Tharmar Ganesh

Factors influencing early complications following Gamma Knife radiosurgery. A prospective study.

Purpose: The factors influencing early complications following Gamma Knife radiosurgery have not been definitely established. We report a prospective study evaluating the incidence of early complications (occurring within 3 months of radiosurgery) and various factors associated with early complications following stereotactic Gamma Knife radiosurgery for intracranial lesions. Patients and Methods: Seventy-nine previously unirradiated consecutive adult patients (82 lesions: arteriovenous malformations 35, benign tumors 43, metastases 4) treated by Gamma Knife radiosurgery were studied between May 1997 and August 1998. The median target volume was 4.8 cm3. The median dose of 15 Gy was prescribed to the 50% isodose. Patients were evaluated clinically and radiologically (with CT/MRI/SPECT) at 3-month intervals for the 1st year and 6 monthly thereafter. Complications were further divided as immediate (occurring within 24 h) or acute (occurring from 1 day to 3 months). Results: Early complications were observed in 19/79 (24.0%) patients. These included immediate in 10 (12.7%) and acute complications in 9 (11.3%) patients and were characterized by headache, nausea/vomiting, vertigo and seizures. No severe early complications were observed. Radiological changes in the form of perilesional edema were seen in 8/82 (9.8%) lesions. Maximum target diameter >25 mm was the only factor significantly associated with early complications by univariate analysis (p = 0.0335). Multivariate analysis revealed maximum target diameter >25 mm and prescribed dose >20 Gy to be significantly associated with early complications (p = 0.0442 and p = 0.0083, respectively). Conclusion: Up to one fourth of the patients undergoing Gamma Knife radiosurgery for intracranial lesions can experience self-limiting early toxicity. The selection of targets with small diameter and volume may reduce the risk of early complications following Gamma Knife radiosurgery.

Impact of different CT slice thickness on clinical target volume for 3D conformal radiation therapy.

The purpose of this study was to present the variation of clinical target volume (CTV) with different computed tomography (CT) slice thicknesses and the impact of CT slice thickness on 3-dimensional (3D) conformal radiotherapy treatment planning. Fifty patients with brain tumors were selected and CT scans with 2.5-, 5-, and 10-mm slice thicknesses were performed with non-ionic contrast enhancement. The patients were selected with tumor volume ranging from 2.54 cc to 222 cc. Three-dimensional treatment planning was performed for all three CT datasets. The target coverage and the isocenter shift between the treatment plans for different slice thickness were correlated with the tumor volume. An important observation from our study revealed that for volume <25 cc, most of the cases were underdosed by 18% with 5-mm slice thickness and 27% with 10-mm slickness. For volume >25 cc, the target underdosage was less than 6.7% for 5-mm slice thickness and 8% for 10-mm slice thickness. For 3D conformal radiotherapy treatment planning (3DCRT), a CT slice thickness of 2.5 mm is optimum for tumor volume <25 cc, and 5 mm is optimum for tumor volume >25 cc.

Reproducibility of tangential breast fields using online electronic portal images

Summary Background Treatment verification and reproducibility plays a major role in radiotherapy to achieve better tumour control. Small uncertainties in daily repositioning of the patients and internal organ motion can lead to discrepancies between the planned and delivered radiation treatments. A factor that influences dose homogeneity and treatment volume is the accuracy of treatment setup. Small deviations in positioning the patient with regard to the beam setup could have a relatively significant impact on the treatment volume and it is imperative to control the setup error during radiotherapy. This study focuses on the importance of inter- and intra-fraction error in tangential breast radiotherapy with an electronic portal imaging device. Aim To study the variation in treatment setup due to intra-fraction and inter-fraction during tangential field breast irradiation. Materials/Methods Twelve patients of carcinoma breast were selected for this study and CT based planning was performed with simple tangential fields. The patients were treated on a 6MV linear accelerator equipped with an electronic portal imaging device (EPID). Portal images were acquired for both medial and lateral tangential fields for 10 fractions and intra- and inter-fraction studies were performed for all the patients. Parameters such as central lung distance (CLD), central beam edge to skin distance (CBESD), central irradiated width (CIW) and cranio-caudal distances (CCD) were measured on the acquired portal image. Results The average systematic differences observed for CLD, CBESD, CCD and CIW were 1.2mm, 2.8mm, 2.07mm and 3.30mm. For intra-fraction motion, the observed standard deviations for CLD, CBESD and CCD were 0.7mm, 0.73mm, and 1.36mm. Similarly the CLD, CBESD, CIW and CCD were analyzed for inter-fraction variation. Conclusions The online portal imaging device is an important tool for ensuring the proper delivery of planned dose. Our results suggest that intra-fraction motion of the breast has less impact on the treatment volume. Regular treatment verification between treatment fractions will help in reducing the normal tissue toxicity and ensures proper dose delivery to the tumour volume.

Simulation of dose to surrounding normal structures in tangential breast radiotherapy due to setup error.

Setup error plays a significant role in the final treatment outcome in radiotherapy. The effect of setup error on the planning target volume (PTV) and surrounding critical structures has been studied and the maximum allowed tolerance in setup error with minimal complications to the surrounding critical structure and acceptable tumor control probability is determined. Twelve patients were selected for this study after breast conservation surgery, wherein 8 patients were right-sided and 4 were left-sided breast. Tangential fields were placed on the 3-dimensional-computed tomography (3D-CT) dataset by isocentric technique and the dose to the PTV, ipsilateral lung (IL), contralateral lung (CLL), contralateral breast (CLB), heart, and liver were then computed from dose-volume histograms (DVHs). The planning isocenter was shifted for 3 and 10 mm in all 3 directions (X, Y, Z) to simulate the setup error encountered during treatment. Dosimetric studies were performed for each patient for PTV according to ICRU 50 guidelines: mean doses to PTV, IL, CLL, heart, CLB, liver, and percentage of lung volume that received a dose of 20 Gy or more (V20); percentage of heart volume that received a dose of 30 Gy or more (V30); and volume of liver that received a dose of 50 Gy or more (V50) were calculated for all of the above-mentioned isocenter shifts and compared to the results with zero isocenter shift. Simulation of different isocenter shifts in all 3 directions showed that the isocentric shifts along the posterior direction had a very significant effect on the dose to the heart, IL, CLL, and CLB, which was followed by the lateral direction. The setup error in isocenter should be strictly kept below 3 mm. The study shows that isocenter verification in the case of tangential fields should be performed to reduce future complications to adjacent normal tissues.

A TECHNIQUE FOR VERIFICATION OF ISOCENTER POSITION IN TANGENTIAL FIELD BREAST IRRADIATION

Treatment verification and reproducibility of the breast treatment portals play a very important role in breast radiotherapy. We propose a simple technique to verify the planned isocenter position during treatment using an electronic portal imaging device. Ten patients were recruited in this study and (CT) computed tomography-based planning was performed with a conventional tangential field technique. For verification purposes, in addition to the standard medial (F1) and lateral (F2) tangential fields, a field (F3) perpendicular to the medial field was used for verification of the treatment portals. Lead markers were placed along the central axis of the 2 defined fields (F1 and F3) and the separation between the markers was measured on the portal images and verified with the marker separation on the digitally reconstructed radiographs (DRRs). Any deviation will identify the shift in the planned isocenter position during treatment. The average deviation observed between the markers measured from the DRR and portal image was 1.6 and 2.1 mm, with a standard deviation of 0.4 and 0.9 mm for fields F1 and F3, respectively. The maximum deviation observed was 3.0 mm for field F3. This technique will be very useful in patient setup for tangential breast radiotherapy.

SU-FF-T-245: Impact of Volume Effect of Detectors in Multiple Points Absolute Dose and the Choice of Plane of Measurement in 2D Planar Dose Verification in Patient Specific IMRT QA

Purpose: To evaluate the volume effect of detectors in multiple points dose in small and large field IMRT verification and to compare the planar dose distribution between fixed‐fields coronal‐plane and multifield axial‐plane verification methods. Method and Materials: Patient‐specific dosimetric verification was conducted for 20 patients of nasopharynxs small‐field, SIB‐IMRT large‐field and prostate tumors. Eclipse‐Helios system, Clinac‐2300C/D linear accelerator, Med‐Tec IMRT phantom and ionchambers of 0.6, 0.13 and 0.015cm3 and MOSFET detectors were used. For each patient, 15 IMRT plans were generated. The multiple points doses were measured at multifield and fixed‐fields on‐axis, 4‐offsets and inhomogeneous points. The agreements between calculated and measured doses were found for small and large field IMRT verification. For relative dosimetry, EDR2 films, Virtual Water phantom for fixed‐fields coronal‐plane and IMRT Body phantom for multifield axial‐plane verifications RITsystem and Vidar scanner were used. The agreements between calculated and measured dose distributions between two methods were compared using various gamma criteria. Results: The agreements between measured and calculated doses using 0.6 and 0.13cm3 chambers at multifield and fixed‐fields on‐axis points were within 4.7% ±2.3. The dose differences of 6% ±1.7, 19.7% ±5.5, and 12.1% ±3.2 at offset1 to 3 points were found. The 0.015cm3 chamber and MOSFET detectors were showed the dose differences of upto 8.4% in large field verification. The mean differences in percentage of pixels passing the gamma criteria of 3mm/5% were 98.2% ±0.9 and 90% ±5.4 for fixed‐fields coronal‐plane and multifield axial‐plane verifications. Conclusions: This study is useful in evaluating the detectors response at high and low dose, outside the fields and inhomogeneous points dose verification. In offset points, 0.6cm3 is more accurate than 0.13cm3 chamber in both small and large field verification. The fixed‐fields coronal‐plane verification is more accurate, however, the multifield axial‐plane is clinically realistic method of verification and to be adopted.

SU‐FF‐T‐178: Dosimetric Evaluation of Penumbra for Conformal and Intensity Modulated Radiotherapy

Purpose: To evaluate the penumbral characteristics as function of collimation systems, conventional detector, depth, energy and MLC travel orientations using diode detectors.Methods and Materials: The experiment was carried out in a Clinac 2300C/D linear accelerator for 6 and 15 MV. For penumbral width determination, cross‐beam profiles were measured using Blue phantom radiation field analyzer, 0.13cm3 ionchamber and diode detectors, CU‐500E dual‐channel electrometer and OmniPro beam‐data acquisition system. The penumbra was measured for both Jaw and MLC‐defined field sizes of 4×4 to 30×30cm2 depths at dmax, 5, 10, 15 and 20cm and also for cross‐plane and in‐plane MLC travel orientations at 100cm SSD. The penumbra width was calculated as the distance between 80%–20% intensity in the profiles. Results: The MLC penumbra was in the range of 3.6mm to 4mm for 6MV and 5.4mm to 7.1mm for 15MV at dmax depth for field sizes of 4×4 to 30×30cm2 as measured by diode detector. The mean differences between Jaw and MLC‐defined fields penumbra were 0.8mm ± 0.2, and 1mm ± 0.2 for 6MV and 1.2mm ±0.2 and 1.8mm ±0.5 for 15MV at dmax and 10cm depths. The maximum difference of 1.9mm ±0.3 for 6MV and 2.2mm ±0.2 for 15MV between diode and 0.13cm3detectors were found. Use of diode detectors, maximum differences of 3.4mm ±3 at 20cm depths between energies and the differences of 3.3mm ± 2.2 for 6 MV and 1.2mm ± 0.3 for 15MV between cross‐plane and in‐plane MLC penumbra were found. Conclusions: The rounded‐end MLC penumbra was larger as compared to Jaw penumbra. The penumbra measured by diode was increasing with energy upto a certain depths and field sizes and then it increases more with low‐energy at larger depths and field sizes as compared to high‐energy. The adequate accounting of penumbra is important in intensity modulated beam dose calculation.

SU‐FF‐T‐51: A Technique for Verification of Isocenter Position in Tangential Field Breast Radiotherapy

Breast irradiation is one of the most challenging problems in radiotherapy due to its complex shape of the target volume and its proximity to the surrounding normal structures. Proper implementation of the planned parameter during treatment is very essential, especially, verification of the planned isocenter position. Small deviations in the positioning of the patient with regard to the beam set up could have a relatively important impact on the treatment volume. Hence, treatment verification and reproducibility of the breast treatment portals is an important step in breast radiotherapy. A simple technique has been proposed in this study to verify the planned isocenter during treatment using electronic portal imaging device. Ten patients were recruited in this study and CT based planning was performed with conventional tangential field technique. For verification purpose, in addition to the standard medial (F1) and lateral (F2) tangential fields, a field (F3) perpendicular to the medial field was used for verification of the treatment portals. Lead markers were placed along the central axis of the two defined fields (F1 & F3) and the separations between the markers were measured on the portal images and verified with the marker separation on the digitally reconstructedradiographs(DRRs). Any deviation will identify the shift in the planned isocenter position during treatment. The average deviation observed between the markers measured from the DRR and portal image was 1.6 mm and 2.1 mm with a standard deviation of 0.4 mm and 0.9 mm for field F1 and F3 respectively. The maximum deviation observed was 3 mm for field F3. This technique will be very useful in patient set‐up for tangential breast radiotherapy.

SU‐FF‐T‐54: Treatment Planning in HDR Intracavitary Brachytherapy: Comparison of Critical Organ Doses as Estimated by Orthogonal Radiographs Based Planning and Image Based Planning

Purpose:Doses to rectum, bladder and Point‐A in patients who received intracavitary HDR brachytherapy treatment were calculated by both orthogonal radiographsreconstruction method and by image based reconstruction method and were compared. Method and Materials: Twenty‐five patients of carcinoma cervix who received intracavitary brachytherapy treatment by micro‐Selectron HDR unit were included in this study. For these patients, in Plato treatment planning system, reconstruction of sources, applicator, organs at risk and Point‐A were done using conventional orthogonal radiographs and by using 1 mm thick CT scanimages. Bladder was identified on radiographs through a Foleys catheter inserted into it filled with 7 cc of radio‐opaque fluid and the rectum through a rectal marker. Doses were calculated at 5 points each on bladder and rectum, for the treatment dose of 7 Gy to Point‐A (right & left). On CTimages, these organs were delineated with the radiologists help and from dose‐volume‐histograms,doses to 1, 2 and 5 cc of these organs were calculated besides dose to Point‐A (right & left). Results: The critical organdoses estimated by the CT method were consistently higher when compared to those by the radiographs method. When compared to the radiographicdose maximum for bladder, doses to 1, 2 and 5 cc of bladder were higher by 89%, 69.7% and 41.6% on an average respectively. For rectum, the corresponding values were 36.6%, 21.7% and 0.4%. However dose to Point‐A did not differ by more than 3% between the two methods. Conclusion: The conventional orthogonal radiography based reconstruction method consistently underestimates the doses to critical organs while the doses to reference points are comparable in these two methods. Long‐term follow‐up in such patients can identify the threshold volumes for these critical organs beyond which complication rates are bound to rise.

SU‐FF‐T‐178: A Study On the Reproducibility of Tangential Breast Fields Using Online Electronic Portal Images

Purpose: To study the reproducibility of tangential breast treatment technique using online portal imaging system.Introduction:Treatment verification and reproducibility is an important step in radiotherapy to achieve a better tumorcontrol. Care should be taken to ensure the same dose to be delivered in the same volume of irradiation. Electronic portal imaging technique plays a vital role in accomplishing the above task by studying the setup error and correct the same before the treatment delivery. Method and Materials: Twelve patients of carcinoma of breast were selected for this study and CT based planning was performed with simple tangential fields. The patients were then treated on a 6MV linear accelerator equipped with an electronic portal imaging device. Portal images were acquired for both medial and lateral tangential fields for 10 fractions and intra and the inter‐fraction studies were performed for all the patients. The parameters such as central lung distance (CLD), Central beam edge to skin distance, central irradiated width and cranio‐caudal distances were measured on the acquired portal image. In the intra‐fraction study lead markers were placed on the patient skin to study the breast movement during treatment.Results: The maximum variation of the marker during the treatment was 1.8 mm with a standard deviation of 0.575 mm. Similarly the CLD, CBESD, CIW and CCD were analyzed for the intra and inter‐fraction variation. Conclusion: Online portal imagingdevice is an important tool for ensuring the proper delivery of planned dose. Our result suggests that intra‐fraction motion of the breast has less impact on the treatment volume.