value with the above CT + RT protocol, with minimal recovery at 12 months following completion of treatment.

Dosimetric verification of gated delivery of electron beams using a 2D ion chamber array.

The purpose of this study was to compare the dosimetric characteristics; such as beam output, symmetry and flatness between gated and non-gated electron beams. Dosimetric verification of gated delivery was carried for all electron beams available on Varian CL 2100CD medical linear accelerator. Measurements were conducted for three dose rates (100 MU/min, 300 MU/min and 600 MU/min) and two respiratory motions (breathing period of 4s and 8s). Real-time position management (RPM) system was used for the gated deliveries. Flatness and symmetry values were measured using Imatrixx 2D ion chamber array device and the beam output was measured using plane parallel ion chamber. These detector systems were placed over QUASAR motion platform which was programmed to simulate the respiratory motion of target. The dosimetric characteristics of gated deliveries were compared with non-gated deliveries. The flatness and symmetry of all the evaluated electron energies did not differ by more than 0.7 % with respect to corresponding non-gated deliveries. The beam output variation of gated electron beam was less than 0.6 % for all electron energies except for 16 MeV (1.4 %). Based on the results of this study, it can be concluded that Varian CL2100 CD is well suitable for gated delivery of non-dynamic electron beams.

Investigation of Inter and Intra-fractional Uncertainties in Lung IMRT delivery

Changes in the tumour position due to inter and intra-fractional motion introduce uncertainty in IMRT delivery. In this work, we experimentally evaluated the effect of inter and intra-fractional motion uncertainties in the IMRT delivery. A lung patient was planned with five field dynamic IMRT for a total dose of 60Gy in 30 fractions using 6MV photon beam at a dose rate of 400MU/min. The plan was delivered on Varian CL2100CD Linear Accelerator with millennium 120 MLC for a single fraction and measured using IMRT MatriXX placed on Quasar motion platform, aligned with respect to isocentre for planar as well as point dose measurements. This measurement was summated for thirty fractions and taken as reference. Thirty measurements were performed for each inter-fractional, intra-fractional and combined effect. The setup errors (Mean±SD) used to simulate the inter-fractional displacements were RL: -0.10±0.27mm, SI: 0.17±0.45mm, AP: -0.04±0.38mm and Rot: 0.02±0.86 degree. The intra-fractional motion was simulated using the motion platform parallel to the MLC leaf motion, for amplitude of 1cm and period of 4s. The planner fluence of inter, intra-fractional motion and combination of both was analyzed against the reference using gamma criteria of 3%/3mm. Similarly the point dose measurements were also compared. The maximum deviation in point dose during a single fraction was -3.8%, 3.1% and -5.9% for interfractional, intra-fractional and combined respectively. The same resulted in deviation of -0.4%, -0.3% and 0.1% respectively when summated for 30 fractions. The percentage of pixels failing the gamma criteria during a single fraction was 28.0%, 16.4% and 28.5% for inter-fractional, intra-fractional and combined respectively. The same revealed 11.4%, 12.4% and 19.2% respectively when summated. Though the point dose deviations were nullified over thirty fractions, the planner fluence variation was observed to be considerable.

Partial Transmission Block for Optimization of Anterior Supraclavicular-Posterior Axillary Boost in the Radiation Therapy of CArcinoma Breast

Radiation therapy of breast often involves an anterior supraclavicular-axillary (SC-AX) portal to irradiate the supraclavicular and axillary contents. However, the fall-off of the dose in this region leads to an inhomogeneity that could result either during the use of a single anterior SC-AX field or even with the concomitant use of a posterior axillary boost. An attempt has been made to circumvent this inhomogeneity by the use of a partial transmission block that could be placed in the anterior SC-AX portal corresponding to the posterior axillary boost field. The details of the quantum of partial transmission block to be used for different axillary separations and the reference depths of the dose prescription has been evaluated and presented.

Midline shield for radiation therapy of carcinoma of the uterine cervix: Should it be “midline” or “individualized”?

This study attempts to evaluate the advantage of individualized midline shield (IMLS) constructed on the basis of uterine geometry and applicator position in terms of the dosimetric consequences to points A-right (AR) and left (AL) as compared to standard midline shield (SMLS) in radiation therapy of carcinoma cervix. Twenty consecutive patients of carcinoma cervix (Stage I, II and III) were treated by external beam radiotherapy (EBRT) (50 Gy/5 weeks/25 fractions) and high-dose rate intracavitary brachytherapy (24 Gy/4 weeks/4 fractions) prescribed at point A. At the completion of 40 Gy by EBRT (phase I), IMLS (5 cm wide) at isocenter was fabricated on the basis of uterine geometry as ascertained by a dummy intracavitary application. The remaining 10 Gy of EBRT was delivered using IMLS (phase II) to effectively minimize and optimize the dose to point A. The dose profiles of IMLS were compared against the corresponding dose profile of a 5 cm SMLS and were found to be dependent on the positional variation of AR and AL with respect to the midline. With IMLS, the dose to AR and AL for the 10 Gy of phase II varied between 21.7-38.87% (30.01 +/- 4.87) and 22.42-35.72% (28.12 +/- 3.79) respectively. However with SMLS, the AR and AL doses would have ranged from 22.03% to 77.26% (34.55 +/- 15.94) for AR and from 20.59% to 96.2% (46.93 +/- 28.15) for AL leading to considerable inhomogeneity. Thus, in protocols incorporating midline shield for radiotherapy of carcinoma cervix, IMLS in place of SMLS could be preferred for achieving a definitive and homogeneous dose to the points AR and AL.

Synchronization of Intra-fractional Motion in Dynamic IMRT Delivery

One of the limitations of breath-hold and gated treatments is the prolongation of treatment time. Hence, it is noteworthy to incorporate the intra-fractional motion into the IMRT delivery without holding or gating the beam. The purpose of this study was to develop a method for synchronization of intra-fractional motion in dynamic IMRT delivery. An in-house program was developed in MATLABTM. A dynamic MLC (DMLC) file was imported into the program from the Eclipse treatment planning system (TPS). The program modifies the DMLC file by incorporating the target motion. In this study, target motion was assumed to have amplitude of 2cm and period of 4s. The modified DMLC file was sent for delivery to the linear accelerator (CL2100CD) equipped with millennium 120 MLC. Dosimetric measurements were carried out using IMRT MatriXX 2D array device which was placed over the QUASAR motion platform. In order to evaluate the accuracy of synchronization of intra-fractional motion in delivery (SIMD), the fluence of TPS DMLC file was measured using static detector (no motion). The IMRT MatriXX was moved in cranio/caudal direction (parallel to MLC leaf motion) for the above assumed target motion. The fluence of SIMD was measured for this moving detector using the modified DMLC file. Further, in order to show the benefit of SIMD, the fluence of non-SIMD was also measured, using the original TPS DMLC file which was delivered to the moving detector. The gamma evaluation criterion of 3% / 3mm was used to compare the SIMD and non-SIMD with static delivery. The percentage of pixels passing the criteria in gamma evaluation was 94.4% for SIMD and 63.17% for non-SIMD. This demonstrates that SIMD was able to compensate the intra-fractional motion and could deliver the fluence similar to static.