Forward‐planning intensity‐modulated radiotherapy technique for prostate cancer

Abstract

In this study, we present an intensity‐modulated radiotherapy technique based on forward planning dose calculations to provide a concave dose distribution to the prostate and seminal vesicles by means of modified dynamic arc therapy (M‐DAT). Dynamic arcs (350 degrees) conforming to the beam's eye view of the prostate and seminal vesicles while shielding the rectum, combined with two lateral oblique conformal fields (15 degrees with respect to laterals) fitting the prostate only, were applied to deliver doses of 78 Gy and 61.23 Gy in 39 fractions to the prostate and seminal vesicles respectively. Dynamic wedges (45 degrees of thick end, anteriorly oriented) were used with conformal beams to adjust the dose homogeneity to the prostate, although in some cases, hard wedges (30 degrees of thick part, inferiorly oriented) were used with arcs to adjust the dose coverage to the seminal vesicles. The M‐DAT was applied to 10 patients in supine and 10 patients in prone positioning to determine the proper patient positioning for optimum protection of the rectum. The M‐DAT was compared with the simplified intensity‐modulated arc therapy (SIMAT) technique, composed of three phases of bilateral dynamic arcs. The mean rectal dose in M‐DAT for prone patients was 22.5±5.1 Gy; in M‐DAT and SIMAT for supine patients, it was 30.2±5.1 Gy and 39.4±6.0 Gy respectively. The doses to 15%, 25%, 35%, and 50% of the rectum volume in M‐DAT for prone patients were 44.5±10.2 Gy, 33.0±8.2 Gy, 25.3±6.4 Gy, and 16.3±5.6 Gy respectively. These values were lower than those in M‐DAT and in SIMAT for supine patients by 7.7%, 18.2%, 22.4%, and 28.5% and by 25.0%, 32.1%, 34.9%, and 41.9% of the prescribed dose (78 Gy) respectively. Ion chamber measurements showed good agreement of the calculated and measured isocentric dose (maximum deviation of 3.5%). Accuracy of the dose distribution calculation was evaluated by film dosimetry using a gamma index, allowing 3% dose variation and 4 mm distance to agreement as the individual acceptance criteria in prostate and seminal vesicle levels alike for all supine and prone patients. We found that fewer than 10% of the pixels in the dose distribution of the calculated area of 10×10−cm failed the acceptance criteria. These pixels were observed mainly in the low‐dose regions, particularly at the level of the seminal vesicles.In conclusion, the single‐phase M‐DAT technique with patients in the prone position was found to provide the intended coverage of the prescribed doses to the prostate and seminal vesicles with improved protection for the rectum. Accordingly, M‐DAT has replaced non‐modulated conformal radiotherapy or SIMAT as the standard treatment for prostate cancer in our department.PACS number: 87.53.Tf

Implementation of a wedged-dynamic arc therapy technique for head and neck cancer

“…Film calibration was performed with five kodak extended dose range (EDR2) films (Gammex rmi, Middleton, WI) and 30 cm × cm 30 × 30 cm solid water phantom (PTW-29672: PTW) typically as previously published. [7,8] For each plan a pinpoint ion chamber (PTW 31006, 0.015 cm 3 : PTW) located at the central axis of the cylindrical phantom was used to verify the isocenter point dose, while an EDR2 film was inserted axially into the Alderson phantom at the region of the primary tumor to carry out the 2D dose measurement. The measured and calculated dose planes were imported and compared using Varisoft software (version 3.1: PTW).…”

“…This technique had been published and applied successfully for the treatment of prostate cancer. [7,8] When a static wedge is attached to the rotating gantry the technique is known as wedged dynamic arc therapy (W-DAT). Consequently, one can consider this technique as one step forward of the 3D-CRT toward to provide the complex concave dose distribution.…”

Comparison of intensity‐modulated radiotherapy and forward‐planning dynamic arc therapy techniques for prostate cancer

“…In our previous work, the M-DAT technique was introduced for the treatment of the prostate plus seminal vesicles with differential doses. (5) However, in the present work, the technique is modified for treatment of the prostate only. Accordingly, in the M-DAT technique used here, the two 350-degree dynamic arcs are designed to fit the PPO only and to shield the VRV.…”

“…Modified dynamic arc therapy (M-DAT) was introduced as an extension of SIMAT. (5) This technique uses full arcs that fit to the prostate plus seminal vesicles and that shield the rectum, combined with two lateral posterior oblique wedged conformal fields of low weighting that fit to the prostate only. It was concluded that, as compared with the three-phase SIMAT technique, a single M-DAT treatment phase produces less bladder protection in the region of lower doses with consistently greater sparing of the rectum in regions of higher and lower doses.…”

“…It was concluded that, as compared with the three-phase SIMAT technique, a single M-DAT treatment phase produces less bladder protection in the region of lower doses with consistently greater sparing of the rectum in regions of higher and lower doses. (5) In the present work, we are proposing a bilateral wedged dynamic arc therapy (BW-DAT) technique to achieve a concave dose distribution to the prostate only. This technique uses two bilateral pairs of half rotation (180 degrees) around wedged arcs that fit the prostate only and shield the rectum.…”