Imrt In Prostate Cancer
Carcinoma Prostate is the 4th commonest cancer in India
, with a median age of 72 years at diagnosis. The incidence in U.S. is 265,000 new cases per year with 36,000 deaths every year. Radiation treatment and radio curability in prostate cancer depends upon the dose of radiation given to prostate. In a meta analysis of 22 trials covering 11927 patients, it was seen that radiation dose of more than 70 Gy yields better response. Increasing the dose beyond this increases the rates of clinical, pathological and biochemical response.Kupelian etal (IJROBP, 61, 415-9, 2005) published data of 1325 patients from 9 institutions with T1 and T2 lesions of the prostate. The 8 years PSA disease Free Survival (PSADFS) was 62% and a dose of 72 Gy was an independent predictor of outcome. The dose response curves for conformal Radiotherapy are sigmoid curves which mean that a higher dose has to be delivered to get higher cure rates. If we look at the PSA nadir i.e. a serum PSA level of less than 1.oo ng/ml, these are seen in 90% of the patients who get more than 75 Gy. A positive biopsy rate after 5 years in patients receiving 80 Gy is only 4%. Thus dose escalation in prostate carcinoma by 3D CRT leads to an intermediate and high risk benefit after 70 Gy in response and as well as toxicity. Zietman Al etal, in 2005 published a comparison between conventional versus high dose 3D- CRT, in 393 patients randomized between conventional (n=197) and 3d CRT (n=196). The study concluded that the 5 year Disease Free Survival (DFS) was 80.4% with 3d CRT versus 61.45 in patients who received 70 Gy. There is level I evidence which shows that with 3D CRT without increase in dose reduces gastrointestinal complications and increasing the dose of radiation from 70-78/79.2 Gy decreases the failure rate.
According to the RTOG trial published by Ryu Jk etal (IJROBP 54, 1036-46) irradiation of prostate ad seminal vesicles does not increase the late gastrointestinal or gastrourinary morbidity. At the PD Hinduja National Hospital, 51 patients have been treated from 1997 to 2002 with the median dose of 72 Gy, the acute rectal toxicity has been seen to be grade 0(4 patients), grade 1(31 patients), and Grade 2 (16 patients) while late toxicity has seen to be Grade 0 (41 patients), grade 1(3 patients), and Grade 2 (2 patients). Thus the level evidence is that increase in dose of radiation over 70 Gy increases the rectal morbidity.
IMRT in prostate cancer implies alterations in the intensity in beamlets within a radiation field. It also includes high levels of target structure dose conformality, automated beam selection inverse treatment planning and concept of cross field homogeneity. Therefore with IMRT one is able to increase the dose of radiation without increasing the side effects which may improve local control of disease. However, it must be kept in mind that with increased volume exposure to modest doses of radiation an increase in radiation induced cancers may occur in the long run.
Prior to starting intensity Modulated Radiotherapy for carcinoma prostate the following basic steps are required (i) preparation of the pelvicst (immobilization device), (ii) CECT of the affected part at 5 mm slices (iii) demarcation of the target volume and tissues at risk (iv) evaluation of dose distribution (v) development of DRRs( Digital Reconstructed radiographs) (vi) generation of portal imaging. The Clinical Target Volume (CTV) for prostate carcinoma includes prostate and seminal vesicles. Seminal Vesicles can be removed from CTV after 50 Gy if probability of their involvement is less than 10%. Planning Treatment Volume (PTV0 is taken as CTV +10 mm margin in the cranio- caudal direction and 7 mm in other directions. The treatment position is supine with knees fixed and a rectal suppository is inserted 15 minutes before RT. The patient takes 750 ml of water 1 hour before start of RT and 300 ml immediately before RT. Frequent portal images are taken to correct the patients position. A 18 MV photon beam is used with the gantry at 116 and 224 degrees with step and shoot method. The daily treatment time is less than 8 minutes and usually 285 monitor units are delivered. The maximum rectal dose has to be kept below 60 Gy.
Hypofractionated IMRT is when the number of fractions is reduced to 28 and yet the total dose delivered is 70 Gy. It is seen that the rectal and bladder complications are less in hypo- fractioned IMRT but results of long term follow up are not available.
To conclude IMRT in carcinoma prostate is a well established treatment modality. It helps us to escalate the amount of radiation delivered to tumor and ensure lower dose reaching the normal structures.
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