S Srivastava

SU‐FF‐T‐245: Feasibility Study of Focused and Non‐Focused Photon MLC for Electron IMRT

Introduction: Modulated electron radiation therapy (MERT) could be advantageous for some disease sites. Different modes of MERT have been investigated, such as optimization of energy for each entry angle, electron MLC, etc. Feasibility of using photonMLC for MERT is explored in this study. The depth doses, profiles, penumbra (90%–10%), lateral spread (10%–1%) and radiation leakage for energies 6–21 MeV and at various source‐skin‐distance (80, 90, 100 cm) are investigated. Materials & Methods: Using both a Varian (with non‐focused MLC) and a Siemens (focused MLC) accelerators, beam characteristics at dmax are measured for possible beamlets from 1×1cm2 – 10×10 cm2 for each electron energy at 80, 90 and 100 cm SSD. The profiles are collected using film dosimetry in solid water as well as ion chamber and electron diode measurements in a scanning water tank. Results: For both the focused and non‐focus MLC, profiles obtained with 100 cm ssd exhibit a large penumbra (90–10%) in the range 4.2–7.5 cm for energies 6–21 MeV, the lower the energy, the larger the penumbra. At higher energies and 80–90 SSDs, the penumbra is much reduced. For example, for the focused MLC, it is 23 mm at dmax for 18 MeV, while for the non‐focused MLC, it is 11 mm for the 20 MeV. The percent depth dose(PDD) curves though not as steep as that with an electron cone, are clearly more advantageous compared to a photon PDD with smaller exit dose.Conclusions: The key to MERT with existing photonMLC is to reduce the source‐skin distance, while maintaining sufficient clearance for isocentric treatment. Our measurements indicate that beamlets <10×10, electron energies ⩾ 12 MeV and SSD ⩽ 90cm may provide clinical acceptable combinations for MERT with photonMLC. Focused and non‐focused MLC differ slightly in the beam characteristics.

The dosimetric and radiobiological impact of calculation grid size on head and neck IMRT

PURPOSE Small-volume structures usually found in the head and neck may be susceptible to dose-volume averaging, which has not been studied. Here, the impact of calculation grid size on dose distribution for tumor control probability (TCP) and normal tissue complication probability (NTCP) is investigated for head and neck (H&N) intensity modulated radiation therapy (IMRT). METHODS AND MATERIALS IMRT plans were generated for H&N patients with different grid sizes (1-5 mm) to calculate dose and related TCP and NTCP. Dose parameters such as D2%, D50%, D98%, and the homogeneity and conformity indices were calculated. The dose distributions were also compared with measured dose for all IMRT plans. A 1-tailed pair t test was used to analyze the data. RESULTS The mean dose to planning target volume and TCP decreases with increasing grid size, whereas for organs at risk (OARs), mean dose, and NTCP increase with increasing grid size. The average mean dose to planning target volume decreases linearly with grid size, but for OARs such as cochlea, parotid gland, and the spinal cord, mean dose increases with grid size. IMRT dose verification showed that the number of points meeting the gamma criterion of 3%/3 mm increased with decreasing grid sizes. The homogeneity index for the target increased up to 60% and conformity index decreased on average by 3.5% between 1- to 5-mm grid that resulted in decreased TCP and increased NTCP. A 1-tailed pair t test showed significant statistical differences among various grid size calculations compared with 1-mm grids. CONCLUSIONS Based on our findings, the smallest possible grid size should be used for accurate dose calculation in small-volume structures-especially in H&N planning. A smaller calculation grid provides superior dosimetry with improved TCP as well as reduced NTCP, which is more pronounced for smaller OARs.

Impact of rectal balloon-filling materials on the dosimetry of prostate and organs at risk in photon beam therapy

The use of rectal balloon in radiotherapy of prostate cancer is shown to be effective in reducing prostate motion and minimizing rectal volume, thus reducing rectal toxicity. Air‐filled rectal balloon has been used most commonly, but creates dose perturbation at the air‐tissue interface. In this study, we evaluate the effects of rectal balloon‐filling materials on the dose distribution to the target and organs at risk. The dosimetric impact of rectal balloon filling was studied in detail for a typical prostate patient, and the general effect of the balloon filling was investigated from a study of ten prostate patients covering a wide range of anterior–posterior and left–right separations, as well as rectal and bladder volumes. Hounsfield units (HU) of the rectal balloon filling was changed from −1000 HU to 1000 HU at an interval of 250 HU, and the corresponding changes in the relative electron density (RED) was calculated. For each of the HU of the rectal balloon filling, a seven‐field IMRT plan was generated with 6 MV and 15 MV photon beams, respectively. Dosimetric evaluation was performed with the AAA algorithm for inhomogeneity corrections. A detailed study of the rectal balloon filling shows that the GTV, PTV, rectal, and bladder mean dose decreased with increasing values of RED in the rectal balloon. There is significant underdosage in the target volume at the rectum–prostate interface with an air‐filled balloon as compared to that with a water‐filled balloon for both 6 MV and 15 MV beams. While the dosimetric effect of the rectal balloon filling is reduced when averaged over ten patients, generally an air‐filled balloon results in lower minimum dose and lower mean dose in the overlap region (and possibly the PTV) compared to those produced by water‐filled or contrast‐filled balloons. Dose inhomogeneity in the target volume is increased with an air‐filled rectal balloon. Thus a water‐filled or contrast‐filled rectal balloon is preferred to an air‐filled rectal balloon in EBRT of prostate treatment. PACS numbers: 87.55.D‐, 87.55.de, 87.55.dk, 87.55.Gh, 87.55.kd

SU‐E‐T‐358: Penumbral Dose with Limited Scatter in Photon Beams with and Without Flattening Filter

Purpose: Dose outside the active radiation beam plays an important role in estimation of critical organs complications and also an important contributing factor in IMRT plan optimization. In this study, we investigated the effect of scatteringmaterial and characteristics of penumbral dose between conventional accelerator and machine with flattening free filter (FFF). Methods: A Varian True Beam was used that has FFF option for low energy beam. Penumbral dose was measured in a water phantom for all energies (6, 10, and 15 MV) with and without the flattening filter. Beam profiles were collected using a 0.125 cm3ion chamber at various depths, from dmax‐15cm for a set of field sizes. To study the effect of limited scatter in the penumbra, the water tank was shifted to reduce the scattering distance from the tank and field edge systematically from 5–20 cm including tank wall. The measured data were also compared with the Eclipse treatment planning data. Results: For all beams with the flattening filter, penumbral dose vary from 5% to 12% from 5 cm to 15 cm depth. The variation is larger for lower energies and is a linear function of depth for all energies. It increases with depth linearly and decreases with beam energy. For FFF beam the slope is greater than that of a regular beam. The penumbral dose varies by <1% with scattering condition. For the same depth, the penumbra is significantly larger for FFF. There is significant difference between TPS FFF data compared to the measured values. Conclusion: Penumbral dose is dependent on scattering distance, depth, beam energy and flattening filter. FFF provides relatively higher dose which is not properly modeled in treatment planning system. For high degree of precision, data should be collected with large scattering condition and beam modeling for FFF should be adequately performed.

SU-E-T-319: The Effect of Slice Thickness On IMRT Planning

PURPOSE The accuracy of volume estimated of a treatment planning system is investigated in this study. In addition, the effect of slice thickness on IMRT planning is also studied. METHODS The accuracy in volume determination was investigated using a water phantom containing various objects with known volumes ranging from 1-100cm3 . The phantom was scanned with different slice thickness (1-10 mm). The CT data sets were sent to Eclipse TPS for contour delineation and volume calculation. The effect of slice thickness on IMRT planning was studied using a commercial phantom containing four different shaped objects. The phantom was scanned with different slice thickness (1-5 mm). IMRT plans were generated for the different CT datasets to calculate TCP, homogeneity (HI) and conformity indices (CI). RESULTS The variability of volumes with CT slice thickness was significant especially for small volume structures. The minimum and maximum error in the volume estimation is in the range of -2.3% to 92%. On the other hand, with increasing slice thickness, the PTV mean dose and TCP values decreases. Maximum variation of ∼5% was observed in mean dose and ∼2% in TCP with slice thickness change from 1-5 mm. The relative decrease in target volume receiving 95% of prescribed dose is ∼5% slice thickness change from 1-5 mm. HI increases up to 163% and CI decreases by 4% between 1-5 mm slice thickness change, producing highly inhomogeneous and least conformal plan. CONCLUSION Accuracy of volume estimation is dependent on CT slice thickness and the contouring algorithm in a TPS. During TPS commissioning and for all clinical protocols, evaluation of volume should be included to provide the limit of accuracy in DVH calculation. A smaller slice thickness provides superior dosimetry with improved TCP values. Thus, the smallest possible slice thickness should be used for IMRT planning.

SU-E-T-454: Impact of Calculation Grid Size On Dosimetry and Radiobiological Parameters for Head and Neck IMRT

PURPOSE IMRT has become standard of care for complex treatments to optimize dose to target and spare normal tissues. However, the impact of calculation grid size is not widely known especially dose distribution, tumor control probability (TCP) and normal tissue complication probability (NTCP) which is investigated in this study. METHODS Ten head and neck IMRT patients treated with 6 MV photons were chosen for this study. Using Eclipse TPS, treatment plans were generated for different grid sizes in the range 1-5 mm for the same optimization criterion with specific dose-volume constraints. The dose volume histogram (DVH) was calculated for all IMRT plans and dosimetric data were compared. ICRU-83 dose points such as D2%, D50%, D98%, as well as the homogeneity and conformity indices (HI, CI) were calculated. In addition, TCP and NTCP were calculated from DVH data. RESULTS The PTV mean dose and TCP decreases with increasing grid size with an average decrease in mean dose by 2% and TCP by 3% respectively. Increasing grid size from 1-5 mm grid size, the average mean dose and NTCP for left parotid was increased by 6.0% and 8.0% respectively. Similar patterns were observed for other OARs such as cochlea, parotids and spinal cord. The HI increases up to 60% and CI decreases on average by 3.5% between 1 and 5 mm grid that resulted in decreased TCP and increased NTCP values. The number of points meeting the gamma criteria of ±3% dose difference and ±3mm DTA was higher with a 1 mm on average (97.2%) than with a 5 mm grid (91.3%). CONCLUSION A smaller calculation grid provides superior dosimetry with improved TCP and reduced NTCP values. The effect is more pronounced for smaller OARs. Thus, the smallest possible grid size should be used for accurate dose calculation especially in H&N planning.

SU-E-T-243: Analysis of Planner Dependence in IMRT

Purpose: To establish trends in the observed variations within large sets of IMRT treatment plans. Past data of comparable treatments are statistically analyzed to identify clinical preferences and establish best practices for future decision‐making processes. Methods: We propose three statistical measures of variations in the DVH of 100 head and neck cancer patients from IU‐Simon Cancer Center: a) Standard deviation for delivered dose, std(D), to specific volumes, b) standard deviation for volume, std(V), receiving a specific of dose, and c) a steepness measure for DVH and 1‐standard deviation at specific dose points as a measure for falloff preference. Results: The three methods show considerable variation amongst DVHs: a) Negligible dose deviation until 70% volume, a steady increase to std(D)=22% at 100% volume, followed by a steady decrease to std(D)=0% at 115% volume. On the other hand, b) standard deviation of volume is rather constant at 3% until at high volumes (100%), where it increases to 15%. c) Average steepness of DVH over the 100 patients is negligible until 94% dose, after which it falls steadily to −13 at 104% dose. It then steadily climbs back to 0 at 110% dose. This change depicts the dose falloff on a typical DVH. The 1 standard deviation spread is most evident between 96% and 110% dose, with a maximum of 7.2 at 103% dose. Furthermore, there is a ripple effect at 85% dose with a standard deviation of 1.8 that is distinct from the usual trend. Conclusion: Unanimous decisions for DVHs exist only at lower values of volume and dose. However, significant variations are found in the falloff region of the DVH. For this region, the increased variations in the steepness suggest that individual preferences are more divergent. A closer unanimity in future decision‐making may lead to enhanced planner independence of IMRT treatments. Joint Purdue‐IU Seed Grant

U‐E‐T‐620: Radiobiological Implication of Margin for Target Expansion in Head and Neck IMRT with Daily IGRT

PURPOSE To account for organ motion and set up uncertainties a margin is added to the clinical target volume (CTV) to form the Planning Target Volume (PTV). There exists significant institutional variability of margins employed between CTV to PTV. The margin used has significant implication for extra tissue (PTV-CTV) dose and directly related with normal tissue complication. This study quantifies the setup uncertainties to optimize the PTV margin and its radiobiological implication for target expansion in H&N cancer. METHODS Nine Nine H&N patients treated with IMRT and daily IGRT with on-board-imaging (OBI) were chosen for this study. Using Eclipse treatment planning system (TPS), treatment plans were generated for different margins ranging from 0 to 10 mm subject to the same optimization criterion for PTV and OAR using 6 MV photon beam. The DVH, extra tissue volume and total MU were recorded for all these IMRT plans. NTCP was calculated using the Lyman Kutcher Burman model from DVH data. The daily setup errors from OBI for the entire treatment were also analyzed. RESULTS Analysis of the 9 patients setup with over 800 data points showed that 98% of the points are within ±5mm using daily IGRT. With increasing margin, the PTV volume increases linearly. There is a 4-fold relative increase in extra tissue volume for margin increase from 0-10 mm With increasing PTV margin the NTCP also increases linearly for the parotid glands. Similar patterns were noticed for all other OARs. CONCLUSION With OBI the setup uncertainty could be easily achieved within ±5 mm for 98% of the H&N treatments. Increase in PTV margin increases NTCP. It is concluded that PTV margin should be limited to ±5mm to reduce extra volume irradiation and also reduces NTCP of OARs.

SU‐E‐T‐632: Study the Effect of Grid Size On Head and Neck IMRT Dosimetry

PURPOSE In IMRT, ICRU-83 requires dose prescription to be at D50% also as mean dose. Dosimetry is impacted by the calculational grid size that is studied for head and neck IMRT with special consideration on the tumor control probability (TCP) and normal tissue complication probability (NTCP). METHODS Five head and neck patients treated with IMRT and daily IGRT were chosen for this study. Using Eclipse treatment planning system (TPS) treatment plans were generated for different grid sizes in the range 1.0 mm-5.0 mm for the same optimization criterion with a 6MV photon beam. The desired dose volume constraints were kept the same to provide uniform dose to the PTV. The dose volume histogram (DVH) was recorded for all IMRT plans and dosimetric data were compared. ICRU-83 dose points such as D2%, D50% and D98% were also calculated. In addition, TCP and NTCP were calculated using LQ Poisson model and Lyman Kutcher Burman model respectively from DVH data. RESULTS The PTV mean dose decreases with increasing grid size from 1mm to 5mm with an average decrease of 1.5%. It is also found that TCP decreases with increase in grid size with an average decrease of 1.6%. Mean dose and NTCP for the right parotid increases with increase in grid size with an average increase in mean dose by 1.6% and NTCP by 4.8%. Similar patterns were observed for all other OARs: left parotid, mandible, brainstem and spinal cord. CONCLUSION It is concluded that smallest possible grid size should be used for accurate dose calculation in head and neck IMRT planning. Smallest calculational grid also improves TCP for target volumes and reduces NTCP for normal tissues. The grid size effect is most important for smaller structures such as cochlea, optic chiasm and lens.

SU‐E‐T‐604: Inter Planner Dosimetric Variations in IMRT

PURPOSE To study inter-planner variations in IMRT dosimetry with identical dose prescription in IMRT optimization for estimation of quality of treatment plans by using dose volume histogram (DVH). METHODS Five treatment planners: p1, p2, p3, p4 and p5 with Eclipse TPS and Varian accelerator were chosen. The 3D data sets including PTV and OAR of a prostate case was sent with instructions for beam orientations, energy and dose-volume constraints. Each planner performed the treatment planning process with inhomogeneity corrections by using the same dose constraints. DVH and isodose distributions were compared to examine the inter-planner variations. RESULTS There were large variations in the IMRT plans reflected by the DVH among the planners even with identical dose constraints in IMRT optimization. Clinically constraint#1 (D95-V95) dose criteria could be considered as the most important one. Two planners did not meet the criteria, whereas planner 4 met all of the dose criteria for PTV and OARs. The inter-planner variations in PTV as large as 20% were found in this study. Every time optimization is performed even with the same constraints, the DVH output was significantly different due to the differences in constraint weights. CONCLUSIONS There exists a significant inter planner variation for all cases studied indicating that even for the same constraints the outcome cannot be guaranteed to be identical due to cost function convergence. It seems feasible to control the over-dosage for all planners. However we see large deviations in plans with respect to different constraints. Planner 4 met all the dose criteria for PTV and OARs indicating that it is possible to plan and meet all the dose criteria. This approach could be used as gold standard.