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Susceptibility of Hypofractionated Radiotherapy of Prostate Cancer to Interfractional Motion

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M Moteabbed

M Moteabbed*, A Trofimov , G Sharp , A Zietman , J Efstathiou , H Lu , Massachusetts General Hospital, Boston, MA


SU-I-GPD-J-71 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To compare the effect of interfractional motion on conventional and hypofractionated pencil beam scanning (PBS) proton therapy and volumetric modulated arc therapy (VMAT) for prostate cancer.

Methods: For 6 patients with low/intermediate risk prostate cancer participating in a randomized clinical trial, serial CT scans were acquired weekly within an hour of treatment. All patients received proton therapy at 79.2Gy(RBE) to prostate and 50.4 Gy(RBE) to proximal seminal vesicles in 44 fractions, delivered by 2 lateral-opposed beams. New PBS plans were created for hypofractionation, featuring a combination of lateral and anterior-oblique beams to limit dose to the femora. VMAT plans were created using a single arc. All plans were recomputed on serial CTs after aligning the images using fiducial markers per clinical protocols. Planned dose was scaled to total dose of 51.6 Gy in 12 fractions and 36.25 Gy in 5 fractions. Fractional doses on weekly scans were similarly scaled and summed using deformable dose accumulation to approximate the delivered dose. Biologically equivalent dose to 2Gy(EQD2) was calculated assuming α/β of 1.5 Gy for prostate and 3 Gy for bladder and rectum.

Results: Target dose degradation was comparable for all fractionation schemes within each modality. The mean prostate D98 for 44/12/5 fractions decreased by 0.12/0.31/0.29 Gy(EQD2) for PBS and 0.52/0.59/0.55 Gy(EQD2) for VMAT, respectively. The maximum prostate D98 reduction was 1.12 Gy(EQD2) for a VMAT case. For bladder and rectum, the planned biological equivalent maximum dose was similar among all fractionation schemes. The mean bladder D2 was reduced by 1.8/3.7 Gy(EQD2) for PBS/VMAT. The mean rectum D2 increased by 0.68/0.98 Gy(EQD2) for 12/5-fraction PBS.

Conclusion: Although the median loss of target coverage was larger for hypofractionated VMAT than PBS, the delivered dose was within clinical tolerance for both modalities. No significant differences in robustness were identified between various fractionation schedules.

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