The Dosimetric Impact of Prostate Rotations During Electromagnetically Guided External-Beam Radiation Therapy

Abstract

Purpose Study the impact of daily rotations and translations of the prostate on dosimetric coverage during RT. Methods and Materials Real–time tracking data for 26 patients were obtained during RT. IMRT plans meeting RTOG0126 dosimetric criteria were created with 0, 2, 3, and 5 mm PTV margins. Daily translations and rotations were used to reconstruct prostate delivered dose from the planned dose. D95 and V79 are computed from the delivered dose to evaluate target coverage and the adequacy of PTV margins. Prostate equivalent rotation is a new metric introduced in this study to quantify prostate rotations by accounting for prostate shape and length of rotational lever-arm. Results Large variations in prostate delivered dose were seen among patients. Adequate target coverage was met in 39%, 65%, and 84% of the patients for plans with 2, 3, and 5 mm PTV margins, respectively. While no correlations between prostate delivered dose and daily rotations are seen, the data shown clear correlation with prostate equivalent rotation. Conclusions Prostate rotations during RT could cause significant underdosing even if daily translations were managed. These rotations should be managed with rotational tolerances based on prostate equivalent rotations.

Comparison between electromagnetic transponders and radiographic imaging for prostate localization: A pelvic phantom study with rotations and translations

“…These studies show that target rotations can cause significant target under‐dosing even when translations are corrected for, with negative effects complexly linked to a combination of the magnitude of rotation, target shape and the distance between the target and transponders’ rotational centroid (eccentricity) 20, 22. As such, while there is not yet a universally recognized trigger value for action, validation of the integrity of Calypso‐derived rotational offset information is valuable, particularly if these values are to be used in the future as evidence to assist in clinical decision‐making.…”

“…This however is not standard practice for prostate radiotherapy, although it is commonly used for stereotactic work. The negative impact of prostate rotation on dosimetric coverage has been widely recognized 20, 21, 33. Consequently, the discovery of large rotational offsets upon prostate localization in the clinic will impact clinical decision‐making.…”

“…To date, there have been several studies that have utilized Calypso‐generated rotational offsets to evaluate PTV margins and dosimetric coverage for prostate radiotherapy 20, 21, 22, 23. These studies show that target rotations can cause significant target under‐dosing even when translations are corrected for, with negative effects complexly linked to a combination of the magnitude of rotation, target shape and the distance between the target and transponders’ rotational centroid (eccentricity) 20, 22.…”

“…For example, the results of one study using this rotational offset information suggested that inter‐ and intrafraction prostatic rotations may result in target underdosing in up to 61% of patients depending on the PTV expansion used 20. The same study above also proposed that this information may be useful in the future in developing a metric to predict target coverage in the clinic prior to radiotherapy treatment.…”

“…In spite of this paucity of evidence evaluating the accuracy of transponder‐based systems like Calypso in measuring rotational offsets, the authors note that a number of recent studies have used Calypso‐generated localization data to draw conclusions on margin calculations and dosimetric coverage 20, 21, 22, 23. For example, the results of one study using this rotational offset information suggested that inter‐ and intrafraction prostatic rotations may result in target underdosing in up to 61% of patients depending on the PTV expansion used 20.…”