Daily Setup Shifts Research Articles

SU‐F‐T‐516: Effects of Inter‐Fraction Organ Displacement/deformation On the Delivered Doses to the Heart, Esophagus, and Lungs in Patients Receiving Thoracic Radiotherapy

Purpose:To quantitatively assess the effects of inter‐fraction changes in organ shape and location on the delivered dose distribution to the organs at risk (OAR) in lung cancer patients.Methods:This study analyzes treatment data of 10 patients, who were treated to 60Gy in 30 fractions. In each fraction a cone beam CT (CBCT) was acquired. Each CBCT was registered with the planning CT using deformable registration tools within MIM Software. The daily setup shifts were used to translate the planned dose distribution on the deformed planning CT. The structures of lungs, esophagus and heart were re‐delineated by a physician on each CBCT. The doses delivered to each OAR, reflecting changes in the position and shape variations, were recomputed. Resultant daily dose volume histograms (DVHs) for OARs were computed and compared to those from the planning CT.Results:Based on the findings of two patients and 24 CBCTs analyzed so far, higher doses are delivered to the lungs and esophagus compared to the treatment plan. The dose differences per fraction between the delivered doses and those in the treatment plan are: for patient 1, lung mean dose = 5.3±1.3cGy and esophagus mean dose = 3.4±3.5cGy. For patient 2, lung mean dose = 12.0±3.9cGy and esophagus mean dose = 34.2±7.5cGy. Regarding the maximum dose to heart, the results varied (−18.9±22.0cGy for patient1 and 53.0±62.2cGy for patient2).Conclusion:The dosimetric effects of inter‐fractional anatomical variations could be estimated using deformable image registration and manual organ segmentation for each CBCT. A considerable dose distribution variation between fractions was observed for the OARs. These changes are currently not taken into account while treating the patients and these may explain cases with severe side effects even when the treatment plan looks satisfactory. These results suggest the need for automated daily dose tracking and accumulation.

Daily setup uncertainty analysis for craniospinal irradiation using helical tomotherapy

PurposeSetup deviations of a craniospinal irradiation (CSI) patient cohort previously treated with helical tomotherapy were used to derive the optimal planning target volume (PTV) margin for CSI patients treated in the supine position. Methods and MaterialsThe daily setup shifts of 27 CSI tomotherapy patients as determined using pretreatment megavoltage computed tomography verification for a total of 454 fractions were evaluated. Translational displacements in the mediolateral (ML), craniocaudal (CC), and anteroposterior (AP) directions were used to assess the systematic and random setup errors, and to derive the PTV margin required when daily image guidance is unavailable. ResultsThe individual patient mean shifts and the corresponding standard deviations in the ML, CC, and AP directions were −0.5 ± 2.1 mm, 1.0 ± 2.7 mm, and 0.7 ± 1.1 mm, respectively. The random errors in the corresponding directions were 1.9 mm, 1.9 mm, and 2.2 mm. The PTV margins required in the absence of daily imaging were 3.7 mm to the right, 4.7 mm to the left, 4.4 mm inferior, 6.4 mm superior, 1.6 mm posterior, and 3.0 mm anterior. ConclusionsThe current PTV margin of 3.0 mm is sufficient to ensure clinical target volume coverage for CSI tomotherapy patients treated with daily megavoltage computed tomography imaging. When pretreatment verification imaging is not used to lower the setup uncertainty, a larger PTV margin of up to 6.4 mm in the CC direction will be needed.

SU‐E‐T‐778: Application of Robust Fluence Approach in IMRT under Clinical Daily Shifts

Purpose: To verify the applicability of robust fluence approach in IMRT under clinical daily shifts. Method and Materials:Clinical daily setup shifts throughout the prostate IMRT treatments have been acquired and analyzed with OBI and CBCT images. Two patient shifts have been chosen to represent the “good” and “bad” scenarios — more and less of Gaussian‐like distributions of the shift statistics with average standard deviation of 3.5–4 mm. The robust fluencies have been obtained by deconvolution of the static fluence with the corresponding shifts approximated by a Gaussian distribution. To each scenario the doses taken into account the shifts of a typical AP field to a water phantom have been calculated with the static fluence and corresponding robust fluence. The static dose without shifts which is the desired dose was set as the benchmark for comparison. Results: The robust fluence approach can achieve good dose conformality to the static dose, especially for the Gaussian‐like distribution of the shifts. For the shifts having large deviation from the Gaussian distribution (although it is statistically rare) the dose by robust fluence suffers a shift which can be covered by a 2.5 mm margin which is much smaller than a margin‐only approach to keep the same dose conformality for this scenario.This conformality provides the possibility to remove or reduce the margin usually added to target to take into account the geometrical uncertainty. Conclusions: Robust fluence approach is promising to remove or reduce the CTV‐PTV margin and consequently reduce the mean dose to surrounding normal tissue.

Systematic and Random Dosimetric Impact Studies on 100 Helical Tomotherapy Cancer Cases

To assess the dosimetric consequence of inter-fractional setup shifts on helical tomotherapy plans with self-developed independent dose calculation software MU-Tomo. A 2nd dose validation software for helical TomoTherapy , called MU-Tomo, has been developed to independently validates point dose upon archived patient documents, initial coordinates and planned dose of the point of calculation, and common dosimetric functions. This work has studied a hundred helical tomotherapy patients from five different treatment sites (30 prostate, 26 head and neck, 18 lung, 17 pelvis, and 9 brain patients). The daily setup shifts were quantified and grouped into systematic (mean daily setup shift for each patient) and random shifts (shifts after corresponding systematic shifts subtraction). Both Systematic and random shifts were incorporated into MU-Tomo to evaluate the systematic and random variations of dosimetric consequences, separately. Systematic variations showed dose deviation with the largest one -10.02% compared to the planned dose and overall SD 3%. Mean random variations showed dose deviation with the largest one -5.65% compared to the planned dose and overall SD 1.9%. According to the ANOVA analyses, random dosimetric variations were found significantly different among specific patient, while systematic dosimetric variations were significantly different between head and neck and brain group and body group. No significant differences were discovered among specific patients for systematic variations, and no significant differences were observed within each of the two groups for random variations. Dosimetric consequences are not significantly correlated with treatment fraction number according to the Pearson correlation analysis. By comparing doses without any shift and with the random shift, the overall dosimetric impacts to each patient are small with the mean value -0.0053% and SD of 1.11%, and 99% of the averaged variation results were within 3.5%. For helical tomotherapy modality, the overall dosimetric impact from random variations is small; instead, systematic shifts cause more dosimetric impact.

MO‐EE‐A3‐04: Development and Dosimetric Studies of Independent Dose Validation Software on Helical Tomotherapy

Purpose: To develop a 2nd dose validation software for helical TomoTherapy, study the sensitivity of the commission data variation on the final dosimetry impact, and inter‐fraction setup uncertainty effect for patient quality assurance. Method and Materials: A 2nd dose validation software for helical TomoTherapy, called MU‐Tomo, has been developed to independently validates point dose upon archived patient documents, initial coordinates and planned dose of point of calculation, and common dosimetric functions. MU‐Tomo has been validated with a hundred cancer cases (30 prostate, 26 head&neck, 18 lung, 17 pelvis, and 9 brain patients). Sensitivity studies were performed by oscillating fluctuation regions of off‐axis profiles, shifting, and rotating profiles. Daily setup shifts were quantified into systematic and random shifts to evaluate dosimetric variations, separately. Results: For dose validation, 98% of dose differences are within ±5% with mean 0.20%±2.06%. Sensitivity studies show linear response by oscillating OARy, 15 times larger dose variation by shifting OARy than OARx, and less than 1.5% difference by rotating OARx in ±6° and more than 5% in ±1° by rotating OARy. Systematic variations are up to −10.02%±3.00%. Mean random variations are up to −5.65%±1.90%. ANOVA analyses show significant differences among patient random dosimetric variations and systematic dosimetric variations between head&neck‐brain group and body group. Variations are not significantly correlated with treatment fraction number with the Pearson correlation analysis. The overall random dosimetric impacts to each patient are ‐ 0.0053%±1.11%. Conclusion: MU‐Tomo, has been developed for TomoTherapy dose validation. Sensitivity studies on fifty patients have been evaluated that OARy profiles are more sensitive than OARx in dose calculation. Dosimetric consequences due to inter‐fractional setup shifts on a hundred helical tomotherapy patients were assessed.Conflict of Interest: This project was supported in part by Oncology Data Systems, Inc., Oklahoma City, OK, USA.

DOSIMETRIC IMPACT OF DAILY SETUP VARIATIONS DURING TREATMENT OF CANINE NASAL TUMORS USING INTENSITY‐MODULATED RADIATION THERAPY

Intensity-modulated radiation therapy (IMRT) can be employed to yield precise dose distributions that tightly conform to targets and reduce high doses to normal structures by generating steep dose gradients. Because of these sharp gradients, daily setup variations may have an adverse effect on clinical outcome such that an adjacent normal structure may be overdosed and/or the target may be underdosed. This study provides a detailed analysis of the impact of daily setup variations on optimized IMRT canine nasal tumor treatment plans when variations are not accounted for due to the lack of image guidance. Setup histories of ten patients with nasal tumors previously treated using helical tomotherapy were replanned retrospectively to study the impact of daily setup variations on IMRT dose distributions. Daily setup shifts were applied to IMRT plans on a fraction-by-fraction basis. Using mattress immobilization and laser alignment, mean setup error magnitude in any single dimension was at least 2.5 mm (0-10.0 mm). With inclusions of all three translational coordinates, mean composite offset vector was 5.9 +/- 3.3 mm. Due to variations, a loss of equivalent uniform dose for target volumes of up to 5.6% was noted which corresponded to a potential loss in tumor control probability of 39.5%. Overdosing of eyes and brain was noted by increases in mean normalized total dose and highest normalized dose given to 2% of the volume. Findings suggest that successful implementation of canine nasal IMRT requires daily image guidance to ensure accurate delivery of precise IMRT distributions when non-rigid immobilization techniques are utilized. Unrecognized geographical misses may result in tumor recurrence and/or radiation toxicities to the eyes and brain.

Assessing the Dosimetric Consequence of Inter-fractional Setup Shifts on Helical TomoTherapy Plans with Independent Dose Calculation

This work studied on dosimetric impact due to inter-fractional uncertainties for one hundred patients from five different treatment sites (30 prostate, 26 head & neck, 18 lung, 17 pelvis, and 9 brain patients) for Tomotherapy modality. Daily setup shifts were quantified and grouped into systematic (mean daily setup shifts) and random shifts (fractionbased shifts with corresponding systematic shift subtraction). Both systematic and random shifts were incorporated into in-house independent point dose calculation software, MU-Tomo, to separately evaluate the systematic and random dosimetric variations. Systematic dosimetric variations showed large dose deviation, with the largest difference at -10.02% compared to the planned dose and 3% standard deviation. Mean random dosimetric variations showed relatively small dose deviation with the largest at -5.65% compared to the planned dose and 1.9% standard deviation. Furthermore, different treatment sites were sorted into the head & neck and brain group, and the body group including lung, pelvis, and prostate cancers. According to ANOVA analyses, random dosimetric variations were found significantly different between patients treated at the same treatment site, while systematic dosimetric variations were significantly different between the head & neck and brain group and the body group. No significant differences were discovered among specific patients for systematic dosimetric variations, and no significant differences were observed within each of the two groups for random dosimetric variations. Dosimetric consequences are not significantly correlated with treatment fraction number according to the Pearson correlation analysis. By comparing doses without any shift against those with the random shift, overall dosimetric impacts to each patient were found to be very small with the mean value -0.0053% and standard deviation of 1.11%. Ninety-nine percentage of the averaged variation results were within 3.5%. This implies that overall dosimetric impact from random variations is small; instead, dosimetric impact is more affected by systematic shifts.