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Dosimetric Effect of Unplanned Rectal Gas Cavities for 1.5-T MR-Linac Guided Prostate SBRT

M Wronski1*, M Ruschin1, C Mccann1, M Davidson1, B Keller1, A Kim1, J Detsky2, A Loblaw2, K Wong3, M Campbell3, D Vesprini2, (1) Sunnybrook Odette Cancer Centre, Department Of Medical Physics, Toronto, ON, CA, (2) Sunnybrook Odette Cancer Centre, Department Of Radiation Oncology, Toronto , ON, CA, (3) Sunnybrook Odette Cancer Centre, Department Of Radiation Therapy, Toronto, ON, CA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: investigate the effect of unplanned rectal gas cavities on clinically relevant dosimetric criteria in prostate SBRT patients treated on a 1.5-T MR-Linac (MRL).

Methods: prostate SBRT patients were treated on MRL with a prescribed dose of 40 Gy in 5 fractions. Daily IMRT treatment planning was performed in Monaco using 11 beams. The planning target volume (PTV) consisted of the prostate and proximal 1 cm seminal vesicles with an added 4 mm margin. Any rectal gas cavities were overridden with an electron density of 1 g/cc to yield plans insensitive to potential changes in gas volume and location during treatment delivery. Retrospectively, an unplanned gas cavity was included in the plans by overriding the rectum to a density of 0 g/cc with the exception of a region within 3 mm of the rectal contour which represented the rectal wall. To quantify the combined effect of lack of attenuation, electronic disequilibrium and a possible electron return effect at the interface between the unplanned gas cavity and rectal wall, clinically-relevant dose-volume criteria were extracted for rectal wall and PTV.

Results: presence of an unplanned gas cavity resulted in minor changes of 0.02±0.84% and 0.04±0.23cc in rectal wall maximum dose and V38Gy, respectively. The PTV volume receiving 105% of the prescribed dose increased by 5.8±4.9%, most likely due to a reduction in photon attenuation for the posterior IMRT beams. All dosimetric constraints were met with the rectal gas present with the exception of 2/5 patient plans where the PTV V105% exceeded a 10% tolerance. These findings are conservative, as the gas cavity is unlikely to remain in place for the entire treatment.

Conclusion: rectal gas cavities are unlikely to result in clinically significant dosimetric changes for prostate SBRT patients treated with 11 IMRT fields on a 1.5-T MRL.


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