Session: Treatment Planning - Salle de bal et foyer

June 25, 2022 from 12:00pm EST to 1:00pm EST

Scientific Session – Treatment Planning
Saturday, June 25, 2022, 12:00-13:00

Scientific Session Treatment Planning – Presentation 1

Initial setup and target tracking during treatment using Calypso electromagnetic system for prostate SBRT

Sankar Venkataraman, Susannah Hickling, Rashmi Koul
CancerCare Manitoba

Purpose: Continuous monitoring of the prostate or its surrogate during treatment for stereotactic body radiation therapy (SBRT) is required for treatment accuracy. This study aims to assess the initial setup and intrafraction motion of the prostate using real-time electromagnetic tracking.

Methods: Thirty-nine patients were treated with Calypso-guided prostate SBRT. The three implanted beacon transponders (SUP, MID and INF) reflect the electromagnetic signal from the array placed over the patient. Based on signal strength, the displacement of the treatment isocentre from the machine isocentre was calculated for initial setup and continuous tracking. CBCT was acquired only for verification of beacon locations and regional anatomy. The intertransponder distance throughout the treatment course and the target deviations in all three directions were calculated. A tracking limit of 3 mm was used for gating during treatment.

Results: Of the 190 treatment fractions, 154 fractions (81%) had no couch shifts applied during treatment, indicating the Calypso-tracked target motion was within 3 mm. Of the remaining 36 fractions, 33 fractions had 1 couch shift and 3 fractions had 2 couch shifts with a maximum displacement of 4 mm. The initial positioning error from pre-treatment CBCT was within 3 mm in all but two fractions. The average intertransponder distance between the planning CT and all treatment fractions was 0.07±0.06 cm for SUP to MID, 0.07±0.06 cm for MID to INF and 0.08±0.06 cm for INF to SUP.

Conclusion: Calypso-guided prostate SBRT for initial setup and target tracking during treatment is feasible and this gated treatment technique improves treatment accuracy.


Scientific Session Treatment Planning – Presentation 2

Initial Clinical Experience Planning and Treating with 4pi Dynamic-Wave Trajectory VMAT for a Cardiac Radiosurgery (CaRS) Program

Steven Thomas, Alanah M. Bergman, Tania Karan, Amir Pourmoughaddas, Isaac Tai, Hardeep Sahota, Cheryl Mark, Marc Deyell, Devin Schellenberg
BC Cancer, Kaiser Permanente-Livermore, St. Paul's Hospital

Purpose: To report on the initial experience of an on-trial cardiac radiosurgery (CaRS) program by describing treatment margins, 4pi dynamic wave trajectory VMAT planning/QA and delivery.

Methods: A collaboration between cardiology and radiotherapy departments resulted in a trial treating ventricular tachycardia with radiosurgery using a dose of 25Gy/1 fraction. A combination of CT, PET and MR imaging plus electroanatomic mapping was used for target definition. To determine motion margins, 4DCT and fluoroscopy imaging was employed. The markerless dynamic tumour tracking (MDTT) module of the Brainlab Vero4DRT was also used to detect and quantify the motion of patients’ implantable-cardioverter-defibrillator (ICD) lead during a pre-treatment assessment. An in-house program was used to isolate cardiac and respiratory motion components from the fluoroscopy images and validate the MDTT ICD motion prediction. 4pi dynamic wave VMAT trajectories (Dynamic-Wave-Arc) plans were created in Raystation (v.7, Raysearch), with geometric avoidance of organs at risk facilitated by an in-house trajectory optimization program. Delivery QA was performed prior to patient treatment. CBCT and fluoroscopic imaging was used for image guidance during treatment.

Results: Two patients have been treated with CaRS using patient-specific 6-arc dynamic-wave-arc VMAT. The trajectories geometrically avoided the stomach. The average monitor units per plan was 6330MU. Motion analysis provided internal target volume (ITV) margins of 5 mm for both patients. First BEAM-ON to last BEAM-OFF time was on average 37min.  Total on-bed time was on average 69 min.

Conclusion: CaRS was successfully delivered to two patients using patient-specific motion margins and 4pi dynamic wave trajectory VMAT.


Scientific Session Treatment Planning – Presentation 3

Evaluating magnetic resonance imaging (MRI)-only simulation and planning for prostate stereotactic body radiotherapy (SBRT)

Melanie Davidson, Joe A Presutti, Ananth Ravi, Ling Ho, Andrew Loblaw, Melanie TM Davidson
Odette Cancer Centre, Sunnybrook Health Sciences Centre - University of Toronto Department of Radiation Oncology

Purpose: Commercial packages deriving synthetic computed tomography (sCT) images from MRI images are becoming available for radiotherapy planning.  This study evaluates the suitability of the Philips MRCAT (Magnetic Resonance for Calculating Attenuation) Prostate package for prostate SBRT.

Methods: Twelve prostate SBRT patients underwent CT and MRI simulation.  Differences in HUs between MRCAT-sCT and CT were captured in a new CT-to-density table.  Plans were computed on both images, accounting for differences in body contour and rectal gas.  Dose was compared using prostate SBRT dose-volume evaluation metrics and gamma analysis (1%/1mm, thresholds= 10%, 20%, 50%, 80%, 90%, to highlight lower and higher dose disagreements). 

Results:  Differences in HU between MRCAT-sCT and CT beyond 0 HU (in bone) were captured in an updated CT-to-density table. Identical plans computed on CT and MRCAT-sCT with our institutional CT-to-density table had reasonable overall dosimetric agreement (gamma pass rates 1%/1mm/10% were >98%). Lower gamma pass rates at 90% threshold (as low as 66.5%) indicates a disagreement at higher doses, also reflected in dose-volume metrics.  When calculations were redone using a CT-to-density table adjusted for the observed HU disagreement between MRCAT-sCT and CT, gamma pass rates (1%/1mm/90%) all increased >98.9% and DVH metrics fell within 2% or 0.4cc of their CT counterparts.

Conclusion: Higher doses for prostate SBRT have been linked to the toxicity of adjacent OARs.  Achieving good high-dose accuracy is thus critical to the safe clinical implementation of an MRI-only planning approach.  Similar dosimetry to CT was achieved with MRCAT Prostate using a new CT-to-density table.


Scientific Session Treatment Planning – Presentation 4

Assessment of intra-fraction motion and PTV margins for single-fraction lung stereotactic ablative radiation therapy (SABR)

Clara Fallone, Clare Summers, Wladyslawa Cwajna, Alasdair Syme
Nova Scotia Health

Purpose: To assess intra-fraction motion and explore reducing PTV (planning target volume) margins in single-fraction lung stereotactic ablative radiation therapy (SABR).

Methods: Thirteen NSCLC patients were treated using linac-based single-fraction lung SABR. A Cone-beam computed tomography (CBCT) image was attained pre-treatment, intra-treatment (between each arc), and post-treatment. The couch correction offsets from the reference CT image were obtained in the vertical, longitudinal, and lateral directions. The scenarios of not applying intra-treatment couch corrections and completing intra-treatment couch corrections were modeled. Motion deviations from the reference image were calculated for the two scenarios, and maximum and average deviations were computed. A van Herk equation was utilized to compute PTV margins for both scenarios.

Results: PTV margins in the vertical, longitudinal, and lateral directions were [4.9, 3.7, 2.4] mm when not applying couch corrections, and [1.8, 1.1, 0.8] mm when applying corrections. Other variability factors add to uncertainty; thus, these calculated values are lower bounds of the PTV margins. The maximum and average intra-fraction deviation 3D vectors, respectively, were 9.9 mm and 1.4 mm when modelling no intra-fraction couch corrections, and 4.9 mm and 0.3 mm when modelling the corrections. Unidirectional movement in the negative vertical direction occurred when intra-fraction corrections were not applied. In 3/13 and 2/13 patients, applying intra-treatment couch corrections yielded larger maximum and average deviations, respectively.

Conclusions: Intra-treatment imaging and motion correction may potentially reduce PTV margin requirements and compensate for observed unidirectional vertical movement. Non-isotropic margins should be considered.


Scientific Session Treatment Planning – Presentation 5

Dosimetric Outcomes for Adaptive Prostate SBRT on MR-Linac: The Time Taken for Contouring and Re-planning Makes a Difference

Eyesha Younus, Andrew Loblaw, Patrick Cheung, William Chu, Chia-Lin Tseng, Danny Vesprini, Jay Detsky, Stanley Liu, Hans Chung, Melanie TM Davidson, Matt Wronski, Mark Ruschin
Sunnybrook Health Sciences Centre

Purpose: Two approaches to MR-Linac are: (1)rigid shift or adapt-to-position (ATP) and (2) full replan or adapt-to-shape (ATS). While ATS enables customization of daily treatment plans, it is unclear how anatomical changes occurring during re-contouring and re-planning impact dosimetry. Here, we compare target volume and organ-at-risk (OAR) dosimetry for both approaches.

Methods: Ten prostate cancer patients treated on MR-Linac to 40Gy/5 (50 fractions total) at our institution were included in the analysis. During ATS, physicians re-contour on a localization MR image (MR-Loc) and a new plan (ATS-loc) is generated. A verification MRI (MR-Ver) is acquired prior to beam-on to check target position. We compared three plans per fraction:  (1) ATS plan (ATS-loc); (2) ATP plan (ATP-Loc) retrospectively calculated on MR-Loc; and (3)ATS-Loc re-calculated on MR-Ver (ATS-Ver). Key target and OAR dose-volume metrics were analyzed on fractional and dose-accumulated plans.

Results:  The target metric was met in 100% (50/50), 82%(41/50), and 84%(42/50) of ATS-Loc, ATP-Loc and ATS-Ver fractions, respectively. Of the total 150  OAR evaluations (50 fractions x 3 OAR constraints), there were 4, 25, and 12 violations for ATS-Loc, ATP-Loc, and ATS-Ver. On dose-accumulated plans, OAR violations were seen on 1, 3 and none of the patients for ATS-Loc, ATP-Loc, and ATS-Ver.

Conclusion: The ATS strategy improves dosimetry over ATP, especially with regard to OAR sparing. Target coverage violations increase when re-calculating on MR-Ver but are still favorable relative to ATP. Future work will investigate if applying a shift after ATS can mitigate degradation of dosimetry due to anatomical changes.


Scientific Session Treatment Planning – Presentation 6

Characterizing Interfractional Changes in Head and Neck Organ-At-Risk DVHs and Their Association with Patient-Reported Outcomes

Sarah Weppler, Owen Paetkau, Wendy Smith
Tom Baker Cancer Centre, University of Calgary

Purpose: For cancer sites such as head and neck (H&N), plan quality may degrade throughout the treatment course due to interfractional weight loss and/or tumour shrinkage. We characterize changes in H&N organ-at-risk (OAR) DVH parameters and their associations with patient-reported outcomes.

Methods: Using a previously validated deformable image registration workflow, we estimated the weekly delivered OAR DVHs for 60 H&N patients treated with curative-intent chemoradiotherapy (70 Gy/33 fractions). A subset of 23 patients completed MDASI, MDADI, and XQ patient-reported outcomes surveys. Lag-1 differences in DVHs between the initial treatment plan and weekly DVH estimates characterized interfractional dose increases. From these differences, we identified which DVH parameters exhibited the greatest changes and compared average DVH parameter increases between patient groups reporting none/mild vs. moderate/severe outcomes.

Results:  The largest average weekly DVH increases occurred for the spared parotid gland (D15%-D28%: 0.50 Gy/week) and pharyngeal constrictor (D9%-D34%: 0.15 Gy/week); changes in brainstem and spinal cord DVHs were minimal (D1%: <0.08 Gy/week). For patients reporting none/mild vs. moderate/severe symptoms, weekly DVH parameters differed most for the spared parotid gland: (D33%, 0.5 Gy/week) vs. (D33%, 2.0 Gy/week) for the MDASI choking/coughing item; (D33%, 0.6 Gy/week) vs. (D33%, 0.8 Gy/week) for the MDASI dry mouth item; and (D32%, 0.6 Gy/week) vs. (D35%, 1.3 Gy/week) for the XQ.

Conclusion: Increased high doses to the spared parotid gland and pharyngeal constrictor were observed, consistent with previously reported results of geometric shifts of the parotid glands towards high dose regions, and overall dose increases due to patient weight loss.


Scientific Session Treatment Planning – Presentation 7

Patient-specific treatment planning for intraoperative radiotherapy of glioblastoma

David Santiago Ayala Alvarez, Marij Popovic, Veng Jean Heng, Peter G. F. Watson, Michael D. C. Evans, Jan Seuntjens
Department of Physics and Medical Physics Unit - McGill University, Department of Oncology and Medical Physics Unit - McGill University

Purpose: The INTRAGO clinical trial assesses survival in glioblastoma patients treated with intraoperative radiotherapy (IORT) using the INTRABEAM. However, dose estimation relies on in-water depth-dose curves from the manufacturer. This method fails to accurately represent the geometry and tissue heterogeneities of the clinical scenario and does not permit dose addition of IORT with external radiotherapy (EBRT). Exceeding the dose constraints to organs at risk (OAR) can result in therapeutic complications. This work presents a treatment planning framework that allows more accurate calculation of IORT dose and its sum with EBRT plans.

Methods: IORT dose was calculated with Monte Carlo (MC) on 8 clinical cases using the in-house planning system BREMS. Manufacturer-provided dose data was compared with the MC dose simulations reported to water and to tissue. The IORT dose, converted to EQD2, and EBRT dose were added in BREMS. The dose to the OARs was evaluated.

Results:  The dose constraints were significantly exceeded in some OARs when the EBRT dose was combined with the MC calculated IORT dose. For instance, the dose to the brainstem, which was on average 50.3 Gy for the EBRT-only plan, was found to be on average 65.6 and 69.1 Gy when combined with IORT in water and in tissue, respectively. This exceeds the 60 Gy dose constraint, which was not evident with the current INTRAGO standard procedure.

Conclusion: This work presents a comprehensive planning framework for patient-specific dose calculation for IORT, and the tools to combine it with adjuvant EBRT plans.