Dosimetric effects of the prone and supine positions on image guided localized prostate cancer radiotherapy

Abstract

Purpose: To compare target coverage and doses to rectum and bladder in IMRT of localized prostate cancer in the supine versus prone position, with the inclusion of image guidance.

Materials and methods: Twenty patients with early stage localized prostate carcinoma who received external beam radiotherapy in the supine and prone positions underwent approximately 10 serial CT examinations in their respective treatment position in non-consecutive days, except for one patient who was treated prone but serially imaged supine. The prostate, bladder and rectum were contoured on all CT scans. A PTV was generated on the first scan of each patient's CT series by expanding the prostate with a 5mm margin and an IMRT plan was created. The resultant IMRT plan was then applied to that patient's remaining serial CT scans by aligning the initial CT image set with the subsequent serial CT image sets using (1) skin marks, (2) bony anatomy and (3) center of mass of the prostate. The dosimetric results from these three alignments were compared between the supine and prone groups. To account for the uncertainties associated with prostate delineation and intra-fractional geometric changes, a fictional "daily PTV" was generated by expanding the prostate with a 3mm margin on each serial CT scan. Thus, a more realistic target coverage index, V95, was quantified as the fraction of the daily PTV receiving at least 95% of the prescription dose. Dose-volume measures of the organs at risk were also compared. The fraction of the daily PTV contained by the initial PTV after each alignment method was quantified on each patient's serial CT scan, and is defined as PTV overlap index.

Results: As expected, alignment based on skin marks yielded unacceptable dose coverage for both groups of patients. Under bony alignment, the target coverage index, V95, was 97.3% and 93.6% for prone and supine patients (p<0.0001), respectively. The mean PTV overlap indices were 90.7% and 84.7% for prone and supine patients (p<0.0002), respectively. In the supine position 36% of cases showed a V95<95% after bony alignment, while only 12.5% of prone patients with V95<95% following bony alignment. Under soft-tissue alignment matching the center of mass of the prostate, the mean V95 was 99.3% and 98.6% (p<0.03) and the PTV overlap index was 97.7% and 94.8% (p<0.0002) for prone and supine groups, respectively.

Conclusions: Soft-tissue alignment combined with 5mm planning margins is appropriate in minimizing treatment planning and delivery uncertainties in both the supine and prone positions. Alignment based on bony structures showed improved results over the use of skin marks for both supine and prone setups. Under bony alignment, the dose coverage and PTV overlap index for prone setup were statistically better than for supine setup, illustrating a more consistent geometric relationship between the prostate and the pelvic bony structures when patients were treated in the prone position.