Traditional radiation planning methods

Traditional radiation planning methods involve choosing the number, direction and arrangement of radiation beams and determining the resultant dose distribution. The plan chosen is the one that provides the best dose coverage to the target while minimizing dose to the surrounding normal tissues is chosen and this is known as forward planning. Intensity modulated radiation therapy is currently the most advanced approach for delivering highly conformal radiation therapy. This technique can achieve even more tightly conformal dose distributions through the use of nonuniform intensity radiation beams. Each beam is divided into multiple segments or beamlets Each beamlet can have different intensity thereby modulating the intensity of each beam. In contrast to the traditional method of radiation planning, IMRT works backward by choosing the desired dose distribution first and then determining the required number of beams and intensities needed to achieve this dose distribution.

This is known as inverse planning. The target volume and critical normal organs are defined and the upper and lower dose limits for the target and normal structures selected. The quality of the plan is assessed using DVHs (Figure 5). At most high volume institutions, IMRT has become the new standard of care for the treatment of men with prostate cancer using EBRT. Intensity modulated radiation therapy is a powerful technique for escalating dose and reducing toxicity. Central to achieving this goal is the recognition and adoption of strict normal tissue constraints.
Rectal side effects are manifest within 2-4 years,18 while bladder and erectile tissue side effects mature over a much longer time course.

Objective DVH criteria have clearly been associated with rectal toxicity.19, 20 The prostate (± seminal vesicles) represents the CTV. The prostate, particularly at the bladder/prostate base superiorly and the prostate apex/urogenital diaphragm inferiorly is better defined on MRI than CT (Figure 6). Also, the boundaries of the sur- rounding normal structures (bladder, rectum, penile bulb) are more easily defined on MRI. In fact, several studies have suggested that CT overestimates the prostate volume by 30-40% over MRI. Conditions for plan acceptance should include that 95-100% of the PTV receives the prescription dose. The CTV should receive 100% of the prescription dose.

The maximum dose to the PTV should not exceed the prescribed dose by more than 17% and <1% of the PTV should receive less than 65 Gy (usually <0.5%). The FCCC PTV margins are 8 mm in all dimensions, except posteriorly at the prostaterectal interface, where a 5 mm margin is planned. These margins are acceptable only when daily localization is performed. The FCCC normal tissue constraints are derived in part from the M.D. Anderson Cancer Center (MDACC) randomized trial. In that study there was a dramatic increase in ≥grade 2 rectal reactions when ≥25% of the rectal volume received ≥70 Gy. The constraints for the rectum are ≤17% and ≤35% of the rectum receives ≥65 Gy and ≥40 Gy, re- spectively. Constraints to the bladder and erectile tissues are less well-defined. However, recent data from FCCC has demonstrated the ability to limit the dose to the erectile bodies without sacrificing prostate dose homo- geneity criteria (maximum prostate dose <120% prescribed dose) and rectal tolerance criteria.
Treatment results Individual institutions defined bNED control following RT differently prior to the development of the ASTRO definition of PSA control. Comparison of treatment results was difficult if not impossible. Statistically significant differences in bNED control could be seen by merely changing the definition of bNED control when keeping all other variables stable. A conference of prostate cancer experts was held at the request of the American Society of Therapeutic Radiology and Oncology (ASTRO) in 1996 and a unified definition of bNED control was developed for reporting successes or failures following irradiation.

The definition selected by this group was designed for clinical practice as well as research trials and did not require a specific single value for post-treatment nadir PSA to determine success or failure. The ASTRO definition defined failure as 3 consecutive rises in post-treatment PSA after achieving a nadir. The date of failure (DOF) was backdated to the time midway between the post-treatment PSA nadir and the first of the consecutive rises in PSA.

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