Session Item

Sunday
August 29
16:00 - 16:20
Plenary
C Regaud Award
1720
Award lecture
Interdisciplinary
00:00 - 00:00
Reconstruction of intra-fractional real-time prostate movement and its effect on dose distribution
PO-1620

Abstract

Reconstruction of intra-fractional real-time prostate movement and its effect on dose distribution
Authors: JÄRVINEN|, Lauri(1)*[ljjarv@gmail.com];Tenhunen|, Mikko(1);Myllykangas|, Mikko(1);Persson|, Mikael(2);Traneus|, Erik(2);Sjöberg|, Oscar(3);Ekström|, Per(3);
(1)Helsinki University Hospital, Cancer Center, Helsinki, Finland;(2)Raysearch Laboratories AB, Raysearch Laboratories AB, Stockholm, Sweden;(3)Micropos Medical AB, Micropos Medical AB, Gothenburg, Sweden;
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Purpose or Objective

Stereotactic body radiation therapy (SBRT) of prostate cancer (PCa) has recently been shown well tolerated in phase I and II trials indicating favorable normal tissue complication probability. An ongoing discussion suggests that the α/β ratio of PCa could be even lower than the α/β ratios of the surrounding organs, which supports use of hypofractionation for increased tumor control probability (TCP). As the number of treatment fractions is reduced, the risk of a TCP compromising geometric miss increases, which is the focus of the present study.

Material and Methods

The selected five patients with local PCa received 7 Gy twice a week in 5 fractions. Real-time monitoring of prostate motion was manifested using the RayPilot system (Micropos Medical AB, Gothenburg, Sweden), which consists of a treatment table top overlay containing an integrated receiving antenna array, and a wired transperineally implanted electromagnetic transmitter. The target was matched using a CBCT. Treatment was cancelled if radial prostate movement exceeded 3 mm from the CBCT match point (the origin).

The dose reconstruction was implemented using the scripting interface of a research version of RayStation 9A treatment planning system (RaySearch Laboratories AB, Stockholm, Sweden) reading 3D-coordinates of a RayPilot time series representing prostate movement, and k-means clustering up to 30 clusters. In this retrospective study, the prostate was allowed to move more than 3 mm. Dose distribution was reconstructed for each cluster and mapped back to the original CT image. CT images were created for each cluster by translation of a rigid prostate (CTV). Femoral heads were chosen to remain rigid and fixed. Soft tissues, and to some extent also pelvic bones, were deformed and transferred in relation to the prostate movement.

Results

Since the treatment system did not record beam on times in relation to the movement data, worst case scenarios were chosen to study what could have happened without real-time monitoring during one fraction. The method allowed prostate movement induced D95 reductions to be analysed per fraction series. Assuming a stationary prostate for the other four fractions, D95 reductions for the five patients were 7.6, 19.4, 0.3, 0.1 and 4.4 %. This represents a minimum error for the treatment. A maximum error would occur if the prostate movement was similar in all the five fractions. In that case the D95 reductions would be 38, 97, 1, 1 and 22 %.

Fig.1. Recorded prostate movement data. The crosshair represents the point of origin for movement data simulating a match based on CBCT image. The red and green stripes correspond to beam on times for the two arcs of the VMAT plan.


Fig.2. Original and reconstructed dose distributions of a cranial CT slice. The lack of dose in the PTV is the result of a cranial movement.


Fig.2. Original and reconstructed dose distributions of a cranial CT slice. The lack of dose in the PTV is the result of a cranial movement.
Conclusion

Prostate movement for 5 times 7 Gy fractionated PCa radiotherapy without intra-fractional monitoring could lower D95 of PTV as much as 19.4 % based on actual prostate movement in only a single fraction.