P Sriram

A patient-specific quality assurance study on absolute dose verification using ionization chambers of different volumes in RapidArc treatments

The recalculation of 1 fraction from a patient treatment plan on a phantom and subsequent measurements have become the norms for measurement-based verification, which combines the quality assurance recommendations that deal with the treatment planning system and the beam delivery system. This type of evaluation has prompted attention to measurement equipment and techniques. Ionization chambers are considered the gold standard because of their precision, availability, and relative ease of use. This study evaluates and compares 5 different ionization chambers: phantom combinations for verification in routine patient-specific quality assurance of RapidArc treatments. Fifteen different RapidArc plans conforming to the clinical standards were selected for the study. Verification plans were then created for each treatment plan with different chamber-phantom combinations scanned by computed tomography. This includes Medtec intensity modulated radiation therapy (IMRT) phantom with micro-ionization chamber (0.007 cm(3)) and pinpoint chamber (0.015 cm(3)), PTW-Octavius phantom with semiflex chamber (0.125 cm(3)) and 2D array (0.125 cm(3)), and indigenously made Circular wax phantom with 0.6 cm(3) chamber. The measured isocenter absolute dose was compared with the treatment planning system (TPS) plan. The micro-ionization chamber shows more deviations when compared with semiflex and 0.6 cm(3) with a maximum variation of -4.76%, -1.49%, and 2.23% for micro-ionization, semiflex, and farmer chambers, respectively. The positive variations indicate that the chamber with larger volume overestimates. Farmer chamber shows higher deviation when compared with 0.125 cm(3). In general the deviation was found to be <1% with the semiflex and farmer chambers. A maximum variation of 2% was observed for the 0.007 cm(3) ionization chamber, except in a few cases. Pinpoint chamber underestimates the calculated isocenter dose by a maximum of 4.8%. Absolute dose measurements using the semiflex ionization chamber with intermediate volume (0.125 cm(3)) shows good agreement with the TPS calculated among the detectors used in this study. Positioning is very important when using smaller volume chambers because they are more sensitive to geometrical errors within the treatment fields. It is also suggested to average the dose over the sensitive volume for larger-volume chambers. The ionization chamber-phantom combinations used in this study can be used interchangeably for routine RapidArc patient-specific quality assurance with a satisfactory accuracy for clinical practice.

SU‐E‐T‐134: Patient Specific Quality Assurance of RapidArc Pre Treatment Plans Using Semiflex 0.125 Cc Ionization Chamber

PURPOSE To evaluate the Patient specific pre-treatment quality assurance for hundred RapidArc plans using semiflex (0.125cc) ionization chambers. METHODS Absolute point dose were measured for head and neck, thorax and abdomen cases using semiflex (0.125 cc) ionization chamber. Verification plan was created for each treatment plan in eclipse 8.6 treatment planning system with the semiflex ionization chamber and the octavius phantom. Measurements were performed on a Varian Clinac2100C/D linear accelerator equipped with a millennium 120 leaf collimator. All the results were compared with the fluence measurements using 2D Seven29 ion chamber array combined with octavius phantom. RESULTS Positive absolute mean dose variation of 0.56 % was observed with thorax cases with a standard deviation (SD) of ± 1.13 between the plans with a range of -1.78% to 2.70%. Negative percentage dose errors were found with head and neck and abdomen cases, with a mean variation of -0.43 % (SD ± 1.50), (range -3.25 % to 2.85 %) and -0.35 % (SD ± 1.48), (range -3.10 % to 2.65 %) for head and neck and abdomen cases respectively. Relative dose measurements with 2D array agreed well with the TPS calculate for all the cases. The maximum percentage value failed in gamma analysis was found to be 4.95, 4.75, and 4.88 for head and neck, thorax, and abdomen cases respectively. In all the cases analysed the percentage dose points failed the gamma criteria was less than 5%. CONCLUSIONS On the basis of the studies performed it can be concluded that the semiflex ionization chamber having a volume of 0.125cc can be used efficiently for measuring the pre-treatment quality assurance of RapidArc plans for all the sites. The results provide an overall accuracy when compared to fluence measurement done using 2D array seven29.

SU-E-T-724: A Phantom Study on IMRT and RapidArc Optimization Strategies for Midline and Peripheral Tumours and Its Clinical Validation by Comparing with Different Treatment Plans

Purpose: To evaluate the optimization strategies for midline and peripheral tumours for IMRT and RapidArc treatments using phantom and its clinical validation by comparing with different plans. Methods: Homogeneous phantom was CT scanned and PTV was delineated for two different positions (midline and periphery). Two organs at risk with different shapes (organ at risk 1, organ at risk 2) were created. Planning was done for IMRT and RapidArc with OAR‐1, placed at distance of 0.5cm and 2cm. Also OAR‐2, placed at a distance of 1cm and 2.5cm from the border of PTV along the central axis. The phantom study was clinically verified by comparing different treatment plans. Results: Dose homogeneity was almost similar for tumours in the midline where OARs are far. RapidArc plans show superior dose homogeneity, when the target is situated at the periphery and OARs are very near to PTV (homogeneity index 2.67 for RapidArc and homogeneity index 4.03 for IMRT). Target coverage was better for all RapidArc plans with maximum conformity index 1.01. The sparing of OAR in terms of the maximum dose was better in RapidArc. A considerable reduction in OAR mean dose (12.37% for OAR‐1 and 10.23% for OAR‐2) was observed with RapidArc technique for peripheral tumors. For healthy tissue no significant changes were observed in terms of the mean dose and integral dose. But RapidArc plans show a reduction in the volume of the healthy tissue irradiation above V10Gy for targets at the periphery and OAR near. Conclusions: Either IMRT or RapidArc can be chosen for tumours in the mid line. Particularly Rapid Arc treatment can be recommended for tumours which are situated at the periphery and organs at risk in close proximity. The clinical validation with different treatment plans well supported the phantom study.

SU‐E‐T‐464: A Patient Specific Quality Assurance Study on Absolute Dose Verification Using Ionization Chambers of Different Volumes in RapidArc Treatments

Purpose: To evaluate and compare five different ionization chamber ‐ phantom combinations for verification in routine patient specific quality assurance of RapidArc treatments. Methods: Fifteen different RapidArc plans conforming to the clinical standards were selected for the study. Verification plan was subsequently created for each treatment plans with different chamber‐phantom combinations CT scanned. This includes Medtec‐ IMRT phantom with micro ionization chamber (0.007cm3) and pinpoint chamber (0.015cm3), PTW‐Octavius phantom with semiflex chamber (0.125cm3) and 2D array (0.125cm3) and indigenously made Circular wax phantom with 0.6 cm3 chamber. The measured isocentre absolute dose was compared with the TPS planned. Results: Micro ionization chamber shows more deviations when compared to semiflex and 0.6 cm3 with a maximum variation of −4.76%,−1.49% and 2.23% for micro ionization, semiflex and farmer chambers respectively. The positive variations indicate that the chamber with larger volume slightly overestimates. Farmer chamber shows higher deviation when compared to 0.125cm3. In general the deviation was found to be less than 1% with semiflex and farmer chamber. A maximum variation of 2% between the plans was observed for 0.007cm3 ionization chamber, except for few cases. Pinpoint chamber underestimates the calculated isocentre dose by maximum 4%. Conclusions: Absolute dose measurements using semiflex ionization chamber with intermediate volume (0.125cm3) shows good agreement with the TPS calculated among the detectors used in this study. Positioning is very important when using smaller volume chambers as they are more sensitive to geometrical errors within the treatment fields. Also it is suggested to average the dose over the sensitive volume for larger volume chambers. The ionization chamber‐phantom combinations used in this study can be used interchangeably for routine RapidArc patient specific quality assurance with a satisfactory accuracy for clinical practices.

SU‐E‐T‐779: Adaptive RapidArc Treatment Planning for Esophageal Cancers Using Cone Beam Computed Tomography

Purpose: To assess the potential of cone beam CT(CBCT) derived adaptive RapidArc treatment for esophageal cancers in reducing the dose to organs at risk (OAR). Methods: Five patients with esophageal cancer were CT scanned in free breathing pattern. The PTV is generated by adding a 3D margin of 1 cm to the CTV as per ICRU 62 recommendations. The double arc RapidArc plan (Clin_RA) was generated for the PTV. Patients were setup using lasers and tattoos and kV‐CBCT scan was acquired daily during first week of therapy, then weekly. Setup errors > 5 mm only were corrected. These images were exported to the Eclipse TPS. The adaptive CTV which includes tumor and involved nodes was delineated in each CBCTimage set for the length of PTV. The composite CTV from first week CBCTs was generated using Boolean union operator and 5 mm margin was added circumferentially to generate adaptive PTV (PTV1). Adaptive RapidArc plan (Adap_RA) was generated. NTCP and DVH of the OARs of the two plans were compared. Similarly, PTV2 is generated from weekly CBCTs. PTV2 was evaluated for the coverage of 95% isodose of Adap_RA plan. Results: The PTV1 and PTV2 volumes covered by 95% isodose in adaptive plans were 93.16±1.8% and 94.66±1.45% respectively. The lung V10Gy, V20Gy and mean dose in Adap_RA plan was reduced by 17.36% (p=0.001), 34.86% (p=0.002) and 16.74% (p<0.001) respectively in comparison with Clin_RA. The Adap_RA plan reduces the heart D35% and mean dose by 17.03% (p=0.002) and 17.04% (p=0.002). No significant reduction in spinal cord and liver doses were observed. NTCP for the lung (0.42% vs. 0.08%) and heart (1.41% vs. 0.092%) was reduced significantly in adaptive plans. Conclusions: The adaptive re‐planning strategy based on the first week CBCT dataset significantly reduces the doses and NTCP to OARs.

SU‐E‐T‐231: Exit Fluence Analysis in RapidArc Using EPID

Purpose: In measuring the exit fluences, there are several sources of deviations which include the changes in the entrance fluence, changes in the detector response and patient orientation or geometry. The purpose of this work is to quantify these sources of the errors. Methods: The quantification of the errors caused by the machine delivery is done by comparing arc picket fence test for a period of 30 days. To quantify the sources of error due leaf and gantry positions and positioning of the patient a RapidArc plan created for the pelvis site was delivered with and without rando phantom and the exit portal images were measured. The day to day exit fluence variation in the patient anatomy were analysed by comparing the daily exit dose images during the course of treatment. For comparison, the first fraction image is used as the reference image. Standard deviation and the gamma analysis were used to quantify the errors between fractions. The gamma criterion used for analysis is 3% Dose Difference and 3 mm Distance to Agreement. Results: The maximum gamma value and Standard deviation for the picket fence test fields was 3.3 and 1.35 respectively. The area failing the gamma criteria was less than 0.1%. The delivery of the RapidArc plans without phantom shows a maximum standard deviation of 1.85 and the maximum gamma value of 0.59. The maximum gamma value for the RapidArc plan delivered with the phantom was found to be 1.2. The largest observed fluence deviation during the delivery for patient was 5.7% and the maximum standard deviation was 4.1%. Conclusions: From this study, it is found that the variation in exit fluence due to patient interfraction organ motion found to be significant than the deviations caused by the machine and detectors.

SU‐GG‐T‐223: Characterization of Responses of 2D‐Array Seven29 Detector and Its Combined Use with Octavius Phantom for the Patient Specific Quality Assurance in Rapid Arc Treatment Delivery

Purpose: To study the dosimetric characteristics of 2D Seven29 ion chamber array and henceforth to perform the patient specific QA for RapidArc treatment delivery in combination with Octavius phantom. Method and Materials: The dosimetric characteristics of PTW 2D seven29 array such as linearity, reproducibility, dose rate, output factors, and directional dependency were evaluated for 6 and 15 MV X‐rays using CLINAC 2100. Performance of detector for measuring clinical dose maps for open and wedge modulated fields and MLC QA were studied. The pre‐treatment patient specific QA of RapidArc for ten different clinical cases was performed. Results: The detector response to dose was linear within the range of 2–500 MUs. Standard deviation for short and long term (5 months) reproducibility were 0.1% and ±1% respectively. The detector response to dose rates was independent with a standard deviation of ±0.7 for 6MV and ±0.5 for 15MV. Output factor showed no significant deviation. The directional dependency for static fields is found to be less than 1.0% when the array is irradiated from the front side. Variation of −4.9% for 6MV and − 5.95% for 15MV was found when the array is irradiated parallel to the beam axis. Less than 4% variation was observed when the beam is incident through the rear of the array for both the energies. MLC and wedge modulated fields matched very well with ion chamber and film measurements. For pre‐treatment QA of RapidArc, Gamma analysis shows that 95% of evaluated dose points in planned and delivered fluence agree for 3mm DTA, 3% dose difference (except for two studies). Conclusion: This study concludes that 2D Array Seven29 is a reliable and accurate dosimeter. The combination of 2D Array with Octavius phantom proved to be a fast and reliable method for pretreatment verification of RapidArc with a satisfactory accuracy for clinical practices.

SU‐GG‐T‐35: A Study on Effect of Inter‐Fractional Anatomical Changes on IMRT and RapidArc Dose Distribution for Carcinoma of Cervix Uteri

Purpose: To investigate the effect of internal anatomical changes on dose delivered by IMRT and RapidArc (RA) for the patients with carcinoma of uterine cervix using kV‐CBCT. Method and Materials: Five patients were taken for this study. In CTimages, target (CTV minus nodes), PTV, bladder, rectum and femoral head were contoured. IMRT with seven fields of equal gantry spacing and dual arc RapidArc with gantry angles: 181–179 and 179–181 degree were done to deliver 50.4Gy to PTV in 28 fractions. Pre‐fraction CBCT were acquired weekly and target and OARs were delineated. The use of CBCT for dose calculation was validated prior to the study. From the CT plan, verification plans were created on CBCTimages. The dose variation in target and OARs between the CT and periodical CBCT (pCBCT) based IMRT and RA plans were analyzed. Results: The mean (±SD) of pCBCT based IMRT plans target D98% and D2% doses were 98.33%±0.92% and 99.92%±1.28% relative to CT based IMRT plan. Similarly for RA, it was 98.53%±0.78% and 100.27%±1.03% respectively. For rectum, the percentage difference between CT and pCBCT based IMRT plans mean dose, D2% and D30% were 2.17%, 0.64% and 1.02% respectively. For RA, it was 2.30%, 0.72% and 0.70% respectively. For bladder, the percentage difference between CT and pCBCT based IMRT plans mean and D35% doses were 1.07% and 0.26%. For RA, it was 1.22%, and −0.04% respectively. For femoral head, the percentage difference between CT and pCBCT IMRT plans D2% dose is −0.46% and for RA, it was −0.35%. Conclusion: Both IMRT and RA plans shows similar dose variation due to the anatomical changes over weeks. The dose variation in target was minimal. However the OAR doses show a substantial variability over weeks.