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Background and purposeMany dose-limiting normal tissues in radiotherapy (RT) display considerable internal motion between fractions over a course of treatment, potentially reducing the appropriateness of using planned dose distributions to predict morbidity. Accounting explicitly for rectal motion could improve the predictive power of modelling rectal morbidity. To test this, we simulated the effect of motion in two cohorts.Materials and methodsThe included patients (232 and 159 cases) received RT for prostate cancer to 70 and 74 Gy. Motion-inclusive dose distributions were introduced as simulations of random or systematic motion to the planned dose distributions. Six rectal morbidity endpoints were analysed. A probit model using the QUANTEC recommended parameters was also applied to the cohorts.ResultsThe differences in associations using the planned over the motion-inclusive dose distributions were modest. Statistically significant associations were obtained with four of the endpoints, mainly at high doses (55-70 Gy), using both the planned and the motion-inclusive dose distributions, primarily when simulating random motion. The strongest associations were observed for GI toxicity and rectal bleeding (Rs=0.12-0.21; Rs=0.11-0.20). Applying the probit model, significant associations were found for tenesmus and rectal bleeding (Rs=0.13, p=0.02).ConclusionEqually strong associations with rectal morbidity were observed at high doses (>55 Gy), for the planned and the simulated dose distributions including in particular random rectal motion. Future studies should explore patient-specific descriptions of rectal motion to achieve improved predictive power.

In the present study, we aimed to evaluate effects of bladder filling on dose-volume distributions for bladder, rectum, planning target volume (PTV), and prostate in radiation therapy of prostate cancer. Patients (n = 21) were scanned with a full bladder, and after 1 hour, having been allowed to void, with an empty bladder. Radiotherapy plans were generated using a four-field box technique and dose of 70 Gy in 35 fractions. First, plans obtained for full- and empty-bladder scans were compared. Second, situations in which a patient was planned on full bladder but was treated on empty bladder, and vice versa, were simulated, assuming that patients were aligned to external tattoos. Doses to the prostate [equivalent uniform dose (EUD)], bladder and rectum [effective dose (Deff)], and normal tissue complication probability (NTCP) were compared. Dose to the small bowel was examined. Mean bladder volume was 354.3 cm3 when full and 118.2 cm3 when empty. Median prostate EUD was 70 Gy for plans based on full- and empty-bladder scans alike. The median rectal Deff was 55.6 Gy for full-bladder anatomy and 56.8 Gy for empty-bladder anatomy, and the corresponding bladder Deff was 29.0 Gy and 49.3 Gy respectively. In 1 patient, part of the small bowel (7.5 cm3) received more than 50 Gy with full-bladder anatomy, and in 6 patients, part (2.5 cm3-30 cm3) received more than 50 Gy with empty-bladder anatomy. Bladder filling had no significant impact on prostate EUD or rectal Deff. A minimal volume of the small bowel received more than 50 Gy in both groups, which is below dose tolerance. The bladder Deff was higher with empty-bladder anatomy; however, the predicted complication rates were clinically insignificant. When the multileaf collimator pattern was applied in reverse, substantial underdosing of the planning target volume (PTV) was observed, particularly for patients with prostate shifts in excess of 0.5 cm in any one direction. However, the prostate shifts showed no correlation with bladder filling, and therefore the PTV underdosing also cannot be related to bladder filling. For some patients, bladder dose-volume constraints were not fulfilled in the worst-case scenario-that is, when a patient planned with full bladder consistently arrived for treatment with an empty bladder.

Using internal fiducial markers and electronic portal imaging (EPID) to realign patients has been shown to significantly reduce positioning uncertainties in prostate radiation treatment. This creates the possibility of decreasing the planning target volume (PTV) margin added on the clinical target volume (CTV), which in turn may allow for dose escalation. We compared the outcome of two plans: 70Gy/35fx, 10 mm PTV margin without patient realignment (Reference Plan) vs. 78Gy/39fx, 5 mm PTV margin with patient realignment (Escalated Plan). Four-field-oblique (gantry angles 35 degrees, 90 degrees, 270 degrees, 176 degrees, 325 degrees) beam arrangement was used. Monte Carlo code was used to simulate the daily organ motion. Dose to each organ was calculated. Tumour control probability (TCP) and the effective dose to critical organ (Deff) were calculated using the biologically normalized dose-volume histograms. By comparing the biological factors, we found that the prescription dose can be escalated to 78Gy/39fx with a 5 mm PTV margin when using internal fiducial markers and EPID. Based on the available dose-response data for intermediate risk prostate patients, this will result in a 20% increase of local control and significantly reduced rectal complications provided that less serial dose-volume behaviour of rectum is proven.

In the present study, we aimed to evaluate effects of bladder filling on dose-volume distributions for bladder, rectum, planning target volume (PTV), and prostate in radiation therapy of prostate cancer. Patients (n = 21) were scanned with a full bladder, and after 1 hour, having been allowed to void, with an empty bladder. Radiotherapy plans were generated using a four-field box technique and dose of 70 Gy in 35 fractions. First, plans obtained for full- and empty-bladder scans were compared. Second, situations in which a patient was planned on full bladder but was treated on empty bladder, and vice versa, were simulated, assuming that patients were aligned to external tattoos. Doses to the prostate [equivalent uniform dose (EUD)], bladder and rectum [effective dose (Deff)], and normal tissue complication probability (NTCP) were compared. Dose to the small bowel was examined. Mean bladder volume was 354.3 cm3 when full and 118.2 cm3 when empty. Median prostate EUD was 70 Gy for plans based on full- and empty-bladder scans alike. The median rectal Deff was 55.6 Gy for full-bladder anatomy and 56.8 Gy for empty-bladder anatomy, and the corresponding bladder Deff was 29.0 Gy and 49.3 Gy respectively. In 1 patient, part of the small bowel (7.5 cm3) received more than 50 Gy with full-bladder anatomy, and in 6 patients, part (2.5 cm3-30 cm3) received more than 50 Gy with empty-bladder anatomy. Bladder filling had no significant impact on prostate EUD or rectal Deff. A minimal volume of the small bowel received more than 50 Gy in both groups, which is below dose tolerance. The bladder Deff was higher with empty-bladder anatomy; however, the predicted complication rates were clinically insignificant. When the multileaf collimator pattern was applied in reverse, substantial underdosing of the planning target volume (PTV) was observed, particularly for patients with prostate shifts in excess of 0.5 cm in any one direction. However, the prostate shifts showed no correlation with bladder filling, and therefore the PTV underdosing also cannot be related to bladder filling. For some patients, bladder dose-volume constraints were not fulfilled in the worst-case scenario-that is, when a patient planned with full bladder consistently arrived for treatment with an empty bladder.

Using internal fiducial markers and electronic portal imaging (EPID) to realign patients has been shown to significantly reduce positioning uncertainties in prostate radiation treatment. This creates the possibility of decreasing the planning target volume (PTV) margin added on the clinical target volume (CTV), which in turn may allow for dose escalation. We compared the outcome of two plans: 70Gy/35fx, 10 mm PTV margin without patient realignment (Reference Plan) vs. 78Gy/39fx, 5 mm PTV margin with patient realignment (Escalated Plan). Four-field-oblique (gantry angles 35 degrees, 90 degrees, 270 degrees, 176 degrees, 325 degrees) beam arrangement was used. Monte Carlo code was used to simulate the daily organ motion. Dose to each organ was calculated. Tumour control probability (TCP) and the effective dose to critical organ (Deff) were calculated using the biologically normalized dose-volume histograms. By comparing the biological factors, we found that the prescription dose can be escalated to 78Gy/39fx with a 5 mm PTV margin when using internal fiducial markers and EPID. Based on the available dose-response data for intermediate risk prostate patients, this will result in a 20% increase of local control and significantly reduced rectal complications provided that less serial dose-volume behaviour of rectum is proven.

Purpose: To assess the feasibility of using the diaphragm as a surrogate for liver targets during MDTT. Methods: Diaphragm as surrogate for markers: a dome‐shaped phantom with implanted markers was fabricated and underwent dual‐orthogonal fluoroscopy sequences on the Vero4DRT linac. Ten patients participated in an IRB‐approved, feasibility study to assess the MDTT workflow. All images were analyzed using an in‐house program to back‐project the diaphragm/markers position to the isocenter plane. ExacTrac imager log files were analyzed. Diaphragm as tracking structure for MDTT: The phantom "diaphragm" was contoured as a markerless tracking structure (MTS) and exported to Vero4DRT/ExacTrac. A single field plan was delivered to the phantom film plane under static and MDTT conditions. In the patient study, the diaphragm tracking structure was contoured on CT breath‐hold‐exhale datasets. The MDTT workflow was applied until just prior to MV beam‐on. Results: Diaphragm as surrogate for markers: phantom data confirmed the in‐house 3D back‐projection program was functioning as intended. In patients, the diaphragm/marker relative positions had a mean ± RMS difference of 0.70 ± 0.89, 1.08 ± 1.26, and 0.96 ± 1.06 mm in ML, SI, and AP directions. Diaphragm as tracking structure for MDTT: Building a respiratory‐correlation model using the diaphragm as surrogate for the implanted markers was successful in phantom/patients. During the tracking verification imaging step, the phantom mean ± SD difference between the image‐detected and predicted "diaphragm" position was 0.52 ± 0.18 mm. The 2D film gamma (2%/2 mm) comparison (static to MDTT deliveries) was 98.2%. In patients, the mean difference between the image‐detected and predicted diaphragm position was 2.02 ± 0.92 mm. The planning target margin contribution from MDTT diaphragm tracking is 2.2, 5.0, and 4.7 mm in the ML, SI, and AP directions. Conclusion: In phantom/patients, the diaphragm motion correlated well with markers' motion and could be used as a surrogate. MDTT workflows using the diaphragm as the MTS is feasible using the Vero4DRT linac and could replace the need for implanted markers for liver radiotherapy. [ABSTRACT FROM AUTHOR]